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• 12 satellite-based and 10 model-based soil moisture (SM) products are evaluated. • First assessment of 10-year subsurface SM products and estimate method on the Tibet. • SMAP products provide better estimation of surface and subsurface SM on the Plateau. • Combination of CDF matching and ExpF methods improves model-based SM estimations. • Latest version of CCI products presents degraded performance than its predecessor. This research is the first attempt to extensively assess the accuracy of commonly used surface and subsurface soil moisture (SM) datasets in reproducing temporal dynamics and trend changes SM observations for a long period on the Tibetan Plateau (TP). To this end, 12 satellite-based and 10 model-based SM products are evaluated using a 10-year upscaled SM dataset obtained from two in situ monitoring networks situated respectively in humid and arid climate zones over the TP. Moreover, the ability of using the outperforming satellite-based surface SM data in combination with cumulative distribution function (CDF) matching and exponential filter (ExpF) methods to improve model-based SM profile estimations is investigated for the first time. Among the 12 satellite-based products, SMAP_L3 provides the best surface SM retrievals, with the DCA and SCA-V products showing superior performance in the humid and arid areas, respectively. Consequently, the harmonized products produced based on the SMAP data (i.e., SMOSMAP and NNsm) also give relatively higher accuracy. It is surprised to find that the latest version of both CCI passive and active products presents degraded performance compared to its predecessor (i.e. v5.3). Among the 10 model-based products, SMAP_L4 with assimilation of L-band brightness temperature (TB) measurements provides relatively better estimations of surface and subsurface SM on the TP. Above findings highlight the superiority of utilizing the L-band TB measurements especially from the SMAP satellite to generate surface SM and improve the model-based SM estimations via data assimilation. Alternatively, it is further demonstrated that incorporating surface SM data from SMAP_L3, in conjunction with both CDF matching and ExpF methods, significantly enhances the precision of model-based SM estimations. This may open up a new possibility for improving the model-based SM profile data across the TP. [ABSTRACT FROM AUTHOR]

• A MGWR-based SMAP soil moisture product downscaling method is proposed. • The method considers the scale differences in the impact of surface variables on soil moisture. • Field validation shows higher accuracy from the downscaled SM than the SMAP SM itself. • This proposed downscaling method performs better than the traditional GWR and MW methods in the semiarid area. Passive microwave remote sensing missions such as Soil Moisture Active Passive (SMAP) are widely used for global soil moisture (SM) estimation. But the low spatial resolution of passive microwave SM products limits their applications at regional and local scales (normally 1–10 km). Using high-resolution auxiliary data to downscale passive microwave SM products is the primary way to obtain high-resolution SM data. However, the existing SM downscaling methods do not entirely account for the scale differences in the impact of various variables on the distribution of SM. To solve this problem, this paper introduced MGWR (Multiscale Geographically Weighted Regression) as a novel way to analyze the scale differences of normalized difference vegetation index (NDVI) and land surface temperature (LST) on the spatial pattern of SM. Accordingly, a new spatial downscaling algorithm for SMAP SM products is proposed and compared with the geographically weighted regression (GWR) algorithm and moving window algorithm (MW). The algorithm was applied to data taken during the SMAP Validation Experiment 2016 (SMAPVEX16), and the downscaled SM evaluated with in situ SM and aircraft observations. These results show that: 1) the SM downscaling conversion function, based on the MGWR, effectively reveals how different surface parameters relate to SM, with LST influencing SM globally and NDVI affecting it locally, 2) compared with GWR and MW, the 1 km SM obtained by the MGWR-based downscaling method showed better spatial agreement with airborne SM in the semiarid region with the ubRMSE decreasing by 0.023 m3/m3 and 0.026 m3/m3 for GWR and MW respectively, 3) when evaluating the 1 km SM with in situ SM in the semiarid region, the SM obtained through MGWR exhibited lower ubRMSE values as compared to both GWR and MW. Specifically, the median ubRMSE values for MGWR, GWR, and MW were 0.043 m3/m3, 0.053 m3/m3, and 0.094 m3/m3, respectively. In conclusion, this study demonstrates that MGWR can effectively improve the accuracy of downscaled SM in semiarid regions. [ABSTRACT FROM AUTHOR]

• Central Asia drought propagation time were longer early and late in the growing season. • Drought thresholds in Central and Western Central Asia were often below 30 mm during 1948–2022. • Soil moisture drought intensity was positively correlated with drought propagation characteristics. • VPD and AI are the main drivers of accelerated drought propagation. Drought propagation time (PT) and threshold (TR) play crucial roles in real-time monitoring of drought progression and severity under climate change, for which there is currently no unified method or standard for quantitative characterization. Central Asia (CA), a prototypical arid region, is frequently plagued by droughts that profoundly impact its ecology and socioeconomic framework, leaving a lack of comprehension about the transition from meteorological to soil moisture drought as for the complex impact of ground-atmosphere interaction. Therefore, this study revealed high-resolution PT and TR across CA based on precipitation and soil moisture datasets by employing a Bayesian causal framework for drought propagation modeling, in which the sliding window with the random forest method was integrated to investigate the dynamic changes and driving factors of drought propagation over recent decades. The conclusions drawn are as follows: (1) in the southeastern hilly and eastern plateau, a prolonged PT (i.e., longer than 100 days) was experienced during the extremities of the growing season, while most areas had a shorter mid-season PT (i.e., less than 40 days), in which the soil moisture drought intensity was positively correlated with longer PTs; (2) from 1948 to 2022, the central and western regions in CA generally displayed lower TR values than those in the northeast, seldom exceeding 30 mm, reflecting the regional precipitation gradient and the positive relationship between TR and severe soil moisture droughts; and (3) the increase in vapor pressure deficit (VPD), the decrease in aridity index (AI) for the reduced precipitation, and the considerable impact of vegetation transpiration on soil moisture consumption contributed to accelerated drought propagation. Overall, this study reveals the complex dynamics and mechanisms of drought propagation amid environmental changes, laying the foundation for innovative drought early warning systems and emergency management approaches in CA. [ABSTRACT FROM AUTHOR]

• Rare examples of highly instrumented observatories in African drylands. • Large water surplus during monsoon not represented by annual scale of aridity index. • Overland flow is rare and root zone achieves field capacity across hillslopes. • Stable isotopes indicate recharge season timing, not a link to heavy rainfall. • Recharge is diffuse in semi-arid Africa validating soil moisture balance model use. We use two comprehensively instrumented field observatories to understand groundwater recharge processes in African drylands. The observatories are located on crystalline basement geology in semi-arid parts of Ghana and Burkina Faso, aridity indices 0.43 and 0.29, respectively, and we report 2017–2019 observations. Groundwater recharge was quantified by inverse water table fluctuation models using specific yield estimates derived from magnetic resonance soundings. Evidence for recharge drivers and mechanisms comes from high resolution meteorological observations, soil moisture (logged hourly and weekly along hillslope transects), overland flow plots, river stage, and stable isotopes of O and H in rainfall events and groundwater. Groundwater recharge varied between 87 and 175 mm/y, i.e. 7–15 % of annual rainfall. Rainfall was twice the volume of water lost via actual evapotranspiration across the four peak months (Jun-Sep) of the monsoon. This seasonal water surplus of ∼ 350 mm/y is not characterised by the annual scale of the aridity index. Overland flow was rare and soil moisture deficits were overcome at all monitoring locations. Large rainfall events only produced appreciable recharge when the antecedent soil moisture was close to field capacity, yet always produced large responses in river stage. Stable isotopes of O and H in groundwater indicate no evidence of evapotranspiration prior to infiltration and their composition is akin to depleted isotopic rainfall at the monsoon peak. Stable isotopes indicate recharge season timing and not a relationship between intense rainfall and groundwater recharge. We contend that the mechanism for groundwater recharge is predominantly diffuse in these semi-arid African settings. [ABSTRACT FROM AUTHOR]

• Spatial distribution of drought onset time decreased with an increase in regional wetness in China. • Drought onset speed was higher in the southern humid region than that in the northern region. • Drought onset was accelerated after 1994, particularly in regions with precipitation range from 300 mm to 600 mm. • The greater the warming magnitude, the wetter the region, the faster the drought onset. • Increased water deficiency was the main reason for the accelerating drought onset. This study investigated the effects of warming on the drought onset in China using meteorological data and soil moisture data from 1980 to 2020. The variation characteristics of drought onset time and onset speed were analyzed systematically. The differences in drought onset time and onset speed during the period 1995–2020 and the period 1980–1994 were compared, and possible impact factors were analyzed. Results indicated that the spatial distribution of drought onset time decreased with an increase in regional wetness in China. In arid regions, onset time at 73 % of sites was over 35 days, and about 13 % was among 60–90 days. In semi-arid and sub-humid regions, the onset time was 25–40 days, and in humid regions, it was 20–30 days. After 1994, namely the significant temperature rise period, the onset time has remarkably shortened in parts of the southern Northeast, North China, the eastern Northwest, the western Southwest, the middle of Tibet, the lower reaches of the Yangtze River, and the coastal region of South China. The shortening was most obvious in sub-humid regions. The greater the warming magnitude, the wetter the region, the faster the drought onset speed. Drought onset speed (γ) was higher in the southern humid region than that in the northern region, and γ for southern Northeast China, northern North China, and eastern Northwest China was higher than that for the rest of North China. After 1994, drought onset speed was accelerated in most regions, especially in the semi-arid and sub-humid regions with precipitation in the range of 300–600 mm. Under the combined effect of increased evapotranspiration and reduced precipitation, the region has been the hot spot with increased drought speed after obvious warming in China. [ABSTRACT FROM AUTHOR]

• Determine amount, frequency, and duration of dew/hoar frost using weighing lysimeters. • Compare characteristics of dew/hoar frost between Artemisia ordosica and bare soil. • Dew provides 30% of water for Artemisia ordosica during extreme drought periods. Dew and hoar frost are important water resources in arid and semi-arid regions. However, knowledge of the characteristics of dew/hoar frost formation in the typical shrub canopy of Northwestern China and its importance for vegetation survival is still not clear as it is difficult to observe. In this study, we investigated dew/hoar frost formation characteristics under both Artemisia ordosica and bare soil conditions based on weighing lysimeters in Mu Us Sandy Land, Northwest of China. The results show that dew and hoar frost occurred 214 days for Artemisia ordosica and 162 days for bare soil. The hoar frost formation mainly occurs from October to March, while dew formation occurs every month. The amount of hoar frost was 19.6 mm under Artemisia ordosica condition and 17.4 mm under bare soil condition, and the amount of dew was 27.5 mm of Artemisia ordosica and 18.4 mm of bare soil. The most common daily amount of dew/hoar frost is between 0 and 0.2 mm, which accounts for 60.7 % and 52.5 % of dew/hoar frost formation days for the Artemisia ordosica and bare soil conditions, respectively. The results from boosted regression tree analysis indicated that relative humidity had the greatest influence on dew/hoar frost formation. During soil drought periods (July 15 to August 18, 2021), the soil water content of Artemisia ordosica kept constant at 0.061 cm3/cm3, while the soil water content of bare soil was dropped at the rate of 0.00135 (cm3/cm3)/d. The ratio of dew amount of Artemisia ordosica to rainfall amount was 42.1 %. Despite the small amount of dew, it serves as a frequent and important supplementary water source for the ecosystem. This study provided valuable insights into dew/hoar frost formation, especially its importance for vegetation survival. [ABSTRACT FROM AUTHOR]

• The water release capacity of BSCs was related to their types and water suction. • BSCs maintained lower water release capacity compared to bare soil under higher water suction. • Lower water release capacity of BSCs under higher suction was attributed to the increase of > 0.25 mm aggregates and SOM. • BSCs exhibited a better WRC due to their lower water release capacity under relatively dry conditions. • Lower water release capacity of BSCs is favorable for dryland ecosystem restoration. Water retention capacity (WRC) of soil is the ability of soil to hold and retain water, which reflects soil water availability and soil drought resistance. Biological soil crusts (biocrusts), the ubiquitous living cover in drylands, regulate water holding capacity (WHC) by changing surface soil physico-chemical properties. However, to date, the water release capacity of biocrusts remains unclear, which hinders a deep understanding of the WRC of biocrusts. Therefore, we examined the WRC by analyzing the WHC and water release capacity of five types of biocrusts at different successional stages: light cyanobacteria (LC), dark cyanobacteria (DC), cyanobacteria mixed with a few mosses (CM), moss mixed with small patches of cyanobacteria (MC), and moss (MS) biocrust. Bare soil (CK) was used as the control. The results showed that there was no significant difference in WHC between CK and the five types of biocrusts. However, biocrusts displayed different water release capacities at different soil water suctions. Biocrusts maintained a higher water release capacity when the soil water suction ranged from 200 cm H 2 O to 800 cm H 2 O (the soil volumetric water content (θ v) ranged from 32% to 22%); furthermore, there were no significant differences in the water release capacity of different types of biocrusts. The biocrusts exerted a lower water release capacity compared to bare soil when the soil water suction ranged from 800 cm H 2 O to 3000 cm H 2 O (θ v ranged from 22% to 11%), which was due to the increases in the > 0.25 mm aggregate content and the soil organic matter (SOM) contents. Biocrusts exhibited a better WRC due to their lower water release capacity under relatively dry conditions (11% < θ v < 22%), which enables biocrusts to maintain longer wet duration and benefits their development. Hence, good development of biocrusts is considered beneficial for the restoration, stability and sustainability of drylands with limited water resources are limited. [ABSTRACT FROM AUTHOR]

• A proportionality coefficient is proposed to approximate the GRACE TWS by the SMS. • Using multi-source SMS products can efficiently reconstruct continuous TWSA. Since 2002, the terrestrial water storage anomalies (TWSA) derived from the Gravity Recovery and Climate Experiment (GRACE) and its successor, GRACE Follow-on (GRACE-FO), have been widely used in water storage studies. Interannual and subseasonal variabilities in terrestrial water storage are critical for understanding the change of water cycle and interpretation of GRACE error. In this study, we evaluate the interannual plus subseasonal terms of global TWSA from GRACE satellite data and soil moisture storage estimated from multi-source soil moisture data. Results show that soil moisture change dominated total water storage change in most regions, especially in relatively arid regions. We further reconstruct the continuous TWSA by combing the trend and seasonal terms from the GRACE(-FO) TWSA data with the interannual and subseasonal variabilities of multi-source soil moisture products on a global scale from April 2002 to December 2021. Our reconstruction result is further compared with the TWSA from GRACE(-FO) and two recent reconstruction products. The comparisons show that the reconstruction result has great consistency with GRACE(-FO), with 82% of the grids showing Nash-Sutcliffe efficiency coefficient values greater than 0.6 based on JPL Mascon solution. This study emphasizes that the information from soil moisture can provide an efficient approach for tracking the interannual and subseasonal variabilities of long-term terrestrial water storage. [ABSTRACT FROM AUTHOR]

• Heatwaves in CA are becoming earlier, more frequent, more intense, more persistent. • Heatwaves are more frequently observed in the west but less in the central CA. • Soil moisture depletion is the main factor in formulating heatwaves in CA. Heatwaves pose increasing threats on water resources for ecosystems and human consumption especially in arid region, such as Central Asia (CA). As the hinterland of Eurasia, water scarcity and land degradation in CA have increased its vulnerability to heatwaves. While the frequency of heatwaves has been examined, more heatwave metrics such as timing are less known, possible influences of soil moisture on heatwave changes in this region are not fully understood. Using reanalysis and observational datasets, here we examine the spatial–temporal changes of heatwaves in CA during 1981–2020 and the role of various associated factors including soil moisture. The results show that the extreme heatwaves occurred more frequently in northwestern Kazakhstan. Temporally, heatwaves have been intensifying with significant increases in frequency (0.10 events/year), intensity (0.81 °C/year), and duration (0.08 days/year) since the 1980 s, and these trends have accelerated since 1997. The first yearly heatwave event tended to occur earlier in more recent decades, indicating an advanced heatwave season. We also found that the heatwave occurrences were significantly correlated with hydro-meteorological factors. In northwestern of CA, extreme heatwaves are exacerbated by a rapid decrease in soil moisture resulting from reduced precipitation and increased solar net radiation in this area. Our findings provide a scientific basis for understanding the mechanisms and predictions of heatwave in arid regions from the climate change perspective. [ABSTRACT FROM AUTHOR]

• Globally, RZSM showed a decreasing trend over 1981–2017. • RZSM over 50% of global land areas decreased. • Globally, decreased RZSM was mainly induced by the rising temperature. Root zone soil moisture (RZSM) has a direct impact on ecosystem function, vegetation growth and food security, and plays a vital role in global climate system, water and carbon cycles. However, large variations and uncertainties still exist in RZSM across the globe under the warming climate. In this study, we applied comparison map profile (CMP), Theil-Sen regression and partial correlation analysis to investigate the spatial and temporal changes of RZMS and its driving factors from 1981 to 2017 by using three soil moisture products–ERA5, GLDAS and MERRA-2. Results showed that RZSM derived from three products presented a similar spatial pattern that the highest RZSM values occurred in tropical forest and cold areas, followed by subtropical, while the relatively low RZSM values were observed in arid and semiarid regions. Globally, RZSM decreased in all of three datasets with a rate of −0.14 × 10-3 m3 m−3 yr−1 on average (p < 0.001), which was largely correlated with temperature anomalies. Spatially, the RZSM trends greatly varied, with 21–31% of global land areas experiencing a significant decreasing trend and 7–24% for an increasing trend, respectively, confirming their different sensitivities to climate change. Temperature-driven RZSM dominated 19–29% of global land areas and was primarily distributed in northern high-latitude areas. The areas dominated by evapotranspiration were mainly in arid and semiarid areas, accounting for 29–44% of global land areas. Precipitation dominates the remaining 36–45% of global land areas mainly in eastern America and Europe, suggesting variations in the dominance of environmental factors on the spatial patterns of RZSM trend. Our findings will deepen our understanding of the impacts of climate change on the long-term trend of global soil moisture, and will be greatly critical to global soil water resource protection and management under the warming climate. [ABSTRACT FROM AUTHOR]

A decrease in soil water content (SWC) has been observed at the global scale and has led to a reduction in evapotranspiration (ET). However, the trend in SWC in eastern Asia differed from the global trend, and this may be due to the overlapping of warm/wet seasons. Because of the limitation of deep soil water content data, especially in drylands, changes in SWC and the correlation between SWC (particularly deep SWC) and ET remain poorly understood. In this study, spatial distribution of correlation coefficient (r) between SWC and ET were calculated based on reanalysis data and remote sensing data, which were validated by in situ data (2004–2005) and sampling data (2016–2017). Firstly, the correlation between SWC and ET decreased with soil depth. The depth of the sharp decrease was 10 cm based on in situ monitoring data (2005–2008) and spatial sampling data (2016–2017). Secondly, the spatial correlation between SWC and ET varied with different vegetation cover types. The correlation increased from the western typical steppe to the eastern forest. The highest values of r were in a forest-covered area, because it had the highest water uptake capacity. However, the difference between r_10 and r_100 (D_r) showed a different spatial pattern, and the lowest negative D_r value was obtained from a forest–grass transition zone in which the enhanced vegetation index showed a significant increasing trend. The significant negative regression determination coefficient (R2 = 0.9, p < 0.01) between B_ EVI (0–0.1) and D_r indicated that the narrowed D_r in the transition zone was due to the vegetation increase. This meant that more deep soil water was taken up into the atmosphere. Thus, the deep SWC could become unstable and deep soil could become drier, which would be unsustainable for the ecosystem. [ABSTRACT FROM AUTHOR]

• Streamflow is correlated more strongly with snowfall than with rainfall. • Warming has caused peak flow to occur earlier. • The effects of snowmelt on streamflow is subject to rainfall regulation on soil moisture condition. • Spring rainfall and snowmelt play a dominant role in spring and summer flow generation. • The mechanisms for flow generation in autumn and winter are complex and vary across space. Mountain watersheds serve as important water sources for socioeconomic activities in semiarid and arid regions. This study examined the streamflow response to snow regime shifts associated with climate variability in the US Great Basin. To this end, the coupled hydro-ecological simulation system (CHESS), a process-based and distributed model, was applied to four mountain watersheds in the US Great Basin. Historical weather records for the period of 1961–1990 were used to spin-up model simulations so that the soil water and vegetation reached the equilibrium state under long-term climate conditions; the period of 1991–2015 was selected as the study period. The model evaluation suggested that CHESS was able to simulate major hydrological processes in the four mountain watersheds. In addition, it was found that annual streamflow was more strongly correlated with snowfall than with rainfall during 1991–2015 and increased by 0.62–0.76 mm as snowfall increased by 1 mm in the study regions. Recent regional warming advanced the timing of the peak flow and shifts in the snow regime also affected the intra-annual pattern of the soil moisture content. Seasonally, spring rainfall and snowmelt were dominant factors in generating spring streamflow and summer baseflow. The mechanisms for flow generation were more complex and more variable in the four watersheds in autumn and winter than in spring and summer. On an annual basis, shifts in snowmelt resulting from climate variability accounted for more than 60% of the variability in the annual streamflow, surpassing the contribution of rainfall in the four watersheds. Overall, our results suggest that shifts in the snow regime due to climate variability have important implications for the intra-annual distribution and availability of water resources in semiarid and arid regions of the US Great Basin. [ABSTRACT FROM AUTHOR]

Highlights • Assessed impact of soil moisture on runoff in semi-arid area. • High antecedent moisture led to more runoff at plot and catchment scales. • Upscaling plot-scale parameters is difficult given high spatial variability. • Infiltration model performance depends on spatial scale. • Simplistic models may be preferable if data is limited. Abstract Antecedent soil moisture is an important factor in the generation of runoff, but guidance for modeling moisture conditions in semiarid catchments is limited and conflicting. In this study, the impact of antecedent moisture was assessed at the plot scale (2.8 m2) using a portable rainfall simulator, and at the catchment scale based on observed precipitation and discharge for a 2.8 km2 watershed in central New Mexico. Performance of three loss models commonly used for hydrologic analysis in the southwestern U.S. was tested at both scales. High initial moisture content led to substantially higher runoff ratios at both scales, confirming the importance of antecedent soil moisture for runoff predictions in semiarid drainages. Hydrologic parameters estimated based on plot experiments, however, were highly variable and cannot easily be upscaled to the catchment scale. Loss model performance was clearly scale dependent: more simplistic (one- and two parameter) loss methods out-performed a more complex (four parameter) model at the watershed scale, while the opposite held true for test plot simulations. At the catchment scale, all models performed poorly for small runoff events, but yielded acceptable results for storms causing large discharges if antecedent soil moisture was considered. Failing to account for antecedent moisture led to simulated runoff volume errors up to one order of magnitude. [ABSTRACT FROM AUTHOR]

[Display omitted] • The top-down QPEs perform better over the humid and low-altitude regions. • IMERG-F (SM2RAIN-ASCAT) performs best (worst) in detecting drought occurrence. • The drought dynamic evolutions highly depend on the drought indices used. • All QPEs capture drought centroid trajectory better on smaller spatial scales. Satellite-based quantitative precipitation estimates (QPEs) have shown great potential in the precipitation-related extremes estimation. However, little is known about the capability of different QPEs to monitor the dynamic evolutions of drought on various spatio-temporal scales. Here, the drought utility of two top-down QPEs (i.e., Climate Hazards Group InfraRed Precipitation with Stations, CHIRPS; Integrated Multi-SatellitE Retrievals for Global precipitation measurement-Final run, IMERG-F) and one bottom-up QPE (i.e., Soil Moisture to Rain-Advanced SCATterometer, SM2RAIN-ASCAT), is evaluated comprehensively in mainland China, based on three commonly used drought indices (Standardized Precipitation Index, Standardized Precipitation Evapotranspiration Index, Composite index of meteorological drought). The results indicate that the top-down QPEs generally show better performance in detecting drought occurrence over the humid and low-altitude regions in eastern China than the arid and high-altitude regions in western China. IMERG-F performs best in detecting drought occurrence in the entire mainland China, followed by CHIRPS, and the least satisfactory by SM2RAIN-ASCAT. The trajectory of drought centroids and the temporal variability of drought characteristics are highly dependent on both the drought indices selected and QPEs used. IMERG-F and CHIRPS can better reproduce the trajectories of large-scale drought events (e.g., the locations of drought origin and termination) than SM2RAIN-ASCAT. The ability in tracking the drought centroid trajectory of all three QPEs is higher at the smaller spatial scales, also it improves from the semi-arid to humid regions. Our results highlight the superiority of the top-down QPEs in drought monitoring compared with the bottom-up QPE in mainland China, particularly over the humid regions. The application of SM2RAIN-ASCAT in drought monitoring to the large-scale arid areas should be cautious. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Nine environmental variables were analyzed from 70 soil samples. • A 10-fold cross-validation procedure was used. • We evaluated model performance and predictive uncertainty. • BRT model had better properties and spatial visibility than the MLSR. • PWP content decreased from the southwest to the northeast of the study area. Abstract The permanent wilting point (PWP) is the minimal point of soil moisture a plant requires not to wilt. If moisture decreases to this, or any lower point, a plant wilts, and can no longer recover its turgidity when placed in a saturated atmosphere for 12 h. The PWP is an essential physical property that has a powerful influence on other soil properties and is important for agricultural production, including irrigation water use efficiency and crop yield. However, there are few published datasets on the PWP of cultivated land in the arid and semi-arid regions of Northeast China. Direct measurements of PWP are time-consuming and expensive; therefore, the aim of this study was to evaluate how environmental variables could be used in spatial predictions of PWP in an arid and semi-arid region based on boosted regression tree (BRT) and multiple linear stepwise regression (MLSR) analyses. Seventy soil samples and nine covariates (including topography and vegetation variables) were collected and analyzed. Cross-validation procedure was used to evaluate model performance and uncertainty. Accuracy evaluation results showed that the BRT model had better predictive performance than the MLSR. PWP content decreased from the southwest to the northeast of the study area, and average values of both models were 26%. BRT and MLSR models should be compared and calibrated to obtain the best prediction effect of PWP spatial distribution in similar areas. Elevation, slope gradient, profile curvature, topographic wetness index, and land-use variables are the main environmental indicators that should be included when generating PWP maps of regions with a dry, continental climate. Such information could aid local land managers and government agencies with evaluating soil quality and water sequestration potential. In conclusion, this study could be used as a reference for predicting the hydrological parameters of the topsoil in other similar ecological environments. [ABSTRACT FROM AUTHOR]

When restoring dryland ecosystems, growing biological soil crust (biocrust) may greatly change the redistribution of rainfall in layered soils. However, ecohydrological modelling studies generally ignore biocrust and thus, the ecohydrological effects of biocrust on restorations remain largely unexplored. Using a long-term restoration case (located in the southeast edge of the Tengger Desert, northern China), we developed an ecohydrological model with explicit consideration of the infiltration in three layered soils (biocrust, shallow and deep sand layers) to investigate influences of biocrust on restoration dynamics in drylands. The proportion of infiltration that reaches ‘annual grass’ (including biocrust and shallow sand layers, 0–30 cm) and ‘shrub’ layers (30–150 cm) with biocrust significantly increased and decreased relative to the values without biocrust, respectively. Meanwhile, biocrust significantly decreased soil water content in deep sand layer, but not in shallow sand layer. As more water was used by transpiration than evaporation, the ecosystem with biocrust reached a final grass-dominated state (high grass cover of 40%, low shrub cover of 4%) rather than a shrub-dominated state (grass cover of 3%, shrub cover of 20%). This study suggests that we need to account for the roles of biocrust on rainfall infiltration to better understand vegetation and restoration dynamics in dryland ecosystems. [ABSTRACT FROM AUTHOR]

Agricultural irrigation with trans-basin water diversion can effectively relieve the water paucity in arid and semi-arid regions, however, this may be accompanied by eco-environmental problems (e.g., saline soils, rising groundwater levels, water quality problems). The mechanism of soil water movement under irrigation in the unsaturated zone of arid regions is a key scientific problem that should be solved in order to evaluate agricultural water management and further improve current irrigation practices. This study investigated the impact of drip irrigation on soil water movement in the unsaturated zone of a cotton field in an inland arid region (the Karamay Agricultural Development Area), northwest China. Combining in situ observational physical data with temporal variation in stable isotopic compositions of soil water, we described the soil water flow system and mechanism in severe (Plot 1) and mild (Plot 2) saline-alkali cotton fields. The infiltration depths are 0–150 cm for both plots. Drip irrigation scheduling makes no significant contribution to local groundwater recharge, however, groundwater can move into the unsaturated zone through capillary rise during cotton flowering and boll periods. Plot 2 is less prone to having secondary soil salinization than Plot 1 due to the existence of a middle layer (approximately 100 cm thick), which elongated the distance between the root zone and aquifer. Rise in the water table (approximately 60 cm for Plot 1 and 50 cm for Plot 2) could be caused by lateral groundwater flow instead of vertical infiltration. We estimated the soil water storage changes in the unsaturated zone and proposed a conceptual model for deciphering the movement process of soil water. This study provides a scientific basis for determining the rise of groundwater levels and potential development of saline soils and improving agricultural water management in arid regions. [ABSTRACT FROM AUTHOR]

The transport of water and vapour in the desert vadose zone plays a critical role in the overall water and energy balances of near-surface environments in arid regions. However, field measurements in extremely dry environments face many difficulties and challenges, so few studies have examined water and vapour transport processes in the desert vadose zone. The main objective of this study is to analyse the mechanisms of soil water and vapour transport in the desert vadose zone (depth of ∼350 cm) by using measured and modelled data in an extremely arid environment. The field experiments are implemented in an area of the Gobi desert in northwestern China to measure the soil properties, daily soil moisture and temperature, daily water-table depth and temperature, and daily meteorological records from DOYs (Days of Year) 114–212 in 2014 (growing season). The Hydrus-1D model, which simulates the coupled transport of water, vapour and heat in the vadose zone, is employed to simulate the layered soil moisture and temperature regimes and analyse the transport processes of soil water and vapour. The measured results show that the soil water and temperatures near the land surface have visible daily fluctuations across the entire soil profile. Thermal vapour movement is the most important component of the total water flux and the soil temperature gradient is the major driving factor that affects vapour transport in the desert vadose zone. The most active water and heat exchange occurs in the upper soil layer (depths of 0–25 cm). The matric potential change from the precipitation mainly re-draws the spatio-temporal distribution of the isothermal liquid water in the soil near the land surface. The matric potential has little effect on the isothermal vapour and thermal liquid water flux. These findings offer new insights into the liquid water and vapour movement processes in the extremely arid environment. [ABSTRACT FROM AUTHOR]

Indirect measurements of field-scale (hectometer grid-size) spatial-average near-surface soil moisture are becoming increasingly available by exploiting new-generation ground-based and satellite sensors. Nonetheless, modeling applications for water resources management require knowledge of plot-scale (1–5 m grid-size) soil moisture by using measurements through spatially-distributed sensor network systems. Since efforts to fulfill such requirements are not always possible due to time and budget constraints, alternative approaches are desirable. In this study, we explore the feasibility of determining spatial-average soil moisture and soil moisture patterns given the knowledge of long-term records of climate forcing data and topographic attributes. A downscaling approach is proposed that couples two different models: the Eco-Hydrological Bucket and Equilibrium Moisture from Topography. This approach helps identify the relative importance of two compound topographic indexes in explaining the spatial variation of soil moisture patterns, indicating valley- and hillslope-dependence controlled by lateral flow and radiative processes, respectively. The integrated model also detects temporal instability if the dominant type of topographic dependence changes with spatial-average soil moisture. Model application was carried out at three sites in different parts of Italy, each characterized by different environmental conditions. Prior calibration was performed by using sparse and sporadic soil moisture values measured by portable time domain reflectometry devices. Cross-site comparisons offer different interpretations in the explained spatial variation of soil moisture patterns, with time-invariant valley-dependence (site in northern Italy) and hillslope-dependence (site in southern Italy). The sources of soil moisture spatial variation at the site in central Italy are time-variant within the year and the seasonal change of topographic dependence can be conveniently correlated to a climate indicator such as the aridity index. [ABSTRACT FROM AUTHOR]

The objective of this study was to exploit the synergy between SMOS and SMAP based on vegetation optical depth (VOD) to improve brightness temperature (TB) simulations and land surface soil moisture (SM) retrievals in arid regions of the world. In the current operational algorithm of SMAP (level 2), vegetation water content (VWC) is considered as a proxy to compute VOD which is calculated by an empirical conversion function of NDVI. Avoiding the empirical estimation of VOD, the SMOS algorithm is used to retrieve simultaneously SM and VOD from TB observations. The present study attempted to improve SMAP TB simulations and SM retrievals by benefiting from the advantages of the SMOS (L-MEB) algorithm. This was achieved by using a synergy method based on replacing the default value of SMAP VOD with the retrieved value of VOD from the SMOS multi angular and bi-polarization observations of TB. The insitu SM measurements, used as reference SM in this study, were obtained from the International Soil Moisture Network (ISMN) over 180 stations located in arid regions of the world. Furthermore, four stations were randomly selected to analyze the temporal variations in VOD and SM. Results of the synergy method showed that the accuracy of the TB simulations and SM retrievals was respectively improved at 144 and 124 stations (out of a total of 180 stations) in terms of coefficient of determination ( R 2 ) and unbiased root mean squared error (UbRMSE). Analyzing the temporal variations in VOD showed that the SMOS VOD, conversely to the SMAP VOD, can better illustrate the presence of herbaceous plants and may be a better indicator of the seasonal changes in the vegetation density and biomass over the year. [ABSTRACT FROM AUTHOR]

• Precipitation, NDVI, and LUCC dominated recipitation the spatial heterogeneity of SM. • Precipitation changes caused up to 40% of the drying up of SM. • SM anomalies at 28–289 cm mainly caused by one-month-old precipitation. • The influences of climate and vegetation were observed to decrease with soil depth. As an essential state variable of the earth's terrestrial system, soil moisture (SM) is highly significant regarding hydrological processes, agricultural production, land management in response to issues like soil erosion etc. However, the net effects of different environmental factors on the heterogeneity and variations of SM remain unclear, due to the complex interactions between variables. Indeed, investigating such effects in the highly climate-vulnerable Mongolian Plateau is imperative for water resource management and climate change adaptation. In this study, we analyzed the contributions of different environmental factors on the spatial heterogeneity and variations of SM in the Mongolian Plateau between 1982 and 2020, using a geographic detector model (GDM) and a novel nonlinear Granger causality model. The results of the GDM analysis demonstrated that precipitation and vegetation were the main controls relevant to SM heterogeneity, with their explanatory powers (Q statistics of GDM) found to be higher than 0.67. In two breakpoints—one in the early 1990′s and another in 2007—SM demonstrated a pattern of increase, then a decrease, subsequently followed by an increase (insignificant decrease in SM at 100–289 cm depth only). Precipitation was identified as the Granger causal of SM variations over 33.39–97.65% of the plateau's vegetated area, leading to the drying up of 40% of its SM. The greatest contribution to SM in 28–289 cm of the plateau was observed as coming from one-month-old precipitation, due to the lag in the manifestation of effects. Further, rising temperatures were found to have an immediate influence (2.73–5.23%) on SM in 14.61–54.90% of the whole plateau's vegetated area, an impact which was relatively high (>14%) in its northeastern wetting zone. Vegetation change posed a relatively weak effect (<2%) on SM over a limited area of the plateau (27.75–35.42%). Although lesser than the impacts of general climate changes, the contributions of climatic extremes could not be neglected, ultimately accounting for up to 10% of SM changes. In summary, this study provides new insights into the individual contributions of various environmental factors on the spatial heterogeneity and variations of SM in the Mongolian Plateau, offering vital information for policymaking regarding climate change mitigation and adaptation, sustainable use of water resources, and ecological restoration for arid and semi-arid region. [ABSTRACT FROM AUTHOR]

Woody plant cover has increased 10-fold over the last 140+ years in many parts of the semi-arid western USA. Woody plant cover can alter the timing and amount of plant available moisture in the soil and saprolite. To assess spatiotemporal subsurface moisture dynamics over two water years in a snow-dominated western juniper stand we compared moisture dynamics horizontally across a discontinuous canopy, and vertically in soil and saprolite. We monitored soil moisture at 15 and 60 cm and conducted periodic electromagnetic induction and electrical resistivity tomography surveys aimed at sensing moisture changes within the root zone and saprolite. Timing of soil moisture dry down at 15 cm was very similar between canopy patches and interspace. Conversely, dry down at 60 cm occurred 22 days earlier in the interspace than under canopy patches. After rainfall, interspaces with more shrubs showed greater increases in soil moisture than interspaces with few shrubs. For the few rainfall events that were large enough to increase soil moisture at 60 cm, increases in moisture occurred almost exclusively below the canopy. Soil water holding capacity from 0 to 150 cm was a primary driver of areas that were associated with the greatest change in distributed electrical conductivity - an indicator of changes in soil moisture - across the growing season. Vegetation was also correlated with a greater seasonal change in electrical conductivity at these depths. The seasonal change in resistivity suggested moisture extraction by juniper well into the saprolite, as deep as 12 m below the surface. This change in deep subsurface resistivity primarily occurred below medium and large juniper trees. This study suggests how tree roots are both increasing infiltration below their canopy while also extracting moisture at depths of upwards of 12 m. Information from this study can help improve our understanding of juniper resilience to drought and the hydrologic impacts of semi-arid land cover change. [ABSTRACT FROM AUTHOR]

• Performance of ERA5, ECV, FLDAS, and GLDAS SM products was assessed in Central Asia. • ERA5 SM product has the highest correlation and lowest RMSE with in-situ stations. • GLDAS has the lowest uncertainty followed by FLDAS and ERA5 SM products. • GLDAS performs better than the other three SM products in capturing drought events. Reanalysis and remotely sensed soil moisture (SM) products are important for monitoring hydrology and the terrestrial carbon cycle, especially in water-limited arid regions. However, it is essential to assess the reliability of SM products before they are used. In this study, surface SM from the European Reanalysis-Interim v5 (ERA5), Essential Climate Variables (ECV), the Famine Early Warning Systems Network (FEWS) Land Data Assimilation System (FLDAS), and the Global Land Data Assimilation System (GLDAS) were assessed based on in-situ data and the Three-Cornered Hat (TCH) approach in a typical arid region, Central Asia (CA). The validation based on in-situ data indicated that all three products underestimated the observed surface SM, except for ERA5. The ERA5 SM was closest to the in-situ observations, averaging a higher correlation coefficient of 0.59, followed by the GLDAS, ECV, and FLDAS. The TCH method revealed low uncertainties (<0.05 m3/m3) for all four SM products, and high uncertainties were found at high altitudes. For ERA5 and ECV SM products, the uncertainty is high in the shrublands and forests, respectively. FLDAS and GLDAS SM products performed better in terms of uncertainty compared to ERA5 and ECV, with an average uncertainty below 0.03 m3/m3, except for forests. The above assessment results indicate that, compared with the other three SM products, ERA5 has a better performance and more potential for application in CA. There was an upward trend in surface SM from 1982 to 2018 in CA. The relationship between the four SM products and aridity index and drought events showed that GLDAS could capture the hydrothermal conditions and drought events, followed by FLDAS, ERA5, and ECV. Our research is intended to provide some reference for the application of gridded SM products to ecosystem and water resources monitoring in arid regions. [ABSTRACT FROM AUTHOR]

• Projections show a clear reduction of soil moisture through the entire annual cycle. • Soil moisture deficits up to 6x (7x) are projected to occur for mid- (end-) of-century. • Semi-arid climate is expected to cover almost 2/3 of the mainland in the RCP8.5. Climate change constitutes a major threat for all the Mediterranean countries due to the combination of large precipitation reductions and temperature increases and the higher frequency of climate extremes, especially driving water scarcity and all the derived multi-sectoral impacts. Portugal, as most of the Mediterranean countries, already endures larger frequencies of droughts and deficits in soil moisture and water storage. In the current study, the future projections of soil moisture are examined using a multi-model EURO-CORDEX regional climate ensemble, in agreement with three future emission scenarios (RCP2.6, 4.5 and 8.5). The drivers of future soil moisture dynamics are also analysed and its effect on relative humidity and evaporation rates. As expected, the projections show a clear reduction of soil moisture through the entire annual cycle, in response to the large decrease in precipitation and temperature increase, via a massive growth of potential evapotranspiration. The overall total soil moisture decreases ranges from −5% for the RCP2.6 to −20% (-10%) for the RCP8.5 (RCP4.5), w.r.t. the present climate. In the historical period, soil moisture deficits rarely reach values 3x over the standard deviation, but projections reveal that for the RCP4.5 (RCP8.5) for the mid-century deficits up to 5x (6x) are projected to occur, and for the end-of-century even 7x for the RCP8.5. The annual cycle of soil moisture is in present and future climate determined by precipitation and potential evapotranspiration, and deficit is both enhanced and covers a wider monthly window in the future, especially for the RCP8.5. The surface humidity also decreases importantly, up to −4% and −8% in spring and summer in the end-of-the-century, in agreement with RCP4.5 and RCP8.5, respectively. Resulting from the projected changes in precipitation and potential evapotranspiration, the typical semi-arid climate, which in present climate is confined to a small south-eastern region of Portugal, is expected to cover almost 2/3 of the mainland in the case of RCP8.5. Finally, this study was developed in the framework of the National Roadmap for Adaptation XXI - Portuguese Territorial Climate Change Vulnerability Assessment for XXI Century (RNA2100) project and aims at delivering a deeper and different featuring of terrestrial water for adaptation purposes in a Mediterranean country. [ABSTRACT FROM AUTHOR]

• PRE-VI associations were greater in the Southern hemisphere drylands. • Moderate soil moisture enhanced the PRE-VI association in global dryland. • Decreased soil moisture weakened the PRE-VI association during 1982–2015. Vegetation growth is critical in characterizing ecosystem carbon sequestration. Water availability profoundly affects the interaction between climate change and vegetation dynamics in drylands. However, the associations of the precipitation–vegetation index (PRE-VI) and temperature–vegetation index (TEM-VI) as well as whether and how the soil moisture modulates PRE-VI and TEM-VI are still unclear. Precipitation contributed substantially much more to vegetation dynamics than temperature, and PRE-VI associations were greater in the Southern Hemisphere than those in the Northern Hemisphere in global drylands. The traceability effects of soil moisture on PRE-VI and TEM-VI were investigated, and a convex shape was observed for PRE-VI as soil moisture increased, but not significant trend for TEM-VI. It showed that moderate soil moisture enhanced PRE-VI associations through the maintenance of reasonable surface–atmosphere feedback to counteract the balance of plant growth and energy collocation. In the past three decades, PRE-VI associations decreased with the increase in soil moisture, suggesting the consistency of the spatial and temporal pattern of the effect of soil moisture on PRE-VI associations in water-limited ecosystems. Our results implied that changes in soil moisture reflect changes in climate–vegetation associations and can guide nature-based solutions to improve ecosystem carbon sink. [ABSTRACT FROM AUTHOR]

[Display omitted] • As Populus simonii Carr degradation, the shallow layer SWC recovery, the deep layer SWC was difficult to recover and DSL gradually developed. • Deep soil water deficit cause Populus simonii Carr degradation due to coarser soils and increased bulk density. • Light degradation is adjustment of soil water in response to drought stress, moderate degradation was the sign of DSL formation. Large-scale afforestation exacerbates soil water scarcity in a semiarid region of northern China, causing the formation of a dried soil layer (DSL) and ecosystem degradation, and overmature Populus simonii Carr (>30 years) degradation in the Three North Shelterbelt Program (TNSP) area has recently been observed. Determining the relationship between degraded Populus simonii Carr and soil water deficit is essential for afforestation in semiarid regions. We monitored soil water content (SWC) to a 160 cm depth in an area with five degradation degrees (healthy, light degradation, moderate degradation, severe degradation, and dead) to investigate soil water dynamics and DSL characteristics from May 2018 to December 2021. The results showed that: compared with that of healthy Populus simonii Carr (94.7 ± 26.2 mm), the shallow layer (0–80 cm) soil water storage of severely degraded (114.1 ± 29.9 mm) and dead Populus simonii Carr (128.0 ± 35.7 mm) was restored by 19.4 mm and 33.3 mm, respectively. However, the deep-layer (80–160 cm) SWC was difficult to recover and a DSL gradually developed, the DSL − SWC was only 9.3–11.95 %. The deep-layer relative extractable soil water (REW) of healthy Populus simonii Carr was significantly higher than that of moderately degraded, severely degraded, and dead Populus simonii Carr (P < 0.05). The deep-layer SWC decreased with increasing degradation due to coarser soils and increased BD. A greater bulk density (BD) and higher sand proportion reduced the soil water holding capacity. Poor site conditions reduced the resistance of shelterbelts, causing the degradation and mortality of Populus simonii Carr during drought. Therefore, our results improve the understanding of the soil water dynamics and degradation mechanism of Populus simonii Carr , providing a theoretical basis for managing and maintaining the TNSP. [ABSTRACT FROM AUTHOR]

[Display omitted] • Biocrusts have higher water-holding ability and capacity as compared with bare soils. • Biocrusts decrease homogeneous matrix flow and infiltration depth during penetration. • Biocrusts increase soil moisture both at surface and subsurface soils after infiltration. • Biocrusts raise evaporation rate by 27%−39% and cumulative evaporation amount by 14% • Depth-restricted infiltration and wetter surface soil enhance biocrusts evaporation. Globally widespread biocrusts form a "living skin" on most dryland soils, helping govern fundamental ecosystem functions, particularly hydrological processes. However, the underlying mechanisms of biocrust effects on soil water infiltration and evaporation are still controversial. Biocrusts possibly regulate infiltration and evaporation due to their unique physicochemical properties and influence on surface soil moisture (θ), and these assumptions were verified in this study conducted on the Loess Plateau of China. The differences in vertical soil moisture distribution and flow patterns of infiltration water between moss-dominated biocrusts and bare soil were investigated through dye tracing experiments. The evaporation characteristics of biocrusts and bare soil were also obtained by weighing micro-lysimeters in both simulated and in-situ experiments. Results showed that biocrusts retarded deep soil water infiltration below ∼ 15 cm and significantly (F = 48.15, P < 0.001) decreased the stained area ratio (percentage of stained area to total area at the same soil depth) by 16% in contrast to bare soil. Because the infiltrated water was mostly retained in shallow soil (0–15 cm) in biocrust-covered soil, their ratios of low interaction flow (stained path width < 20 mm) and high interaction flow (20 mm < stained path width < 200 mm) were significantly higher (F ≥ 7.82, P ≤ 0.007) than those of bare soil. After the dye tracing experiments, the biocrusts modestly increased θ by 17% throughout the soil profile (0–50 cm), but they remarkably increased θ by 33%−78% in the top 20 cm of soil. Additionally, the biocrusts increased soil daily evaporation rate (E) by 27%−39% (with only the exception of the beginning) and the cumulative evaporation amount by 14% in contrast to bare soil over an 8-day field measurement. More importantly, the biocrust-affected soil properties, such as the contents of fine particles and organic matter, as well as saturated water content and field capacity, were closely correlated with E (| ρ| ≥ 0.52, P ≤ 0.08). Consequently, it was concluded that biocrusts restrict the depth of rainwater infiltration and increase surface θ , which may greatly enhance surface soil evaporation in the Chinese Loess Plateau. [ABSTRACT FROM AUTHOR]

Summary Knowledge of the spatial patterns of soil–water storage (SWS) in soil profiles is important for understanding the dynamics of soil water between surface and subsurface soil layers in semiarid area. We investigated the depth persistence of the overall and scale-specific spatial patterns of SWS for different soil layers during different seasons. Soil–water contents were measured using a neutron probe on 22 occasions in 2012 and 2013 along a 1340-m transect over several sub-watersheds in the Liudaogou catchment on the Loess Plateau of China. Similarities in the spatial patterns of SWS were analyzed by Spearman’s rank correlations and wavelet coherency. A spatiotemporal analysis indicated that the temporal evolution of the SWS profiles differed between the growing and non-growing seasons and that landscape position and soil texture determined the amount of SWS at each sampling location. Spearman’s rank correlations were significant between any two layers within different seasons, and the correlation coefficients decreased as the distance between layers increased. Clay content controlled the spatial pattern of SWS between layers at large scales. The SWS spatial pattern had a higher depth persistence during the non-growing season than during the growing season, and the soil layer had a larger effect than season on the similarity in SWS spatial patterns. These results can improve our understanding of the hydrological processes in soil profiles and can be of considerable value in the application of hydrological models and in water management. [ABSTRACT FROM AUTHOR]

Summary Climate change forecasts in a semiarid region are of much interest to academics, managers and governments. A significant decrease in annual precipitation and an increase in mean annual air temperature are expected; consequently, changes in the soil water balance and groundwater recharge to aquifers are expected as a response to climate change forecasts. In this context, our study aimed to assess the impact of climate changes on the soil water balance and natural groundwater recharge in a semiarid area (Ventós-Castellar aquifer, SE, Spain). To this end, we selected Global Climate Model HadCM3 after comparing it with two other models (ECHAM4 and CGCM2). The HadCM3 model climate data (air temperature and precipitation in two emission scenarios: A2-high and B2-low; 2011–2099) were coupled to a HYDROBAL hydrological model to determine the soil water balance. The HYDROBAL model results showed that climate change will have a significant impact on the soil water balance in the study area, especially on groundwater recharge during the latter period. In both the A2-high and B2-low scenarios, the selected years to run the HYDROBAL model showed a decrease in water balance components (precipitation, actual evapotranspiration, aquifer recharge and runoff) in relation to the baseline period (1961–1990). Over the projected period (2011–2099), we expect fewer rainfall events (>15 mm), which promote aquifer recharge, longer dry summer seasons and, consequently, reduced average annual recharge that ranged from 3% to 17%; 10–49 mm, if compared to the baseline period. The methodology developed in the present study can be beneficial for assessing the impact of predicted climate change on groundwater recharge, and can help managers and planners to devise strategies for the efficient use and conservation of freshwater resources. [ABSTRACT FROM AUTHOR]

Summary The features of water regimes and nitrogen distribution in hillslope soil could provide information on the design suitability of hillslope shapes near water bodies and knowledge about the interception of nutrients before entering water bodies. Hillslope shape effects on overland flow and subsurface flow have been demonstrated by many previous studies, particularly in arid and semiarid regions. We present results from a 1-year monitoring study of two naturally formed hillslopes (convex and concave) with average gradients of 41% in a humid area (eastern China) to better understand the soil water content (SWC) variations and nitrogen concentration distributions. The SWC at the upper position of the convex hillslope (CVS) varied widely with relatively high coefficients of variance, while those at the lower position of the concave hillslope (CCS) maintained continuous, relatively high values during the study period. Comparatively, the low segment soil profile (top 80 cm) of the CCS maintained the highest soil water storage, followed by the up segment (top 80 cm) of the CCS and the low segment of the CVS, and finally the up segment of the CVS. Lateral subsurface flow discharged out from the 30–80 cm soil layer at the mid positions of the two hillslopes and the lower position of CVS; while the lateral subsurface flow from the upper hillslope recharged into the 30–80 cm soil layer at the upper and lower positions of the CCS and the upper position of CVS. Most of the soil nitrogen infiltrated the hillslope with water. Nitrogen concentrations at 30 cm depth on both the CVS and CCS varied temporally, responding strongly with rainfall events, while nitrogen concentrations at 80 cm depth changed slightly throughout the study period and maintained relatively low values. Nitrogen concentrations remained relatively low at the upper position on the CCS compared with those at the mid and lower positions. Inversely, on the CVS, the upper position had relatively higher or proximate nitrogen concentrations compared to its mid and lower positions. Nitrogen storage in the low segment soil profile of the CCS was higher than its up segment, but they were similar on the CVS. The nitrogen redistributions on the hillslopes were dominantly from water regimes, in particular, from the lateral subsurface flow. Due to the diversity of water regimes in different shaped hillslopes, the interception of lateral subsurface flow and its nitrogen pollution should receive more attention in a humid region. [ABSTRACT FROM AUTHOR]

Summary Soil moisture is a critical state variable affecting a variety of land surface and subsurface processes. Despite the complex interactions between soil moisture and its controlling factors, the phenomenon of temporal stability of soil moisture ( TS SM ) has been widely observed under natural conditions. In this study, the control of vegetation on TS SM is investigated by artificially manipulating surface vegetation (e.g., vegetated and de-vegetated plots) in a native grassland-stabilized sand dune area with similar soil texture and topography. Soil moisture data were collected at the depths of 30 cm (within the root zone) and 110 cm (below the root zone) over a period of four years. Using soil moisture data from the de-vegetated plots as a baseline, TS SM within the root zone is shown to be mainly affected by vegetation phenology at the study site. Therefore, the control of vegetation on TS SM varies on both seasonal and annual time scales. The change in the interseasonal patterns of TS SM is tightly related to plant phenology and the control of vegetation on the ranking of mean relative difference ( MRD ) of soil moisture significantly weakens during non-growing seasons due to diminished root water uptake. It suggests that the timing of sampling schemes (e.g., growing season vs. non-growing season) may alter TS SM patterns. On annual time scales, TS SM is affected by climatic conditions, as the control of vegetation on TS SM becomes stronger under drier conditions. In particular, vegetation tends to create larger contrasts in soil moisture levels between vegetated and de-vegetated plots in drier years. The soil moisture data also provide evidence that vegetation tends to reduce TS SM and increase spatial variability in soil moisture at the study site. The standard deviation of relative difference ( SDRD ) of soil moisture at the 30 cm depth (within the root zone) is considerably larger in the vegetated plots than those in the de-vegetated plots. As such, the effectiveness of using representative locations for monitoring mean soil moisture conditions in the vegetated plots deteriorates. [ABSTRACT FROM AUTHOR]

Summary Soil water is a major driving force for plant community succession in semi-arid area. Many studies have focused on the relationships of species diversity–productivity, but few studies have paid attentions to the effects of soil water content heterogeneity on the plant species diversity in the semi-arid loess regions. To determine relationship of soil water content heterogeneity and plant community structure properties a semi-arid steppe on the Loess Plateau, we conducted a gradient analysis of soil water content variation and above- and below-ground properties of plant communities. Results showed that community coverage, above- and below-ground biomass were significantly and positively related to the surface soil water contents (0–5 cm). Plant diversity (Shannon index and Richness index) were closely correlated to soil water content at the soil depth of 0–10 cm. But plant height, litter biomass and root/shoot ratio were not related to soil water content. These results showed that there is an positive interaction effects for plant diversity and soil water content in the semi-arid grassland communities. Our observations indicate that change of plant species diversity is also an important community responses to soil water content heterogeneity in the semi-arid grassland ecosystem. [ABSTRACT FROM AUTHOR]

Highlights: [•] SEM was used to assess the influences of soil and topographic properties on SWC at two sites. [•] SWC was controlled by both properties but more by soil at SDNWA with a humid continental climate. [•] Only soil properties controlled SWC at LYMQ with a cold semi-arid climate. [•] Stronger influences of soil and topographic properties existed under wetter conditions at SDNWA. [•] Influences of soil and topographic properties varied with soil layers at both sites. [ABSTRACT FROM AUTHOR]

Highlights: [•] The condensed water in the greenhouse comes from underground water. [•] Daily evaporation is only a small part of the water fluxes in the upper soil layers. [•] The daily evaporation speed can be expressed as a sinusoidal curve. [ABSTRACT FROM AUTHOR]

Highlights: [•] Deep soil moisture was compared between two watersheds with different land uses. [•] Introduced vegetation have significantly lower soil moisture. [•] Land use pattern have correlation with deep soil moisture spatial variability. [•] Introduced vegetation diminish spatial heterogeneity of deep soil moisture. [ABSTRACT FROM AUTHOR]

Highlights: [•] The North American monsoon induces a strong seasonality in watershed water and energy dynamics and vegetation greenness. [•] A dense sensor network and imagery from an Unmanned Aerial Vehicle were used to characterize a semiarid watershed. [•] Spatial variations in soil moisture and channel runoff show that internal processes redistribute water in the watershed. [•] Distributed observations improve monthly and daily evapotranspiration estimates relative to single site measurements. [Copyright &y& Elsevier]

• Demonstrating that deep soil resources play key roles in trees' responses to drought. • Apple trees used deep soil water (DSW), causing deep soil desiccation. • Absence of DSW reduced hydrological and physiological performance. In drylands, planted trees generally develop roots through mechanisms that enhance their capacity to forage for resources, particularly water, in deep soils and thus survive droughts. However, it remains unclear how and to what extent, lack of access to deep root water uptake will affect the water balance, growth and production of trees in water-limited ecosystems. To address this gap, we designed a novel experiment to partition soils and roots below 200 cm depth from higher layers in a dryland orchard on China's Loess Plateau to evaluate the response of apple (Malus pumila Mill.) trees to the lack of deep roots and access to deep soil resource in 2019 and 2020 (when a natural extreme drought occurred). We found that without partitioning apple trees clearly used deep soil water, thereby causing more soil desiccation than partitioned trees, especially during the drought. The unavailability of deep soil water resulted in much greater water stress, which cause a decrease of 36 % in transpiration, 20 % in photosynthesis rate and 32 % in mid-day leaf water potential relative to unpartitioned trees. Consequently, the yield and quality of apple trees also clearly decreased 11 % and 15 %. These findings suggest that the absence of uptake of deep soil water by roots reduced trees' drought resistance. However, the partitioned trees had clearly higher water use efficiency (WUE) than the unpartitioned trees, although in both cases WUE was strongly reduced by the extreme drought. Our results demonstrate that deep soil resources and roots play key roles in trees' responses to drought, and are likely to play crucial roles in future ecosystem dynamics. [ABSTRACT FROM AUTHOR]

[Display omitted] • A low-coverage-mode shelterbelt stand can maintain a positive water budget. • Evapotranspiration (ET) had a markedly negative effect to deep drainage (DP). • Soil moisture, precipitation and soil conductivity affected positively daily DP. • Leaf area index, rooting depth and microclimate factors influenced DP by ET. Large-scale afforestation can combat desertification, provide ecosystem services, and mitigate climate change. However, it can also have negative effects on the hydrological cycle and water budget, and thus on water security in dryland regions depending on the afforestation strategy used. Currently, the effects of different afforestation strategies on soil moisture dynamics, evapotranspiration (ET), and water budget at the stand scale are unclear. In this study, we investigated the water budget in two poplar stands, one planted using a low-coverage-mode shelterbelt (LCMS) strategy and the other planted using a conventional strategy (control). Deep drainage (DP), as an indicator of water budget, was estimated using a Hydrus-1D model calibrated with the observed soil moisture at different soil depths (across 0–200 cm) during the growing seasons in 2016, 2017, and 2019. The average DP in the LCMS stand was 107.0–149.6 mm, accounting for 36.5–42.4% of the rainfall occurring during the growing seasons, which was slightly higher than that in the control stand containing younger trees at a higher density. However, DP was more spatially heterogenous in the LCMS stand compared with in the control stand. Specifically, in the LCMS stand, the inter-canopy DP was 195.3–237.5 mm, accounting for 66.6–69.7% of the rainfall, whereas the under-canopy DP was only 18.7–65.0 mm, accounting for 6.4–18.4% of the rainfall during the corresponding period. Structural equation modeling indicated that, on a daily scale, soil moisture had the greatest effect on DP, followed by precipitation and soil saturated hydraulic conductivity. Other climate factors (global radiation, air temperature, vapor pressure deficit and wind speed) and vegetation structure parameters (leaf area index and rooting depth) affected ET, which had a significantly negative relationship with DP. These results help explain the effects of plantation strategy on the water budget, with important implications for artificial forest management, ecological restoration, and protection of local water resources in water-limited environments. [ABSTRACT FROM AUTHOR]

For Debre Markos, Ethiopia, moisture trajectories change from dominantly south and southwest in the wet season to northeast in the dry season. Recycled continental moisture from nearby region (PC) is the most dominant moisture source throughout the year (67%), and the percentage is higher in the wet season (79%) than the dry season (56%). Secondary moisture sources include the south Indian Ocean (SIO) for the wet season and the north Indian Ocean (NIO) for the dry season. The most important control over the precipitation isotopic variation at Debre Markos is the convective intensity at both the study site and the moisture uptake locations. Other important controlling factors include temperature, precipitation amount, rainout and relative humidity. [Display omitted] • NW Ethiopian highland receives most moisture (67%) from nearby continental areas. • Secondary moisture sources include the south (16%) and north (14%) Indian Ocean. • Convective intensity is the most important control over precipitation isotopes. • Other controlling factors include temperature, precipitation, rainout and humidity. Understanding the controlling factors for variations of precipitation isotopes is essential for accurate interpretation of isotope-based paleoclimate proxy data as well as better apprehension of the hydrological cycle. These are particularly important for Ethiopia, an arid region and important location for studying human evolution. Past studies have established the importance of moisture source on isotopic variation in precipitation, but few studies have identified exact moisture sources and quantified their contributions. In this study, we use a Lagrangian moisture diagnostic technique and daily precipitation isotope data collected at Debre Markos on the northwest Ethiopian Highland to quantify the contributions of different moisture sources to precipitation. We also quantitatively establish the relative importance of local climate conditions, climate conditions of moisture uptake locations, and different moisture sources on the isotopic variation in precipitation at the study site. Our results show that recycled continental moisture from adjacent areas is the most dominant moisture source throughout the year (67%), and its contribution is higher in the wet season (79%) than in the dry season (56%). Secondary moisture sources include the southern Indian Ocean (16%) for the wet season and the northern Indian Ocean (14%) for the dry season. Based on correlation analyses, we find that the most important control over the precipitation isotopic variation is the convective intensity at both the study site and the moisture uptake locations. Strong connective activities are linked to low isotopic values, and the seasonal movement of the deep convection region could explain the seasonal precipitation δ18O patterns with low values in the wet season and high values in the dry season. Other climate factors that affect the isotopic composition of precipitation include local temperature, relative humidity, rainfall amount as well as total rainout along the trajectories and relative humidity at moisture uptake locations. Our study could provide new perspectives for interpreting δ18O-based paleoclimate proxies in this region, as well as crucial information for understanding hydrocliamte processes in a region where sustainable water resources are of particular importance for socioeconomic development. [ABSTRACT FROM AUTHOR]

Highlights: [•] Moisture was monitored under two biological soil crusts and adjacent scalped soils. [•] Biological crust removal resulted in decreased moisture at 0.03m during soil wet periods. [•] Decrease in soil moisture was more noticeable in fine than in coarse-textured soils. [•] Soil water loss was faster in scalped than in biologically crusted soils. [•] Moisture was similar in soils with or without biological crusts during soil drying periods. [Copyright &y& Elsevier]

Summary: Soil moisture is an effective water source for plant growth in the semi-arid Loess Plateau of China. Characterizing the response of deep soil moisture to land use and afforestation is important for the sustainability of vegetation restoration in this region. In this paper, the dynamics of soil moisture were quantified to evaluate the effect of land use on soil moisture at a depth of 2m. Specifically, the gravimetric soil moisture content was measured in the soil layer between 0 and 8m for five land use types in the Longtan catchment of the western Loess Plateau. The land use types included traditional farmland, native grassland, and lands converted from traditional farmland (pasture grassland, shrubland and forestland). Results indicate that the deep soil moisture content decreased more than 35% after land use conversion, and a soil moisture deficit appeared in all types of land with introduced vegetation. The introduced vegetation decreased the soil moisture content to levels lower than the reference value representing no human impact in the entire 0–8m soil profile. No significant differences appeared between different land use types and introduced vegetation covers, especially in deeper soil layers, regardless of which plant species were introduced. High planting density was found to be the main reason for the severe deficit of soil moisture. Landscape management activities such as tillage activities, micro-topography reconstruction, and fallowed farmland affected soil moisture in both shallow and deep soil layers. Tillage and micro-topography reconstruction can be used as effective countermeasures to reduce the soil moisture deficit due to their ability to increase soil moisture content. For sustainable vegetation restoration in a vulnerable semi-arid region, the plant density should be optimized with local soil moisture conditions and appropriate landscape management practices. [Copyright &y& Elsevier]

Summary: This paper combines hydrological observations and modelling results of a semi arid catchment in Brazil that could lead to a better understanding of the hydrology of similar catchments in semi-arid regions. The Tapacurá catchment (area 470.5km2) in the Northeast of Brazil was selected for this study. The Distributed Catchment Scale Model, DiCaSM, was calibrated and validated for the stream flows of the Tapacurá catchment. The model performance was further tested by comparing simulated and observed scaled soil moisture. The results showed the ability of the model to simulate the stream flow and the scaled soil moisture. The simulated impacts of climate change of low emission (B1) scenarios, on the worst perspective, indicated the possibility of reduction in surface water availability by −13.90%, −22.63% and −32.91% in groundwater recharge and by −4.98%, −14.28% and −20.58% in surface flows for the time spans 2010–2039, 2040–2069, 2070–2099, respectively. This would cause severe impacts on water supply in the region. Changing the land use, for example by reforestation of part of the catchment area which is currently arable land, would lead to a decrease in both groundwater recharge by −4.2% and stream flow by −2.7%. Changing land use from vegetables to sugar cane would result in decreasing groundwater recharge by almost −11%, and increasing stream flow by almost 5%. The combination of possible impacts of climate change and land use requires a proper plan for water resources management and mitigation strategies. [Copyright &y& Elsevier]

Summary: Knowledge of soil moisture is critical to understanding many of the hydrological processes that are of interest in soil hydrology, meteorology, and ecology research. In the present study, we used rainfall and soil moisture data from 2003 to 2008 measured at grassland and meadow sites in the Qilian Mountains of northwestern China to analyze the response of soil moisture to rainfall event size during the growing season. The responses of soil moisture at the grassland and meadow sites to rainfall event size were similar, although total rainfall, the frequency of large rainfall events, and the vegetation types were different. Soil moisture at depths of 20 and 40cm increased significantly after rainfall events larger than 15 and 20mm, respectively, but the magnitude of these changes varied in response to differences in the duration of the dry interval preceding the rain. Soil moisture at depths from 60 to 80cm in the grassland increased obviously with rainfall events larger than 40mm or after two consecutive large rainfall events, but was not significantly correlated with rainfall event size in the meadow. Soil moisture at depths from 120 to 160cm did not change significantly during the growing season at either site. The soil water storage at depths from 20 to 80cm at both sites increased obviously after rainfall events larger than 20mm. The present results suggest that large rainfall events (>20mm) play a key role in increasing soil water storage in the grassland and meadow ecosystems of these semi-arid mountains. [ABSTRACT FROM AUTHOR]

Summary: Soil moisture (θ) is a fundamental hydrological state variable and its spatial pattern is important for understanding hydrological processes. Determination of small catchment-scale soil moisture status and distribution at intermediate scales (0.01–1km2) is challenging. Primarily because multi-point measurements using sensors are often impractical, while remote sensing resolution is often too coarse. Geophysical methods, e.g. electromagnetic induction (EMI), offer potential for bridging this gap. Our objective was to test the use of time-lapse EMI surveys to separate the influences of ‘static’ soil variables, e.g. texture/mineralogy, from ‘dynamic’, e.g. changes in soil moisture. A novel differencing approach is used for estimating relative changes in soil moisture, subtracting the bulk soil electrical conductivity (ECa) of the driest seasonal soil map, named the residual ECa, from the ECa collected during subsequent wetting. This approach allows us to remove the effects of spatially distributed mineral electrical surface charge (ECs) and other factors, and improve estimation of water content. Comparing results with TDR determined soil moisture, we improve the correlation from r 2 =0.28 to r 2 =0.48. ECa measurements are observed to be correlated in time (r 2 >0.7), but fall into two distinct groups, corresponding to times before and after the onset of stream flow, supporting the concept of preferred soil moisture states. Catchment wetness index predicts areas of convergence resulting in overland flow and stream flow. However, the spatial pattern of soil moisture does not mirror the wetness index, as others have found. We contend that the use of time-lapse EMI imaging provides important insight into the distribution and dynamics of catchment-scale changes in soil moisture, but acknowledge its limitations of requiring moisture dependent contrast of ECa, which will not occur in some soils. [ABSTRACT FROM AUTHOR]

Summary: Surface soil moisture information is needed for monitoring and modeling landscape processes at various spatial scales. Studies have shown that surface reflectance spectra of soils are often jointly controlled by soil moisture content (SMC) and salt content (SSC) but has not been sufficiently addressed. In this study, eight different index types of soil moisture hyperspectral indices were being examined for their sensitivities to SSC based on laboratory controlled experiments with a wide range of SMC and SSC (dataset I, 196 spectra in total). They were then validated using field in situ measurements (dataset II) as well as data extracted from the same locations from a Hyperion image (dataset III). The dD1300,1970 was identified as the robust salt-resistant index to estimate soil moisture, with R 2 of 0.57 and 0.65 corresponding to Datasets I and II, respectively. However, the performance of the index somehow dropped when Dataset III (R 2 =0.29, P <0.01) was used, mostly due to the quality of Hyperion data resulting from the propagated errors from atmospheric corrections. With improved accuracy from atmospheric corrections and development of new satellite-borne sensors, it is foreseen that the dD1300,1970 index identified in this study should have wide application in the mapping of soil moisture in saline soils of arid lands. [Copyright &y& Elsevier]

Summary: Studying surface–atmosphere feedback is often limited by the accuracy of the land surface observations (particularly soil moisture estimates) or the performance of the land surface models. To further our understanding of soil moisture effects on land–atmosphere fluxes, improvements in soil moisture mapping over large regions are necessary. The aim of this study was to obtain accurate soil moisture mapping over a 120 by 100km2 area in West Africa using the considerable amount of measurements available from local and regional scales, recorded during the African monsoon multidisciplinary analysis (AMMA) experiment. The modelling strategy was based on the use of a land surface model (LSM), employed to provide high-resolution soil moisture mapping over the studied area. A microwave emission model was then used to simulate associated microwave brightness temperatures (TB) to compare with the Advanced microwave scanning radiometer (AMSR) at the same spatial (25 by 20km2) and temporal resolution (daily). Discrepancies between observed and simulated TB were analysed and used to calibrate the LSM and the microwave emission models to match the specific hydrology and soil microwave behaviour of the studied area. Nevertheless, a positive bias of the near-surface soil moisture remained and the land surface model was still unable to reproduce the rapid dynamic of the near-surface soil moisture observed at the local and regional scales in this climatic context. To solve this problem, a secondary surface soil layer was added to match in situ soil moisture measurements as well as satellite microwave measurements. Additionally, the choice of the soil permittivity model was found to be of prior importance in order to perform suitable microwave brightness temperatures. Finally, a soil moisture retrieval algorithm based on AMSR and meteosat second generation (MSG) measurements was proposed in order to improve the quality of the soil moisture estimates over the studied area (the root mean square error decreases from 5.4 % vol. to 2.8 % vol). [Copyright &y& Elsevier]

Summary: In irrigated semi-arid and arid regions, accurate knowledge of groundwater recharge is important for the sustainable management of scarce water resources. The Campo de Cartagena area of southeast Spain is a semi-arid region where irrigation return flow accounts for a substantial portion of recharge. In this study we estimated irrigation return flow using a root zone modelling approach in which irrigation, evapotranspiration, and soil moisture dynamics for specific crops and irrigation regimes were simulated with the HYDRUS-1D software package. The model was calibrated using field data collected in an experimental plot. Good agreement was achieved between the HYDRUS-1D simulations and field measurements made under melon and lettuce crops. The simulations indicated that water use by the crops was below potential levels despite regular irrigation. The fraction of applied water (irrigation plus precipitation) going to recharge ranged from 22% for a summer melon crop to 68% for a fall lettuce crop. In total, we estimate that irrigation of annual fruits and vegetables produces 26hm3 y−1 of groundwater recharge to the top unconfined aquifer. This estimate does not include important irrigated perennial crops in the region, such as artichoke and citrus. Overall, the results suggest a greater amount of irrigation return flow in the Campo de Cartagena region than was previously estimated. [Copyright &y& Elsevier]

• A drought index was obtained for drought monitoring based on soil moisture series. • Drought characteristics (e.g., return period) were presented on a regional scale. • Wavelet analysis was adopted to study the area extent characteristics of drought. As the critical parameter of the agricultural drought process, soil moisture (SM) has the potential to monitor agricultural drought, which can provide practical support for water management. In this study, a drought index, namely Soil Dryness Index (SDI), was proposed based on long-term gridded ERA-5 land SM, and its performance was evaluated. Furthermore, we employed the run theory and copula to study the spatiotemporal drought characteristics based on this index in Inner Mongolia. Although the return period of drought events has been assessed in many studies, the drought tends to occur with a nonstationary frequency. The wavelet transform was then applied to drought area proportion time series in sub-regions to study the temporal characteristics of droughts. The results show that SDI performs well in monitoring agricultural drought. There is strong heterogeneity in the spatial distribution of drought events. Frequent and low-severity droughts dominated arid and semi-arid regions, while some high-severity droughts mainly occurred in humid and semi-humid regions (mainly in Hulunbuir and Xing'an). The droughts, especially those with high severity, have become much more frequent in the last two decades. The period from 2000 to 2010 had seen more frequent droughts in recent 40 years, and most 20-year return period droughts occurred during this period. These findings could contribute to a better understanding of spatiotemporal patterns of agricultural droughts and provide a reference for future drought trends in Inner Mongolia. [ABSTRACT FROM AUTHOR]

• Extreme rainfall has increased, with greater increases for more extreme events. • Storm volumes show lesser trends, and there is a shift to shorter rainfall events. • Flood peaks for frequent flooding have decreased across many parts of the world. • Flood peaks for rarer flood events have increased across many parts of the world. • Changes in antecedent soil moisture are modulating changes in flooding. Increases in extreme rainfall intensities as a result of climate change pose a great risk due to the possibility of increases in pluvial flooding, particularly in urban and developed areas. But evidence is emerging that the observed increases in extreme rainfall are not resulting in universal increases in flooding. Indeed, on a global scale, studies consistently find more gauge stations with decreasing rather than increasing trends in the annual maxima flood magnitude. Here, we aim to improve our understanding of how rainfall and streamflow extremes are changing and why floods are not always observed to increase despite increases in rainfall extremes. To do so, we examine trends in streamflow events using 2776 stations from the Global Runoff Data Centre, with events chosen to isolate the impact of changes to their respective rainfall and antecedent soil moisture. The analysis is limited to stations with 30 years or more of active record with the majority of stations in North America, Europe, Brazil, Oceania, and southern Africa. Consistent with physical reasoning, for more frequent events such as the annual maxima, it is found the peak 1-day rainfall event intensity is increasing approximately a rate of 6–7%/°C, with rarer event intensities increasing at a rate exceeding the Clausius-Clapeyron relation. We find that storm volumes are not increasing as greatly as the peak rainfall and storm durations are decreasing, pointing to an intensification of rainfall events and a peakier temporal pattern. While rainfall is intensifying, the magnitude of frequent floods (those expected to occur on average once per year) are in general decreasing, particularly in tropical and arid regions of the world. We find that this is likely due to a dominance of drying antecedent soil moisture conditions. However, the magnitude of rarer floods has, on average been increasing. We suggest this is because, for these rarer events, the increase in rainfall outweighs the decrease in soil moisture. Our results point to a worst of both world's scenario where small floods, responsible for filling our water supplies, are decreasing, while the large flood events which pose a risk to life and infrastructure, are increasing. [ABSTRACT FROM AUTHOR]

• CLDASv2/CSSPv2 soil moisture simulation is better than ERA5 and GLDASv2.1 over China. • CLDASv2 meteorological forcings are better than that of ERA5 and GLDASv2.1 reanalysis. • CSSPv2 is superior to LSMs of ERA5 and GLDASv2.1 for soil moisture simulation. • The advantage of CLDASv2/CSSPv2 against reanalysis originates mainly from the CSSPv2. Land surface model (LSM) simulations forced by observed meteorological data provide spatially continuous and temporally complete soil moisture estimates, but the influences of models and meteorological forcings are yet to be validated over a large area due to the lack of in situ measurements. In this study, the Conjunctive Surface-Subsurface Process version 2 (CSSPv2) model was driven by three meteorological forcings, namely, CLDASv2.0, ERA5 and GLDASv2.1, to provide soil moisture simulations over China. The validations over 2090 in situ stations during 2012–2017 showed that CLDASv2.0/CSSPv2 soil moisture simulation performed better than ERA5 and GLDASv2.1 reanalysis products, with an increased correlation of 26%–68% and reduced errors of 14%–24% at the daily time scale. The improvements mostly originate from the use of an advanced LSM because CLDASv2.0/CSSPv2 only increased the correlation by 5%–35% and decreased the errors by up to 9% when compared with ERA5/CSSPv2 and GLDASv2.1/CSSPv2. In contrast, ERA5/CSSPv2 and GLDASv2.1/CSSPv2 soil moisture simulations increased the correlations from their alternative reanalysis LSMs by 17%–63%, and decreased the errors by up to 18%. The results are similar when using the SMAP satellite product as the validation data. The influence of the LSM was more obvious over semiarid regions, such as northern China. The influence of meteorological forcing was more significant for soil moisture simulations at the surface layer, while the LSMs played a more critical role for the middle and deep layers, especially during the cold season due to freeze-thaw processes. This study demonstrates the possibility to further improve soil moisture estimates at a large scale with advanced LSMs, even with the emergence of modern reanalyses. [ABSTRACT FROM AUTHOR]