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• The THREW-T model produced good simulation on streamflow, snow, soil moisture, and isotope simultaneously in two basins. • Calibration toward isotope improved the simulations of internal streamflow and soil moisture's spatiotemporal variation. • Calibration toward isotope yielded the most reasonable runoff source apportionment, and reduced the uncertainty. Multiple-objective calibration helps constrain the parameter uncertainties and improve the performances of hydrological models. Previous studies have indicated that calibration toward soil moisture data could improve the streamflow simulation, but its influence on the runoff source apportionment quantification still needs to be analyzed. Meanwhile, although isotope calibration has proved to improve the representation of internal hydrological processes, the value of isotope on the simulation of internal state variables such as soil moisture has yet to be examined. This study utilized the tracer-aided hydrological model THREW-T (Tsinghua Representative Elementary Watershed – Tracer-aided version) in two mountainous basins on the Tibetan Plateau (The Upper Brahmaputra and Upper Yangtze basins) to evaluate the value of soil moisture and isotope data on model calibration. The result shows that: (1) The THREW-T model produced good simulation on streamflow, snow cover area, soil moisture, and stream water isotope simultaneously in the two study areas. Calibration toward soil moisture and isotope caused slight (∼0.03) but statistically significant (p < 0.01) decrease on the Nash-Sutcliffe coefficient of streamflow simulation compared to the baseline calibration variant only toward streamflow. (2) Calibration toward soil moisture brought no improvement to streamflow simulation for the validation period and stations in both basins, only improving soil moisture simulation. However, calibration toward the isotope improved the simulations of internal streamflow and soil moisture's spatiotemporal variation. (3) Different calibration variants resulted in different estimations of the runoff source apportionment, and independent evidence indicated that the results obtained by isotope calibration were most reasonable. Calibrations toward streamflow and soil moisture underestimated and overestimated the contributions from subsurface runoff, respectively. Isotope was the most sensitive objective to the runoff source apportionment and significantly reduced the uncertainty. Our study found a lower value of soil moisture data than the isotope on model calibration. However, we believe that the full potential of soil moisture data was not utilized due to the current limitations in soil moisture simulation and measurement methods, and the development of relevant technologies will make the soil moisture data more valuable for model calibration. [ABSTRACT FROM AUTHOR]

• Seasonal water uptake differed among different tree species. • Deep-rooted tree species had lower growth resistance to drought. • Shallow-rooted tree species was more adaptative to seasonal precipitation areas. • The root structure and deep soil water availability were the key factors influencing the drought resistance of trees. Understanding the water use patterns of different tree species and their adaptability to drought is urgently needed due to global warming. In this study, we investigated the seasonal water uptake patterns of Pinus tabulaeformis and Picea wilsonii on the eastern Tibetan Plateau using stable isotope methods. In addition, tree ring width data was used to evaluate the radial growth resistance of the two species to drought in 2022. The focus was on identifying species differences in water uptake patterns during drought spells and radial growth resistance to drought. The results showed that although Pinus tabulaeformis could increase water uptake from deeper soil layers during drought spells, the radial growth resistance to drought was significantly lower (p < 0.001) than that of Picea wilsonii due to the limited water content in the deep soil layers. In contrast, Picea wilsonii had higher resistance and adaptations to drought based on rapid access to water resources from seasonal precipitation with denser fine roots. Overall, shifting water sources to deeper soils during drought spells was not always effective in alleviating drought stress, but the root structure and deep soil water availability were the key factors influencing the drought resistance of trees. The findings had significant implications for decision-making on the species suitability of plantation projects under future climatic change. [ABSTRACT FROM AUTHOR]

• In-situ soil moisture is used to characterize the spatiotemporal scales of precipitation, especially for those under 10 km. • A sliding window approach is adopted to capture the spatiotemporally varying rainfall events. • The Central TP is dominated by precipitation with relatively short durations (0–6 h) and small spatial extents (< 16 km). The spatiotemporal scale of precipitation is crucial for understanding the local hydrometeorological processes. It also plays a significant role in determining the layout of rain gauges. Convectional rainfalls are typical in the Tibetan Plateau (TP), but the spatiotemporal scales of precipitation events in the TP, particularly at scales less than 10 km, remain unclear, which is mainly due to limited in-situ precipitation observations and the relative coarse spatial resolution of satellite precipitation products. Building upon the strong response of soil moisture changes to precipitation, this study attempts to reveal the spatiotemporal scales of precipitation in the central TP during summertime (June–September) by investigating the inter-station correlation of accumulated positive soil moisture increments obtained from in-situ soil moisture measurements in a mesoscale region of the central TP. The soil moisture-implied spatiotemporal scales are further verified against precipitation from the Integrated Multi-satellitE Retrievals for GPM (IMERG) at spatial scales larger than 10 km. Results show that: 1) the variations of surface soil moisture increase and precipitation are fairly consistent, making positive increments in surface soil moisture an effective surrogate for reflecting the spatiotemporal scales of precipitation in central TP; 2) the spatial extents of precipitation as indicated by the precipitation-induced positive soil moisture increments are positively correlated with the temporal scales; 3) precipitation events with relatively short durations (0–6 h) and spatial extents of less than 16 km (and in particular, 0–3 h for scales less than 10 km) dominate the central TP. This study provides a novel approach for characterizing precipitation scales in regions with scarce in-situ precipitation observations, and is expected to inspire new way of assessing large scale soil moisture and precipitation products. [ABSTRACT FROM AUTHOR]

[Display omitted] • From spatiotemporal dimension to analyze drought propagation of SPEI to SSMI on the TP. • There is a lag time of 2–3 months from SPEI to SSMI and increases from summer to winter. • Climate, vegetation and permafrost type have an impact on drought propagation. • In winter, drought duration decreases in 45% of the TP after drought propagation, and drought severity decreases in 52%, particularly in the Inner TP. • The direction of propagation from SPEI to SSMI is predominantly northwest. Meteorological drought signals trigger different types of droughts by propagating in the water and energy cycle processes. The understanding of the propagation process from meteorological to soil moisture drought is not clear on the Tibetan Plateau (TP). We used the standardized precipitation index (SPEI) and the standardized soil moisture index (SSMI) to represent meteorological and soil moisture drought. By using Mann-Kendall (MK) trend test, wavelet analysis, run theory, Moran's I and drought migration model, we evaluated the trend of SPEI and SSMI on the TP from 1980 to 2018, analyzed the response relationship, explored the temporal (lag time (LT), propagation rate, duration (DD) and severity (DS)) and spatial (spatial autocorrelation, trajectory, direction and distance) properties of drought propagation, and discussed the potential factors. The result indicated that SPEI intensified at winter and annual scales, SSMI mitigated at all time scales. Both of them showed a consistent dry trend in the southeastern TP. In the time dimension, SSMI usually lagged behind SPEI by about 2–3 months, and the LT increased from summer to winter. The LT tended to be shorter in summer in regions characterized by humid and semi-humid climates, meadows and shrublands, and seasonally frozen ground. Propagation rates were greater in the southeast than northwest. After propagation, DD and DS became weaker in winter (45 % and 52 % of regions decreased, respectively) and stronger in summer (80 % and 76 % of regions increased, respectively). In the spatial dimension, the LT of adjacent regions is closely related, dominated by High - High values and Low-Low values. SSMI had shorter migration trajectories and slower migration rates than SPEI. In addition, the direction of drought propagation was predominantly northwest. For the first time, this study has provided insights into the spatial and temporal propagation of meteorological drought to soil moisture drought on the TP and provides a theoretical basis for understanding drought propagation and predicting soil moisture drought. [ABSTRACT FROM AUTHOR]

• A novel algorithm named SM2RAIN-BayesOpt is proposed for estimating rainfall by integrating the SM2RAIN algorithm and Bayesian optimization. • The performance of the SM2RAIN-BayesOpt algorithm is evaluated against rain gauge data in three typical areas. • The SM2RAIN-BayesOpt rainfall estimates is compared with IMERG-V06B, and ERA5 reanalysis rainfall products. • Effects of soil freezing on SM2RAIN-BayesOpt rainfall estimation are investigated. • Effects of land cover on calibration parameters of SM2RAIN-BayesOpt are inspected. The rainfall product derived from the SM2RAIN (Soil Moisture to Rain) algorithm has been widely used. However, there is still a large uncertainty partly due to the soil moisture input and parameters estimation of the SM2RAIN algorithm, which limits the application of the model in alpine regions. Here, the SM2RAIN-BayesOpt algorithm was developed by integrating the SM2RAIN algorithm and Bayesian optimization to improve the estimation of parameters (Z, a, b, T base , T pot), subsequently incorporating SMAP Level-3 soil moisture products for rainfall estimation. The performance of the SM2RAIN-BayesOpt algorithm was evaluated based on observed rainfall data under different environmental conditions in three typical alpine regions, namely Tibetan Plateau, Heihe River Basin, and Shandian River Basin. Moreover, SM2RAIN-BayesOpt, IMERG-V06B, and ERA5 reanalysis rainfall estimates were also compared with in-situ rainfall observations. The results showed that the proposed SM2RAIN-BayesOpt algorithm can obtain more accurate rainfall estimates in all studied areas in terms of different evaluation metrics. It was also found that our proposed SM2RAIN-BayesOpt algorithm performs better in alpine meadows and grassland than in desert and forestland. SM2RAIN-BayesOpt algorithm can considerably improve the accuracy of rainfall estimation, and it is of significant value for rainfall monitoring in alpine regions where observational data are scarce. [ABSTRACT FROM AUTHOR]

Thawing permafrost on the Qinghai-Tibet Plateau (QTP) has great impacts on the local hydrological process by way of causing ground ice to thaw. Until now there is little knowledge on ground ice hydrology near permafrost table under a warming climate. This study applied stable tracers (isotopes and chloride) and hydrograph separation model to quantify the sources of ground ice near permafrost table in continuous permafrost regions of the central QTP. The results indicated that the ground ice near permafrost table was mainly supplied by active layer water and permafrost water, accounting for 58.9 to 87.0% and 13.0 to 41.1%, respectively, which implying that the active layer was the dominant source. The contribution rates from the active layer to the ground ice in alpine meadow (59 to 69%) was less than that in alpine steppe (70 to 87%). It showed well-developed hydrogeochemical depth gradients, presenting depleted isotopes and positive chemical gradients with depth within the soil layer. The effects of evaporation and freeze-out fractionation on the soil water and ground ice were evident. The results provide additional insights into ground ice sources and cycling near permafrost table in permafrost terrain, and would be helpful for improving process-based detailed hydrologic models under the occurring global warming. [ABSTRACT FROM AUTHOR]

• Seasonal positive SM-P and negative SM-T2m feedback detected at TP. • Positive SM-runoff feedback dominated at TP, especially in summer and autumn. • ET linked in the SM-P-T2m feedback, but the nexus of SM-ET-ROF is complex. • Inner TP, Sources of Yellow-Yangtze-Mekong-Salween river basin as SM-hydroclimate interacting hotspot. • Common compound aggregation method is efficient to recognize hotspot among multi-data. Land-atmosphere energy and moisture exchanges exert great impacts on local and regional climate. However, high uncertainties exist in the recent land–atmosphere interactions due to scarce observations and difficulties in modeling at high elevations and cold regions. This study applied multiple high-resolution reanalysis products (ERA5-land, GLDAS Noah and CLSM products) and one regional climate model, RegCM4, to diagnose the interaction pattern and strength of seasonal soil moisture and hydroclimate on the Tibetan Plateau (TP) using a conditional correlation coefficient method and the global land–atmosphere coupling experimental modeling approach. A compound aggregation method is applied to recognize hot spots with strong soil moisture-hydroclimate interactions among multiple data. As a result, the Changtang Plateau in the inner TP, the sources of the Yellow, Yangtze, Mekong, Salween, and Lancang Rivers and part of the Brahmaputra River basin in the south-southeastern TP are diagnosed with strong land–atmosphere interactions in most seasons. In summer, the coupling strength is stronger, and the covered area is much wider and spatially continuous as the climate turns warm. All applied data indicate a positive dominant soil moisture-precipitation feedback, and a negative soil moisture-temperature feedback at the hot spots. Meanwhile, the soil moisture-runoff feedback is strong and positive at most parts of the TP, especially at the hot spots and in summer. Evapotranspiration acts as a positive linkage in the soil moisture-precipitation-temperature interactions, while the nexus of soil moisture, evapotranspiration and runoff is complicated. This study contributes to understanding local land-hydroclimate interactions in the high elevation and cold climate of the TP. Additionally, the found the proposed compound aggregation method efficient for compound studies. [ABSTRACT FROM AUTHOR]

Soil moisture is a key limiting factor in grass growth and restoration in alpine meadow ecosystems on the northern Tibetan Plateau. In the deeper layers, soil moisture influences the processes of freeze-thaw, erosion and water cycle. Cosmic-ray neutron probe (CRNP) is a new method for continuously monitoring mean soil water content (SWC) at hectometer scale, which has been applied in an alpine meadow at a high accuracy. However, with CRNP measuring depth of only 30 cm, depth-scaling is needed for sufficient insight into deep-layer soil moisture. This study evaluated the accuracy of CRNP measurement of SWC in the 2015 and 2016 growing seasons and the performance of temporal stability (TS) analysis in depth-scaling CRNP-estimated SWC. During the study period, 11 field samplings were done for calibration of CRNP-estimated SWC. Using 22 occasions of neutron probe measurements for each of 113 investigated locations, the TS of SWC was analyzed and its performance in depth-scaling CRNP-estimated SWC at five soil depths (10, 20, 30, 40 and 50 cm) was evaluated. The results showed that the mean SWCs to the depth of 50 cm were 12.9 and 17.0%, respectively in 2015 and 2016 growing seasons and were temporally influenced by precipitation and spatially by soil depth. The accuracy of the CRNP-measured SWC was high, with root mean square error and Nash-Sutcliffe efficiency coefficient (NSE) of 2.1% and 0.832, respectively. Representative locations for TS existed in all the soil layers, which increased with increasing soil depth. For the various soil layers, TS-estimated SWC was close to field-measured value. Only a relatively small error and high NSE were noted, suggesting that TS was reliable in application in CRNP depth-scaling. The study provided further scientific basis for the application of CRNP and an effective way of depth-scaling CRNP-estimated mean SWC in alpine meadow ecosystems. [ABSTRACT FROM AUTHOR]

• Apparent Thermal Inertia (ATI)-based soil moisture downscaling methods suffer from the vegetation effect problem. • A vegetation adjusted method for ATI-based satellite soil moisture downscaling is proposed. • The satellite soil moisture product was bias-corrected before downscaling. • The downscaled 1 km resolution soil moisture is nearly unbiased and free of vegetation effect. Many applications such as irrigation management and flood predictions require soil moisture (SM) estimates at fine-resolutions. Soil moisture estimates of current satellite missions are normally at coarse spatial resolutions, such as 25–36 km for Soil Moisture and Ocean Salinity (SMOS) SM data. Many spatial downscaling approaches have been developed to get finer resolution SM. Apparent thermal inertia (ATI), as a reliable approximation of thermal inertia (TI) that could be derived from daily fine-resolution optical/thermal remote sensing (RS) data, has been testified to be highly related to SM. However, the empirical relationship between ATI and SM varies with vegetation cover density. The main purpose of this study is to develop an ATI-based SM downscaling method that is free of vegetation cover effects. The study was conducted in Naqu areas in the central Tibetan Plateau (TP) in southwest China, where spatially intensive in-situ SM measurements were routinely obtained. European Space Agency's Climate Change Initiative (ESA CCI) soil moisture combined product (version 4.2) was used for this study. The fine-resolution (1 km) ATI data were derived from the Collection 6 MODerate resolution Imaging Spectroradiometer (MODIS) products. A modified quantile-quantile (Q-Q) adjustment method was applied to correct the bias of the CCI SM data before downscaling. A non-linear regression model between bias-corrected CCI SM and fine resolution ATI was built to disaggregate the bias-corrected SM to spatial resolution of 1 km. In this model, Enhanced Vegetation Index (EVI) was introduced to globally and quantitatively adjust the vegetation effect in the relationship between SM and ATI. The bias-corrected CCI SM and the downscaled 1 km resolution SM were evaluated using the in-situ SM measurements obtained from the Multi-scale Soil Moisture and Temperature Monitoring Network on the central TP (CTP-SMTMN) in the study area. The results indicate that the proposed modified Q-Q adjustment method effectively reduced the bias of the original CCI SM data (bias = 0 m3/m3). The downscaled 1 km resolution SM shows fine scale spatial variability of soil moisture and preserves the accuracy (R = 0.552) and temporal variability of the bias-corrected CCI SM. The accuracies of the downscaled SM over areas with different vegetation cover show quite good consistency. The uncertainties of the proposed SM spatial downscaling method were comprehensively analyzed. The proposed ATI-based SM downscaling algorithm is applicable to generating representative and consistent high spatial resolution SM over areas with different vegetation cover density. [ABSTRACT FROM AUTHOR]

Soil moisture is a key variable in the exchange of water and energy between the land surface and the atmosphere, especially over the Tibetan Plateau (TP) which is climatically and hydrologically sensitive as the Earth’s ‘third pole’. Large-scale spatially consistent and temporally continuous soil moisture datasets are of great importance to meteorological and hydrological applications, such as weather forecasting and drought monitoring. The Fengyun-3B Microwave Radiation Imager (FY-3B/MWRI) soil moisture product is a relatively new passive microwave product, with the satellite being launched on November 5, 2010. This study validates and reconstructs FY-3B/MWRI soil moisture across the TP. First, the validation is performed using in situ measurements within two in situ soil moisture measurement networks (1° × 1° and 0.25° × 0.25°), and also compared with the Essential Climate Variable (ECV) soil moisture product from multiple active and passive satellite soil moisture products using new merging procedures. Results show that the ascending FY-3B/MWRI product outperforms the descending product. The ascending FY-3B/MWRI product has almost the same correlation as the ECV product with the in situ measurements. The ascending FY-3B/MWRI product has better performance than the ECV product in the frozen season and under the lower NDVI condition. When the NDVI is higher in the unfrozen season, uncertainty in the ascending FY-3B/MWRI product increases with increasing NDVI, but it could still capture the variability in soil moisture. Second, the FY-3B/MWRI soil moisture product is subsequently reconstructed using the back-propagation neural network (BP-NN) based on reconstructed MODIS products, i.e., LST, NDVI, and albedo. The reconstruction method of generating the soil moisture product not only considers the relationship between the soil moisture and NDVI, LST, and albedo, but also the relationship between the soil moisture and four-dimensional variations using the longitude, latitude, DEM and day of year (DOY). Results show that the soil moisture could be well reconstructed with R 2 higher than 0.56, RMSE less than 0.1 cm 3 cm −3 , and Bias less than 0.07 cm 3 cm −3 for both frozen and unfrozen seasons, compared with the in situ measurements at the two networks. Third, the reconstruction method is applied to generate surface soil moisture over the TP. Both original and reconstructed FY-3B/MWRI soil moisture products could be valuable in studying meteorology, hydrology, and ecosystems over the TP. [ABSTRACT FROM AUTHOR]

• Improved an AET process retrieval algorithm by incoporating radiation and soil water constraints. • Determined the most sensitive parameters of P-LSH algorithm using Sobol' sensitivity analysis. • Derived the parameter posterior distributions to reduce P-LSH uncertainty using the DE-MC. Actual evapotranspiration (AET) is an important component of the water cycle on the Tibetan Plateau. Remote sensing retrieval of AET requires further improvement in this region. The objective of this study is to improve the Process-based Land Surface evapotranspiration/Heat fluxes algorithm (P-LSH) by incorporating the impacts of incoming shortwave radiation and soil moisture on canopy conductance and optimizing the parameters. Based on observations from three FLUXNET towers on the Tibetan Plateau, five key parameters in the improved P-LSH algorithm were determined by Sobol' sensitivity analysis. Among them, the parameter b 3 , which defines the relationship of potential canopy conductance (g 0) and Normalized Difference Vegetation Index (NDVI), was identified as the parameter to which the algorithm was most sensitive. We then utilized the Differential Evolution Markov Chain (DE-MC) method to analyze uncertainty of model parameters, and posterior distributions of parameters were used to correct the response curve of g 0 to NDVI in the improved P-LSH algorithm and simulate daily AET. The results show that uncertainty in five parameters is greatly reduced after the Markov process, and compared with the original algorithm, the simulation of the improved P-LSH algorithm is greatly upgraded, specifically reflects in higher R2 (0.93 vs 0.90) and lower RMSE (11.39 W m−2 vs 15.11 W m−2). Finally, although reanalysis-based net radiation, vapor pressure deficit, and wind speed were major sources of uncertainty of reanalysis-driven AET estimates, reanalysis-driven AET estimates did not cause excessive errors compared to tower-driven AET estimates (R2 = 0.87 vs 0.93, RMSE = 15.76 W m−2 vs 11.39 W m−2), which provided a reasonable basis for further application of the improved P-LSH algorithm to larger watershed scales. [ABSTRACT FROM AUTHOR]

Summary Cosmic-ray neutron sensing (CRNS) is a new method for continuously monitoring mean soil water content (SWC) on a hectometer scale. To evaluate the application and accuracy of the method for SWC observation in an alpine meadow ecosystem (AME), we installed the CRNS in a flat meadow near the Naqu prefecture on the northern Tibetan Plateau. We collecting soil samples and applying the system by the oven-drying method. A weather station was also installed near the CRNS for monitoring basic meteorological variables and the soil temperature and water content at various depths. Three Em-50 instruments for monitoring SWC and soil temperature were buried in three sub-quadrats northwest, northeast and southeast of the CRNS at distances of 460, 370 and 373 m, respectively, to observe the variation of SWC at the various depths. The footprint of the CRNS for SWC observation in the meadow was about 580 m, and the mean measuring depth was about 31 cm according to the general calculation equations. The reference neutron flux for dry soil ( N 0 ) had a mean and coefficient of variation of 8686 and 3%, respectively, and remained substantially invariant throughout the measuring period. The five SWCs from the independent field samples almost passed through the SWC trend of the CRNS, the root mean square error (RMSE) was 0.011 m 3 m −3 for the CRNS and oven-drying method. The time series of SWC measured by the CRNS agreed well with the mean SWC series to a depth of 20 cm measured by the weather station. The trend of SWC measured by the Em-50s generally agreed with the trend of SWC measured by the CRNS, but some values and variations of SWC differed between the Em-50s and CRNS data. Because of the good agreement between the CRNS and independent field samples, we suspect that this disagreement is due to an insufficient representativeness of point observations and the distances of the points from the CRNS. The diurnal variation of hourly SWC from the CRNS was sinusoidal during a dry period, peaking at 11:00 and was minimum at 18:00 (Beijing time), with a range of 1%. Overall, the CRNS measured SWC in the AME with an acceptable accuracy, providing a scientific basis for the promotion and application of the CRNS in high, cold ecosystems. [ABSTRACT FROM AUTHOR]

• A consecutive 6 year in-situ evapotranspiration dataset was constructed using eddy covariance. • The seasonal variation of ET was analyzed for the typical alpine meadow from 2014 to 2019. • ET was primarily controlled by the available energy in the typical alpine meadow. • Soil water content affected ET via bulk surface conductance within a certain range. To understand the water, energy, and carbon cycles in the Tibetan Plateau (TP), it is essential to estimate seasonal and inter-annual variations in energy fluxes and evapotranspiration (ET) for alpine meadow ecosystems. The multiyear (2014–2019) energy fluxes and ET, for a typical alpine meadow at Arou station (northeastern TP), and their environmental and biophysical controls were evaluated using the eddy covariance method in this study. Latent heat flux (LE) was the dominant component of energy consumption during the growing season, whereas sensible heat flux (H) dominated energy partitioning during the non-growing season. H showed the opposite trend to LE, while the seasonal variation of soil heat flux (G) was small. The daily ET was primarily controlled by the available energy on the seasonal scale. Soil water content (SWC) and normalized difference vegetation index (NDVI) displayed secondary effects on ET during the non-growing and growing seasons, respectively. The inter-annual ET was relatively stable, ranging from 562.6 to 661.9 mm (coefficient of variation; CV = 7.4 %); this was slightly higher than the annual precipitation despite large variations in inter-annual precipitation (CV = 19.9 %) and was most likely due to snow and frozen ground melting. The cumulative ET in the growing season was about 77 % of the annual ET. There was a nonlinear increase in the daily Priestley–Taylor coefficient (α = ET/ET eq , where ET eq is the equilibrium evaporation) with an increase in bulk surface conductance (g c), which was insensitive to increases in g c that exceeded 15 mm s−1. There was a good relationship between g c and NDVI. This study provides insights into the driving mechanisms of long-term variations in the energy partitioning and biophysical controls on ET in alpine meadow ecosystems. [ABSTRACT FROM AUTHOR]

[Display omitted] • TWS changes in the ETP have two distinct stages. • The TWS changes in the ETP is determined by the relationship between SMS and LWS. • The increase in PET driving the TWSA from an increase to a decrease since 2012. Terrestrial water storage (TWS) in the endorheic Tibetan Plateau (ETP) increased from 2002 to 2012 but decreased in 2012–2016. This study used Gravity Recovery and Climate Experiment (GRACE) data and Global Land Data Assimilation System (GLDAS) data to analyse TWS changes in the ETP in 2012 from increasing to decreasing. The results showed that these TWS changes could be divided into two stages. From April 2002 to August 2012, TWS increased at a rate of 4.43 Gt/yr from the GRACE-Mascons and 1.11 Gt/yr from the GRACE-SH, whereas after September 2012, it declined at a rate of-5.62 Gt/yr from the GRACE-Mascons and −6.99 Gt/yr from the GRACE-SH. The increase in lake water storage (LWS) (7.98 Gt/yr) was higher than the loss of other components from 2002 to 2012, therefore, the LWS gradually dominated the increase in TWS. However, the soil moisture storage (SMS) decreased more significantly (-5.27 Gt/yr) than the increase in LWS (<1 Gt/yr) during 2012–2016, accounting for 66% of the decrease in TWS in the ETP. From a water balance perspective, the relationship between precipitation (P) and evapotranspiration determined the region's changes in TWS. It was found that 90% of the decrease in TWS in the ETP during 2012–2016 was attributed to an increase in potential evapotranspiration (PET), whereas 7% was attributed to a decrease in P. Thus, climate change (P and PET) accounted for 97% of the TWS reduction during 2012–2016. Furthermore, 3% of the decrease in TWS in the region was attributed to land surface changes. [ABSTRACT FROM AUTHOR]

• Lake effect on TWSC was estimated using a newly developed long-term lake volume data. • Contributions of water storage components to TWSC were estimated using both RF model and water balance approach. • The RF method was used to estimate both TWSA and TWSC. • Regional TWSA decreased in the pre-mutation period and then quickly increased in the post-mutation period with an intensified hydrological cycle. Terrestrial water storage (TWS) and its change (TWSC) are extremely sensitive to climate change, and are good indicators of the response of the hydrological system to climate change in the Tibetan Plateau (TP). However, long-term variation in TWS and its components, as well as contributions of these component changes to TWSC, are still not well documented. In this study, we extended monthly TWS anomaly (TWSA) and TWSC from 1989 to 2019 using the random forest (RF) method, analyzed the long-term contribution rates of each component change to TWSC using a mass balance approach, and compared them with the rates estimated by the RF method. Our major findings are: (1) the warming and wetting trend in the entire analysis period (PA) and in the pre-mutation period (P1) was slowed down or even reversed in the post-mutation period (P2), which was caused mainly by the decrease in temperature and precipitation in the north-eastern Inner Basin (IB) and southern Qaidam Basin (QB); (2) the RF method is capable of estimating monthly TWSA and TWSC (0.872 ≤ R2 ≤ 0.957 in training data, 0.528 ≤ R2 ≤ 0.860 in test data). Variable selection has an important impact on the method, and TWSC can be used as an alternative when TWSA estimate is poor; (3) Significant annual TWSA increase (2.20 mm/a, P < 0.05) in the IB during PA comes from a higher increase during P2 (3.62 mm/a, P < 0.05) and a small variation during P1 (−0.65 m/a, P > 0.1). Spatially, the increase is attributed to the higher increase trends (> 4.50 mm/a, P < 0.05) in north-eastern IB during PA and P2, and slight decrease in western and southern IB during PA. The spatial and temporal heterogeneity was mainly caused by the differences in precipitation amount, glacier proportions, and permafrost types. (4) During PA, TWSC in the IB was mainly determined by lake water storage change (LWSC) with a contribution rate of 62 %, while in the QB it was mainly determined by soil water change (−43 %), and LWSC had a contribution rate of −27 %. LWSC impact on TWSC was more significant in the IB, especially in the southeastern sub-regions with regional water storage surplus (TWSC > 0). The overall increase in water storages and fluxes, and exchanges imply an intensified hydrologic cycle in the region. [ABSTRACT FROM AUTHOR]

• A consecutive 6 year in-situ evapotranspiration dataset was constructed using eddy covariance. • The seasonal variation of ET was analyzed for the typical alpine meadow from 2014 to 2019. • ET was primarily controlled by the available energy in the typical alpine meadow. • Soil water content affected ET via bulk surface conductance within a certain range. To understand the water, energy, and carbon cycles in the Tibetan Plateau (TP), it is essential to estimate seasonal and inter-annual variations in energy fluxes and evapotranspiration (ET) for alpine meadow ecosystems. The multiyear (2014–2019) energy fluxes and ET, for a typical alpine meadow at Arou station (northeastern TP), and their environmental and biophysical controls were evaluated using the eddy covariance method in this study. Latent heat flux (LE) was the dominant component of energy consumption during the growing season, whereas sensible heat flux (H) dominated energy partitioning during the non-growing season. H showed the opposite trend to LE, while the seasonal variation of soil heat flux (G) was small. The daily ET was primarily controlled by the available energy on the seasonal scale. Soil water content (SWC) and normalized difference vegetation index (NDVI) displayed secondary effects on ET during the non-growing and growing seasons, respectively. The inter-annual ET was relatively stable, ranging from 562.6 to 661.9 mm (coefficient of variation; CV = 7.4 %); this was slightly higher than the annual precipitation despite large variations in inter-annual precipitation (CV = 19.9 %) and was most likely due to snow and frozen ground melting. The cumulative ET in the growing season was about 77 % of the annual ET. There was a nonlinear increase in the daily Priestley–Taylor coefficient (α = ET/ET eq , where ET eq is the equilibrium evaporation) with an increase in bulk surface conductance (g c), which was insensitive to increases in g c that exceeded 15 mm s−1. There was a good relationship between g c and NDVI. This study provides insights into the driving mechanisms of long-term variations in the energy partitioning and biophysical controls on ET in alpine meadow ecosystems. [ABSTRACT FROM AUTHOR]

[Display omitted] • TWS changes in the ETP have two distinct stages. • The TWS changes in the ETP is determined by the relationship between SMS and LWS. • The increase in PET driving the TWSA from an increase to a decrease since 2012. Terrestrial water storage (TWS) in the endorheic Tibetan Plateau (ETP) increased from 2002 to 2012 but decreased in 2012–2016. This study used Gravity Recovery and Climate Experiment (GRACE) data and Global Land Data Assimilation System (GLDAS) data to analyse TWS changes in the ETP in 2012 from increasing to decreasing. The results showed that these TWS changes could be divided into two stages. From April 2002 to August 2012, TWS increased at a rate of 4.43 Gt/yr from the GRACE-Mascons and 1.11 Gt/yr from the GRACE-SH, whereas after September 2012, it declined at a rate of-5.62 Gt/yr from the GRACE-Mascons and −6.99 Gt/yr from the GRACE-SH. The increase in lake water storage (LWS) (7.98 Gt/yr) was higher than the loss of other components from 2002 to 2012, therefore, the LWS gradually dominated the increase in TWS. However, the soil moisture storage (SMS) decreased more significantly (-5.27 Gt/yr) than the increase in LWS (<1 Gt/yr) during 2012–2016, accounting for 66% of the decrease in TWS in the ETP. From a water balance perspective, the relationship between precipitation (P) and evapotranspiration determined the region's changes in TWS. It was found that 90% of the decrease in TWS in the ETP during 2012–2016 was attributed to an increase in potential evapotranspiration (PET), whereas 7% was attributed to a decrease in P. Thus, climate change (P and PET) accounted for 97% of the TWS reduction during 2012–2016. Furthermore, 3% of the decrease in TWS in the region was attributed to land surface changes. [ABSTRACT FROM AUTHOR]

Summary: Soil moisture content is one of the limiting factors for natural vegetation succession in alpine river valleys on the Tibetan Plateau. However, its spatio-temporal variability and effect on artificial vegetation restoration has rarely been reported. In this paper, we conducted a case study to examine the spatio-temporal variability and vertical characteristics of soil moisture by setting up a 100×120m experimental plot on aeolian sandy land in a riparian ecotone and analyzing the result data using a geo-statistical approach. The soil moisture contents at different depths all showed strong temporal variability, with the mean soil moisture ranging from 4% to 6% in spring, from 6% to 14% in summer and from 9% to 12% in autumn. The Gaussian semi-variogram model was the best fitted theoretical model for the spatial structure of soil moisture contents in different seasons, and soil moisture at the field scale showed strong spatial dependence. The spatial patterns of soil moisture all demonstrated strong similarity between depths with the strongest between 0–20cm and 20–40cm depths in autumn. The soil moisture content over the upper 60cm was the key factor that restricted the seed germination and seedling growth, and it affected vegetation coverage and density. The precipitation, variations of river water level, elevation as well as landform types were the main factors determining the variations of soil moisture content in different seasons. The results have important implications for the ongoing vegetation restoration in the study area. [ABSTRACT FROM AUTHOR]

Summary: Alpine meadow soil is an important ecosystem component of the Qinghai-Tibetan Plateau. However, the alpine meadow soil is undergoing serious degradation mainly due to global climate change, overgrazing, human activities and rodents. In this paper, spatial sequencing was chosen over time succession sequencing to study the changes of soil hydraulic properties under different degrees of alpine meadow degradation. Soil saturated hydraulic conductivity (Ks) and Gardner α both at the surface and at 40–50cm depth were investigated in the field using tension infiltrometers. Soil physical and chemical properties, together with the root index at 0–10cm and 40–50cm soil layer depths were also analyzed. Pearson correlations were adopted to study the relationships among the investigated factors and principal component analysis was performed to identify the dominant factor. Results show that with increasing degree of degradation, soil sand content increased while soil Ks and Gardner α as well as soil clay content, soil porosity decreased in the 0–10cm soil layers, and organic matter and root gravimetric density decreased in both the 0–10cm and 40–50cm soil layers. However, soil moisture showed no significant changes with increasing degradation. With decreasing pressure head, soil unsaturated hydraulic conductivity reduced more slowly under degraded conditions than non-degraded conditions. Soil Ks and Gardner α were significantly correlated (P =0.01) with bulk density, soil porosity, soil organic matter and root gravimetric density. Among these, soil porosity is the dominant factor explaining about 90% of the variability in total infiltration flow. Under non-degraded conditions, the infiltration flow principally depended on the presence of macropores. With increasing degree of degradation, soil macropores quickly changed to mesopores or micropores. The proportion of total infiltration flow through macropores and mesopores significantly decreased with the most substantial decrease observed for the macropores in the 0–10cm soil layer. The substantial decrease of macropores caused a cut in soil moisture and hydraulic conductivity. This study improves the understanding and prediction of alpine meadow soil and ecosystem changes and provides guidelines for improving water flow modeling under the background of global climate change over the Qinghai-Tibetan Plateau and similar regions. [Copyright &y& Elsevier]

Abstract: The reduction of soil water content induced by global warming is expected to affect plant communities worldwide. However, less is known about the consequences of global warming-induced decreases of soil water on alpine wetland ecosystems on the Qinghai-Tibetan Plateau. To determine the responses of a natural alpine wetland community to decreases in soil moisture, we conducted a gradient analysis of soil water using a sequence space-series variation. We used the sequence space-series variation of soil water contents to reflect potential time-series variations by examining the effects of spatial heterogeneity on soil water, as well as determining the changes that would occur in above- and below-ground properties of an alpine wetland community. We found that vegetation aboveground biomass, vegetation cover and height all significantly increased along soil moisture, but species richness decreased. Soil organic carbon, total nitrogen, available nitrogen, total phosphorus and available phosphorus all significantly increased along soil moisture, but soil pH, total potassium and available potassium significantly decreased. Species richness was significantly and negatively correlated to aboveground biomass, vegetation cover and height. Aboveground biomass, vegetation cover and height were all significantly and positively related to soil organic carbon, total N and P, and available N and P, but were negatively related to total K. Conversely, species richness was significantly and negatively related to soil organic carbon, total N and P, and available N and P, but positively related to total K. Our observations indicate that decreased soil water would potentially have a negative influence on the alpine wetland plant communities and soil properties. [Copyright &y& Elsevier]

Summary: A Bayesian method was used to fit the Shuttleworth–Wallace model to half-hourly measurements of evapotranspiration (ET) with the eddy covariance technique from an alpine grassland on the Qinghai-Tibetan plateau during the main growing season in 2008, and probabilistically estimated its parameters and predication uncertainties using both dataset-by-dataset and multi-data procedures. This enabled us to reveal the seasonal variations of some physiology-related parameters and the impacts of constant parameters on the performances of the model. Results indicated that the S–W model using the posterior mean parameter values obtained by different procedures all successfully reproduced the observed responses in ET. However, the seasonal variations in the canopy conductance parameter (g max) should be counted in long-term ET estimating. From simulated results, the daily mean partitioning [i.e. the ratio of the estimated daily soil evaporation (E) over total evapotranspiration (ET); E/ET] was relative low (0.02–0.07 with a mean of 0.04) for the alpine grassland when leaf area index (LAI) was more than 3m2 m−2, and was closed related to LAI and vegetation condition. At the diurnal timescale, the canopy conductance was the main factor control the partitioning of ET. [ABSTRACT FROM AUTHOR]

Summary: Two main types of grasslands on the Qinghai–Tibet Plateau, alpine swamp and alpine meadow, were selected for this study. Monitoring plots were constructed on each type of grassland with varying degrees of vegetation degradation. The impacts of alpine grassland cover changes on the hillslope water cycle were analyzed in terms of runoff generation, precipitation interception, dew water formation, and soil water dynamics of the active layer, etc. The results showed that different types of grasslands led to different runoff generation regimes; namely, runoff varied linearly with precipitation in alpine swamp, whereas in alpine meadow, runoff exhibited an exponential precipitation-dependence. The decrease in vegetation cover in alpine swamp leads to a decrease in soil moisture content in the top 20cm of the soil, a delay in the thawing start time in the spring, and a decrease in both surface runoff and subsurface interflow. In alpine meadow, however, the decrease in vegetation cover led to a significant increase in the depth of topsoil moisture content during the thawing period, earlier occurrence of thawing, and an increase in the runoff generation ratio. The alpine meadow vegetation canopy had a higher maximum interception ratio and saturation precipitation than alpine swamp vegetation. With the decrease in vegetation cover, the rainfall interception ratios decreased by almost an identical range in both the alpine meadow and alpine swamp grasslands. Dew water commonly occurs on alpine grassland, accounting for about 12.5–16.5% of precipitation in the same period, and thus, is an important component of the water cycle. With the degradation of vegetation, surface dew water decreased; however, the ratio of dew water formed in the air to the total amount of dew water rose significantly. At the hillslope scale, the changes of alpine vegetation cover had a great influence on the water cycle, which were partly attributed to that the changes of alpine vegetation cover directly altered the surface energy balance, surface water cycle processes, and the thermal and hydraulic properties of active soil. [Copyright &y& Elsevier]