Actual evapotranspiration (ETa) is an important component of the surface water cycle. The geeSEBAL model is increasingly being used to estimate ETa using high-resolution remote-sensing data (Landsat 4/5/7/8). However, due to surface heterogeneity, there is significant uncertainty. By optimizing the quantile values of the reverse-modelling automatic calibration algorithm (CIMEC) endpoint-component selection algorithm under extreme conditions through 212 global flux sites, we obtained the optimized quantile values of 11 vegetation types of cold- and hot-pixel endpoint components (Ts and NDVI). Based on the observation data of the global FLUXNET tower, the sensitivity of 20 parameters in the improved geeSEBAL model was determined through Sobol’s sensitivity analysis. Among them, the parameters dT and SAVI,hot were confirmed as the most sensitive parameters of the algorithm. Subsequently, we used the differential evolution Markov chain (DE-MC) method to analyse the uncertainty of the parameters in the geeSEBAL model used the posterior distribution of the parameters to modify the sensitive parameter values or ranges in the improved geeSEBAL model and to simulate the daily ETa. The results indicate that by analysing the end element components of the geeSEBAL model (Ts and NDVI), quantile numerical optimization and parameter optimization can be performed. Compared with the original algorithm, the improved geeSEBAL model has significantly improved simulation performance, as shown by higher R2 values, higher NSE values, smaller bias values, and lower RMSE values. The most suitable values of the predefined parameter Zoh were determined, and the reanalysis of meteorological data inputs (relative humidity (RH), temperature (T), wind speed (WS), and net radiation (Rn)) was also found to be an important source of uncertainty for the accurate estimation of ETa. This study indicates that optimizing the quantiles and key parameters of the model end component has certain potential for further improving the accuracy of the geeSEBAL model based on high-resolution remote-sensing data in estimating the ETa for various vegetation types.

Study region: Global and 28 large river basins Study focus: Actual evapotranspiration (ETa) plays a key role in the redistribution of water, carbon and energy. The emergence of many ETa products has made uncertainty assessment increasingly important. The FLUXNET2015 dataset and 28 large watershed water balance datasets were used in this study. The monthly scale products of the ERA5-Land reanalysis data (ERA5), Global Land Data Assimilation System (GLDAS), Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA) and Penman-Monteith-Leuning Model Version 2 (PML) terrestrial evapotranspiration models were evaluated from 2001 to 2017. The differences in composition (vegetation transpiration (T), soil evaporation (Es), canopy interception loss (Ei) and other components (open water and ice and snow sublimation) (Eo)) and vegetation among the four ETa products were compared.New Hydrological Insights for the Region: At the site scale, the ERA5 and MERRA products significantly outperformed the GLDAS and PML products, with the latter exhibiting poorer reliability in site validation. The PML product's basin-scale water balance KGE metric overall outperformed those of the ERA5, GLDAS, and MERRA products, with KGE > 0 in 23 basins. The use of basin-scale data mitigates the impact of local outliers on the simulation results, leading to KGE validation metrics at the basin scale that are overall superior to those obtained from site-scale validation. There were large errors in the estimates of T and Es in the ERA5 product, related to the overestimation of Es and underestimation of T. The boundary between sea and land (used to divide marine evapotranspiration and land evapotranspiration) was unclear in the PML products. Eo/ETa was overestimated, and there were clearly high values at the land margin (Eo peaks as high as 3803 mm/yr). The difference in evapotranspiration components had a considerable influence on the uncertainty of ETa. The vegetation types in the 4 ETa products for DBF***, EBF***, ENF***, MF***, GRA***, and CRO*** all exhibited significant differences at the P<0.001 level. This study contributes to product uncertainty analysis and the determination of ways to improve ETa products.

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Shallow groundwater is an important water source for Haloxylon ammodendron (H. ammodendron). The accurate estimation of evapotranspiration (ETg) from groundwater is of great significance for the water cycle and the maintenance of ecological stability. Using a combination of the water balance method and the groundwater level fluctuation method (WTF), the water balance components (precipitation, soil moisture, groundwater depth, and Bowen ratio meteorological data) in the desert–oasis transition zone were continuously monitored from 2015 to 2018 and the ETg was estimated The results showed that the closed degree of Bowen specific energy after data screening was higher, and the annual actual evapotranspiration (ETa) value could be reliably calculated at 260.87 mm. As the main contributor to water consumption in the growing season, latent heat accounted for 70.16~91.86% of the energy balance. Precipitation had no significant impact on water consumption for H. ammodendron vegetation growth, and the precipitation in the main growing season accounted for 59.44% of the ETa. The groundwater depth in the study area decreased yearly and had a significant impact on the growth of H. ammodendron vegetation. Although the groundwater depth in the study area was greater than 9 m, the ETg, as an important part of the water balance, was found to participate in the evapotranspiration process brought about by H. ammodendron due to the strong root system and supporting capillary water in the soil. The actual evapotranspiration ETa for H. ammodendron in the main growing season was 244.32 mm, and the contribution rate for ETg was as high as 74.78% or approximately 182.35 mm. After the ETg was verified using the water balance method and WTF, R was greater than 0.96, the RMSE range was 1.5931~4.5706, the bias range was −0.15~0.11, and the IOA value was greater than 0.95. The accuracy of the estimation model was high, and the results were relatively accurate. The model can be applied in the desert–oasis transition zone to obtain accurate ETg estimations and provide theoretical guidance and a scientific basis for local water resource management and ecological protection.

Snow-affected mountainous areas are highly vulnerable to climate changes in arid and semi-arid regions. Previous studies have put more emphasis on runoff, however, research about snow hydrological processes is still insufficient. Based on the optimization of calibration strategies, this study aims to provide a more realistic simulation of snow hydrology by coupling both a lumped rainfall-runoff model (GR6J) and a semi-distributed snow model (CemaNeige). Compared with calibration using only runoff, the multi-objective calibration with both runoff and snow not only significantly improved the KGE (Kling-Gupta efficiency) of snow from 0.34/0.47 to 0.67/0.69 during the calibration/validation period, but also slightly improved the KGE of runoff from 0.84/ 0.75 to 0.86/0.78, respectively. Additionally, we also found that the calibration using only runoff may underestimate snow cover (41%) and overestimate snowpack (20%), while there is little discrepancy in snowmelt and runoff. Snowmelt runoff is 57.7 mm accounting for 26% of the annual runoff. In the historical period (1980-2011), the runoff varied with a positive trend of 2.15 mm year-1 (p < 0.1), while its tendency became negative in the future. A 15% to 30% decline in runoff was projected for SSP370 (SSP3-RCP7) and SSP585 (SSP5RCP8.5) in the 2085s (2070-2100) compared with that in the 1985s (1970-2000). Through further study on seasonal variations, we found that the future runoff decline is probably due to mild rise in rain not enough to alleviate the larger AE (actual evaporation), and for the runoff and AE we also found the variations in summer dominated the annual changes. In response to global warming, SCED (snow cover end date) advances while SCOD (snow cover onset date) recedes in all four scenarios as the year increases. Generally, coupling different hydrological models and multi-objective calibration strategies is vital for understanding the hydrological processes in snow-affected catchments.

Riparian phreatophytes in hyperarid areas face selection pressure from limiting groundwater availability and high transpiration demand. We examined whole-plant water use and hydraulic traits in Populus euphratica and Tamarix ramosissima seedlings to understand how they adapt to groundwater variations. These species coexist in the Tarim River floodplain of western China. Measurements were performed on 3-year-old seedlings grown in lysimeters simulating various groundwater depths. P. euphratica had relatively greater leaf area-specific water use due to its comparatively higher sapwood area to leaf area ratio (H-v). A high H-v indicates that its sapwood has a limited capacity to support its leaf area. P. euphratica also showed significantly higher leaf-specific conductivity (k(sl)) than T. ramosissima but both had similar sapwood-specific conductivities (k(ss)). Therefore, it was H-v rather than k(ss) which accounted for the interspecific difference in k(sl). When groundwater was not directly available, k(sl) and H-v in P. euphratica were increased. This response favors water loss control, but limits plant growth. In contrast, T. ramosissima is more capable of using deep groundwater. Stomatal sensitivity to increasing leaf-to-area vapor pressure deficit was also higher in P. euphratica. Overall, P. euphratica is less effective than T. ramosissima at compensating for transpirational water loss at a whole-plant level. For this reason, P. euphratica is restricted to riverbanks, whereas T. ramosissima occurs over a wide range of groundwater depths.

Soil water,as a major factor limiting vegetation in deserts,does not only affect the survival and growth of plants,but also the species abundance and distribution.To research the spatiotemporal variations of soil moisture content in root zone is vital for understanding the recovery,maintenance and the stability of plant communities in deserts.In the Gurbantunggut desert in northwestern China,Haloxylon ammodendron as the dominant species plays an important role in dune stabilization.However,little is known about the effects of plant life stage on the variations of soil moisture content in the root zone for Haloxylon am modendron.In our study,soil water dynamics were monitored for eight typical Haloxylon ammodendrons individuals in an inter-dune low land at the southern edge of Gurbantunggut Desert,each with a soil moisture monitoring point 20-30 cm away from the stem base.Another two points were set in the inter-shrub bare soil.In addition,16 points were positioned two individuals with 10 meters away from each other.Thus there were totally 26 points.From February 2014 to November 2014,in-situ observations for soil moisture within 0-400 cm soil were performed by neutron probe method in combination with oven-drying method to explore the spatiotemporal variations of soil moisture in the root zone of Haloxylon ammodendron individuals at different life stages.Results show that (1) the annual variation of soil water underwent 4 periods,i.e.,recharging period (from later February to early March),equilibrium period (from early April to late May),discharging period (from early June to late October),and stable period (from early November to middle February of the following year);(2) soil water dynamics differed within profiles,with a higher coefficient of variation that decreases rapidly with depth in the top 50 cm soil layer,and a relatively constant coefficient for 50-400 cm soil layers;(3) the averaged soil moisture content at different life stages in spring,summer,fall,as well as in the whole year displayed a pattern in order of dead individual ＞ adult individulal ＞ young individual ＞ bare soil;(4) soil moisture decreased gradually with the increasing distance to the Haloxylon ammodendron base stem within 5 meters;and (5)following rainfall events,the soil moisture content in 10 cm soil increased more in the root zone than in the bare soil.

融雪水土壤入渗量是干旱区沙漠重要的水平衡收入项.2012-2013、2013-2014年两个冬季对古尔班通古特沙漠南缘沙丘西坡、东坡和丘间地降雪前和融雪后的土壤含水率进行监测,根据水量平衡原理计算了沙丘西坡、东坡、丘间地和景观尺度上的融雪水土壤入渗量,并与采用筒测法的实测结果进行比较.结果表明:降雪前土壤含水率较低,未冻层非饱和土壤水对地表冻结层土壤水分的补给可忽略不计;融雪入渗水是表层土壤获得补给的主要水源;冻结期潜水既没有蒸发,积雪融化后潜水也没有获得补给;研究区西坡、东坡、丘间地和景观尺度上的融雪水土壤入渗量分别为20～43、27～43、32～45和26～45 mm.

The groundwater of medium salinity refers to the groundwater with a mineralization degree of 2-7 g/L that can be directly or indirectly used for irrigation. This paper determined the distribution area of the groundwater with medium salinity in Tarim Basin, estimated the total amount of recharge resources and exploitable quantity of the groundwater of medium salinity. The irrigation water quality assessment was conducted on the groundwater of medium salinity by using multiple factors and methods. The salt tolerance parameters and irrigation water mineralization control parameters of the cotton in under-mulch-drip irrigation were determined. The under-mulch-drip irritation technology for cotton in groundwater of medium salinity was developed and applied successfully to the demonstration area. The social, economic and ecological effects of this technology as well as the application prospect were analysed. This paper also puts forward two questions that need to be further studied.

Based on experimental data from the five observation points during the three years, the linear subsected functions and the nonlinear s-shaped functions between the cotton relative yield and soil salt content on the salinized soil about the 0-20cm soil layer and the 0-40cm soil layer in Akesu River Irrigation District were constructed by linear regression and nonlinear least square approximation. Their applicabilities were analyzed and compared and it was found the nonlinear s-shaped function of the 0-20cm soil layer to fit better with the response relationship between the cotton relative yield and the soil salt content on the salinity soil than others in Akesu River Irrigation District.which and the indexes of cotton salt tolerance were definited, and then the indexes of cotton salt tolerance were drawn on with the function with better applicability. From the function, some indexes of salt tolerance，which contained the cotton critical soil salt content, the cotton threshold soil salt content, the soil salt content at the fastest rate of cotton relative yield reduction, and the soil salt content at the 50% cotton relative yield reduction, and so on, were determined, which can be provide as the important references for the agricultural planting, improvement of salinized soil and irrigation with saline water in Akesu River Irrigation District.

Water and energy balance models were developed at the local and basin scale by aggregating a HYDRUS Model across scales in a topographic framework. A spatially distributed approach was followed to aggregate the HYDRUS Model to basin scale. The effects of spatial variability on the estimation of evapotranspiration at basin scale are emphatically discussed. In the present work the main emphasis was focused on the estimation of the spatial variability of both evapotranspiration and the calculation of statistical parameters of soil water storage from local scale to basin scale. Our results illustrate that the changes of the local and area-averaged potential evapotranspiration (ETp), actual evapotranspiration (ET) show a strong seasonal trend where the mean and standard deviation (STD) approximately fit the two-harmonics of Fourier series. Moreover, ETp and ET are random variables which follow the normal distribution with mean and deviation changing with season. A sensitivity analysis of the results shows that to a reasonable degree, the incorporated model can be used to determine the relationship between the actual area-averaged evapotranspiration and the main controlling factors for both bare soil and vegetated surfaces. (C) 2003 Elsevier Science Ltd. All rights reserved.

植物根系吸水特性是SPAC系统中水分运移规律研究的重要内容.现有的根系吸水模型大致可分为3类:经验模型、半经验半理论模型和理论模型.此领域的研究最早始于50年代末60年代初,进入70年代,各种根系吸水模型相继出现.随着计算机科学的发展与普及以及SPAC水分传输动态模拟研究的深人,在80年代涌现出了大批研究成果.未来研究的重点是:注重根系吸水机理研究、修改与完善已有的根系吸水模型、植物根系结构研究、根区水分运移力能关系研究、植物根系吸水特性应用研究和植物根系过程可视化的研究.