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• Reservoir drawdown zone was a hotspot of GHG emissions. • CH 4 fluxes went to zero due to dam decommissioning. • Soil volumetric moisture content and soil bulk density were the major factors contributing to GHG emissions. • The desiccation of reservoir sediments can result in loss of C from reservoir. Reservoirs are a category of artificial ecosystem that play a key role in the global carbon cycle. However, there has been limited research on the impacts of dam decommissioning (DD) on greenhouse gas (GHG) emissions over a reservoir life cycle. This study analyzed the effects of DD on CO 2 and CH 4 fluxes emitted from impounded water zone (area of inundated sediments), the drawdown zone (area of sediment exposure during DD), and the margin zone (area of long-term sediment exposure) before, during, and after the drawdown of a retired reservoir in Inner Mongolia. The margin zone encompassed ± 90 % of the total reservoir area over the study period, whereas the impounded water and drawdown zones accounted for the remainder. CO 2 emissions decreased during DD process in the margin zone (325.24 ± 313.80–128.81 ± 128.78 mg m−2h−1) and impounded water zones (286.00 ± 242.00–0.00 ± 0.00 mg m−2h−1), whereas they increased in the drawdown zone (0.00 ± 0.00–239.52 ± 104.35 mg m−2h−1). CH 4 emissions gradually declined over time after DD in the impounded water zone and this zone was the main contributor of CH 4 fluxes before DD (57.36 ± 12.77 mg m−2h−1), and finally emissions went to zero. Exposure of sediments gradually resulted in a significant increase in CO 2 emissions. The major factors affecting GHG emissions were soil volumetric moisture content (SMC) and soil bulk density (ρb). The CO 2 -eq of the reservoir ecosystem decreased after DD. This happened despite CO 2 emissions increasing because CH 4 , which has a higher global warming potential, decreased following DD. The consideration of DD in the carbon footprint of reservoirs for understanding of reservoir carbon dynamics and the global carbon balance requires further study of the long-term effects of DD on carbon fluxes. [ABSTRACT FROM AUTHOR]

• Model-based polarimetric decomposition method for soil moisture estimation (SME). • Dihedral scattering component is neglected in bare soil and sparse vegetation areas. • SME results of C- and L-band full polarimetric data are analyzed. • SME accuracies of bare soil, sparse, and dense grassland are evaluated. Polarimetric synthetic aperture radar (PolSAR), as an advanced active sensor, can penetrate vegetation to detect ground information. The multimode observation data can provide more information to interpret different scattering mechanisms and have great application potential in soil moisture estimation (SME). In this paper, a modified method of soil moisture inversion from PolSAR data based on a two-component polarimetric decomposition model is introduced. The proposed method neglects the dihedral scattering component in the case of sparse vegetation cover and no-building areas, compensates the orientation angle by considering the influence of surface roughness and terrain slope, and calculates the dielectric constant with the hybrid X-Bragg/Fresnel model. Gaofen-3 (C-band) data from Ordos, Inner Mongolia, and Soil Moisture Experiment 2003 (SMEX03) airborne synthetic aperture radar (AIRSAR) (C- and L-band) data from Southern Oklahoma were applied in the experimental research. Furthermore, the performance and validity limits were assessed by comparing the retrieval results with in situ measurements and Soil Moisture Active Passive (SMAP) soil moisture products. The results reveal a root-mean-square error (RMSE) of 8.66% with an inversion rate of up to 61% when using the Gaofen-3 data from Ordos. For the Southern Oklahoma study area, the inversion RMSE of the AIRSAR C- and L-band data is 8.57% and 8.95%, respectively. The tendency of the inversion results is also consistent with the change trend of the site observations. In the bare soil and sparse vegetation covered areas, the retrieval error is relatively small, while in dense vegetation covered areas, soil moisture is underestimated in both the C- and L-band data, and the former is more underestimated than the latter. [ABSTRACT FROM AUTHOR]

• Synergistic geostationary and polar-orbit satellites were used to estimate spatially continuous soil moisture. • Soil moisture retrievals with different temporal composing periods were investigated. • Estimated soil moisture were assessed with both reanalysis and microwave-based datasets. Soil moisture (SM) is a key variable in the surface energy balance and water cycle, and its spatiotemporal dynamics are of great significance to climate, agriculture and other fields. Optical remote sensing has been widely used to estimate SM with relatively fine spatial resolution. However, optical observations are easily contaminated by clouds, making it difficult to obtain spatially continuous SM over large regions. In the present study, a semimonthly SM dataset over the study area of the entire Inner Mongolia region with nearly full spatial coverage was derived from the synergistic use of China's Feng-Yun (FY) geostationary (FY-4A) and polar-orbit (FY-3D) observations, following a previously developed trapezoid feature space in a pixel-to-pixel manner. A preliminary assessment was conducted to evaluate the performances of the proposed method over two main dominant land cover types (grassland and cropland) in the study region, where the China Meteorological Administration Land Data Assimilation System (CLDAS) and the Soil Moisture Active Passive (SMAP) SM products were provided as references. The results indicated that the estimated SM was well correlated to the referenced SM datasets, with a significant correlation coefficient varying from 0.5 to 0.8. Furthermore, for the grassland (cropland), unbiased root mean square errors of approximately 0.062 (0.097) m3/m3 and 0.055 (0.069) m3/m3 can be found when comparing the estimated SM with the CLDAS and the SMAP product, respectively. [ABSTRACT FROM AUTHOR]

[Display omitted] • The movement of soil water and salt was not synchronous during the freezing period. • Soil salt transport was determined by the salt components and liquid concentration. • The components causing the soil salt change were different in the three saline soils. • Salt components with higher eutectic points could hinder salt transport in freezing. • The solute concentration gradient of salt reached 3.07 mol/(L·m) during freezing. Soil water and salt movement in freezing-thawing periods play an important role in the agricultural ecological environment. Field experiments show a non-synchronized movement of soil water and salt during the freezing-thawing period with no definite mechanism. In this study, the non-synchronization of water and salt movement in the frozen layer (0–1 m) was analyzed from the perspective of soil salt composition combining with the convection–diffusion theory. Field experiments were carried out during two freezing-thawing periods from 2017 to 2019 in Yonglian experimental station of Hetao Irrigation District in Inner Mongolia, China. The contents of soil moisture, salt and its components (Na, K, Ca, Mg, CO 3 , HCO 3 , Cl and SO 4) were measured. The correlation analysis between storage increments of total salt and its components was carried out and the FREZCHEM model was used to calculate the soluble and solid salt components. During the freezing period, the soil water storage increased by 5.42% on average, while the soil salt storage decreased by 18.69%, showing the different migration directions of soil water and salt. The migration direction of each component of salt was not exactly the same with that of the total salt. The main components causing the storage increment of soil salt were different in the three saline soils. The solute concentration gradient of soluble salt reached 3.07 mol/(L·m), which caused stronger diffusion effect to result in downward salt migration, while the convection effect drove the soil water and salt to move upwards. This is the major reason for the non-synchronization movement of soil water and salt during the freezing-thawing period. This study provides new data and perspective to understand the soil water and salt movement during the freezing-thawing period in agricultural areas with shallow groundwater table depth. [ABSTRACT FROM AUTHOR]

Abstract: Since the late 1990s China has promoted the “Conversion of Cropland to Forest and Grassland Program” (CCFGP) as the largest environmental restoration program in the world. Because more water was required for established vegetation, it was assumed that evapotranspiration (ET) and water use in an older restored grassland would be greater than that in a younger restored grassland and that water shortages would then become more severe for further vegetation establishment in a semi-arid steppe due to the extraction of the established vegetation. To verify this hypothesis, a field experiment was carried out in 2008–2009 at seven sites of different ages after restoration. The hypothesis was tested using the Bowen ratio system together with other comparative methods. Results showed that less rainfall would infiltrate the deeper soil layer as the period of time after restoration increased and that the natural steppe site had the shallowest infiltration depth (10cm or less). However, the youngest restored grassland site had the deepest infiltration depth and the highest soil water content. These results also showed that transpiration and ET rates were higher in the natural steppe site and older restored sites than those in the younger restored sites. Furthermore, it was found that the ratio between soil evaporation and ET was generally lower for all seven sites, which indicated that transpiration was a dominant component in ET. These results indicated that more water was used for vegetation establishment in the older restored grassland than that in the younger restored grassland. Our hypothesis was proved to be true. Meanwhile, it was also found that the dominant species changed from annual herbs (species had lower drought tolerance ability) in the younger restored grassland to perennial grasses in the older restored grassland (species had higher drought tolerance ability). Therefore, it was concluded that the natural steppe was able to utilize more water for vegetation establishment and it used water more efficiently than the restored steppe, while the older restored steppe used water more efficiently than the younger restored steppe. Because changes of vegetation type from annual to perennial not only increased evapotranspiration, but also changed the water budget, more attention should be paid to water shortage when undertaking the CCFGP in semi-arid steppes. [Copyright &y& Elsevier]

Summary: The magnitude and seasonal dynamics of evapotranspiration (ET) over a temperate desert steppe in Inner Mongolia, China, were investigated during a growing season (from 1st May to 15th October 2008) using the eddy covariance (EC) technique. Diurnal variations of latent heat flux (LE) at different phenological stages showed similar patterns with the peak value around noon. The sensible heat flux dominated the latent heat flux at midday during the whole growing season. Seasonal variation in ET followed closely the variation in soil water content (SWC) and leaf area index (LAI). The maximum daily ET rate was 3.8mmday−1. Cumulative ET during the study period estimated directly by EC method was 150.9mm, exceeding the precipitation (PPT) received at the site during the same period (133.0mm) by 13.4%. The daily crop coefficient (Kc, calculated as the ratio of the measured ET (from the EC system) to the reference ET (from the Penman–Monteith model)) also showed a distinct seasonal pattern, however its value on average was considerable lower (0.17) during the measurement period than the results reported from other grasslands, suggesting that this steppe was strongly affected by the deficit of available water. SWC was the most important environmental factor controlling ET in this steppe. The correlation analysis between daily Kc and its major environmental controls indicated that SWC, relative humidity (RH), and net radiation (Rn) were the major explanatory variables for daily Kc. Daily Kc showed a linear increase with SWC and Rn, and an exponential increase with RH. Thus, a daily Kc model would be developed as a function of SWC, RH, and Rn. An empirical daily ET model derived from the FAO 56 crop coefficient approach was developed based on this daily Kc model and the Penman–Monteith model. This ET model was validated by the data from the growing season in 2009, and showed a good consistency between the simulated and the measured ET values. Based on the developed ET model, an alternative ET model with the variable of SWC replaced by PPT was established, which may have the advantage of ET estimation under PPT more easily available than SWC. [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]

• Lc-excess was more robust than slope in representing soil evaporation process. • Soil evaporation declined from western to eastern sites in three transects. • Relative humidity was the dominated factors of soil evaporation in Tibet Plateau. • Vegetation coverage was the dominated factors in Loess and Inner Mongolia Plateau. Soil evaporation plays an important role in non-productive water loss and local climate regulation. The deviation of the relationship between δD and δ18O in surface soil water from that in precipitation is used to indicate the integrative results of the soil evaporation process, but the distinction between the slope of the soil water evaporation line (SEL) and line-conditioned excess (lc-excess) as individual indicators has not been identified. Furthermore, spatial variability and interaction effects of climate, soil, and vegetation on soil evaporation in arid and semi-arid grasslands remain poorly understood. We established 3 west-to-east sampling transects along with increasing precipitation and decreasing aridity index gradients in grasslands of the Tibetan (TP, 1500 km), Loess (LP, 600 km), and Inner Mongolia (MP, 1200 km) Plateaus. Slope of SEL and weighted mean lc-excess were calculated according to the δD and δ18O in soil water, which was extracted from soils collected at 10 cm intervals to a depth of 1 m along with soil profiles during the vigorous growing season. We found that lc-excess appeared relatively more robust than slope in representing soil evaporation process because lc-excess was determined by evaporation fractionation factors and soil residual water storage, while slope was regulated only by evaporation fractionation factors. Soil evaporation, as indicated by lc-excess, exhibited decreasing trends from west to east, and was controlled by relative humidity in TP, and by vegetation coverage in LP and MP. Our results highlighted that the soil evaporation process may be more sensitive to climate change in TP, and vegetation restoration in LP and MP. [ABSTRACT FROM AUTHOR]

• A mass balance model for agricultural salt accumulation and leaching is developed. • The model has looser restrictions on the discretization of space and time. • The calculated soil salt are about 20% better when considering the immobile region. Soil salinization has become a widespread problem that seriously affects the sustainable development of agriculture. To deal with the salt accumulation and leaching problems in practical agricultural systems, a new soil water and solute transport model adopting the mass balance scheme is developed in this study. The soil water movement module is based on a previously published soil water mass balance model, named as UBMOD. The mobile-immobile assumption has been adopted to develop the soil solute transport module, which can describe the obvious anomalous transport phenomenon in practice. The advection, chemical reactions of solute, solute transfer between the mobile and immobile regions, and dispersion in the mobile region are considered in the solute transport module, while only chemical reactions and solute transfer between the mobile and immobile regions are considered in the immobile region. Three cases were designed to evaluate the performance of the model by comparing the simulation results with those obtained by experimental results, HYDRUS-1D, and practical observations. The results demonstrate that the developed model can solve the homogeneous and heterogeneous soil water movement and solute transport effectively, and it has looser restrictions on the discretization of space and time than the numerical solution of Richards' equation and can keep the mass balance well, which make it more suitable for practical conditions. Moreover, the real-world application of salt accumulation and leaching problems in Hetao Irrigation District, Inner Mongolia, China shows that the RMSE of the simulated soil salt content with the immobile present region is about 20% better than that when ignoring the immobile present region. Therefore, it is necessary to take the significant anomalous transport into account when studying the salt accumulation and leaching processes in practical agricultural systems. [ABSTRACT FROM AUTHOR]

• Visible and shortwave-infrared satellite data is used for the downscaling scheme. • Downscaled SMAP soil moisture at 1 km are generated for the Inner Mongolia. • The downscaled SMAP is validated against measurements collected from 30 in situ stations over the entire study area. Soil moisture (SM) plays an indispensable role in many practical applications, such as drought monitoring, hydrologic applications and agricultural management. Passive microwave remote sensing has proven capable of capturing changes in SM. However, the coarse spatial resolution (approximately 25–40 km) may greatly limit many regional hydrological and agricultural applications. In this study, we present a SM downscaling method based on the visible and shortwave-infrared (SWIR) remote sensing data toward improved spatial resolution of Soil Moisture Active Passive (SMAP) SM. In the proposed method, the Land Surface Temperature (LST), Normalized Difference Vegetation Index (NDVI), horizontally polarized Brightness Temperature (TBh), vertically polarized Brightness Temperature (TBv) and SWIR reflectance collected in Inner Mongolia from April to September 2018 and April to September 2019 are used as input data and combined with topographic information to downscale the SMAP SM L3 product from the original 36 km to 1 km. Random forest (RF) was used to link the input data and SMAP SM. Finally, 30 in situ station SM measurements distributed in Inner Mongolia and precipitation data were used to verify the downscaled SM. The results show that the correlation range of the downscaled SM and the in situ SM is 0.246 to 0.739, with an average of 0.535. The root mean square error (RMSE) range is 0.02 to 0.152 m3/m3, and the mean RMSE is 0.059 m3/m3. The ubRMSE has a range of 0.02 to 0.06 m3/m3, and the mean performance is 0.04 m3/m3. The bias ranges from −0.145 to 0.087 m3/m3, and the mean bias is 0.008 m3/m3. The downscaled SMAP SM can capture the spatial heterogeneity of the SMAP SM and the dynamic changes in measured soil moisture. The downscaled SMAP SM also has a high spatial correspondence with the original SMAP SM, providing more detailed soil water information than the original 36 km resolution. [ABSTRACT FROM AUTHOR]

• A three-level iteration scheme to solve hydrothermal coupling model is developed. • The model can capture water table depth caused by freeze-thaw processes accurately. • The upward flow of groundwater to freezing soils is estimated. • The deepest impact depth of soil frozen process is 180 cm. • The per-freezing water table 100 cm can ensure required soil hydrothermal conditions. In arid agricultural areas with shallow water table depth, water exchange between vadose zone and groundwater is intensive during the freezing-thawing period, and thus has a significant impact on crop growth. This study is aimed to develop a one-dimensional model that couples heat and variably saturated water in seasonally freezing-thawing agricultural areas. To alleviate numerical oscillations occurring during the phase change at zero temperature, a three-level iteration scheme was developed for solving the water and heat coupling model, which improves numerical stability for variably saturated water and heat modeling. The model accuracy was evaluated by comparing the simulation results with those from SHAW under various soil texture and bottom conditions. The square of correlation coefficient (R 2) of freezing and thawing depth and soil temperature profiles are above 0.95 and the values of root mean square error (RMSE) of total soil water content are lower than 0.05 cm3/cm3, which suggest the model accuracy. The model was further applied to simulate the hydrothermal conditions in the Yonglian irrigation field of Hetao Irrigation District in Inner Mongolia, China. Model calibration and validation were conducted, and the validated model was used to predict hydrothermal changes under scenarios with different soil surface temperatures and pre-freezing water table depths. The results show that the soil freezing process has great impact on the fluctuation of water table depth when the pre-freezing water table depth is shallower than 180 cm. The maximum freezing depth has a negative relation with the pre-freezing water table depth when it is shallower than 180 cm. An empirical formula was established to estimate the upward flow of groundwater to freezing soils. The soil moisture and thermal conditions were analyzed in the germination and seedling stages after the freezing-thawing period, and the analysis leads to a recommended pre-freezing water table depth of 100 cm, which provides suitable soil moisture and thermal conditions for the crop growth. [ABSTRACT FROM AUTHOR]