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Accurate estimation of evapotranspiration (ET) and the partitioning of ET into transpiration (Tr), soil evaporation (Es) and interception (Ei) is critical to understand water cycle and land-atmosphere feedback. In this study, we evaluated the performances of two remote sensing-based ET models at multiple scales, and analyzed the uncertainties in ET partitioning due to the model structures. These two models were Simple Terrestrial Hydrosphere Model (SiTH) developed by our team and the Global Land Evaporation Amsterdam Model (GLEAM). As far as ET were concerned, the two models exhibited relatively good performances at different scales. However, it was found that GLEAM performed relatively poor at evergreen broadleaf forest (R2 = 0.34; RMSE = 0.87 mm day−1; NSE = −0.28). In addition, the seasonal pattern of simulated ET by GLEAM at the tropical rainforest was not consistent with the observations. Furthermore, great discrepancies in ET partitioning were observed between the two models. Generally, GLEAM tended to underestimate Es (slope = 0.02; R2 = 0.004), and overestimate Tr (slope = 1.51; R2 = 0.78) compared to the observations. The underestimations of Es by GLEAM may partly be due to the ignorance of soil evaporation under vegetation canopy. On the contrary, SiTH displayed relatively good performances in estimations of Es (slope = 0.76; R2 = 0.62) and Tr (slope = 0.98; R2 = 0.51). However, both of the two models failed to properly simulate Ei, although GLEAM (slope = 0.55; R2 = 0.83) performed slightly better than SiTH (slope = 0.40; R2 = 0.95). Global multi-year average ratios of Tr, Es, and Ei to ET for GLEAM and SiTH were 0.76, 0.09, 0.15 and 0.67, 0.25, 0.08 respectively. In future studies, it is important to investigate direct observations on different components of ET, especially on the interception, to improve our understanding on the ET processes.

Difficulty in measuring hydraulic conductivity, particularly under dry conditions, calls for methods of predicting the conductivity from easily obtained soil properties. As a complement to the recently published EMFX model, a method based on two specific suction conditions is proposed to estimate saturated film conductivity from the soil water retention curve. This method reduces one fitting parameter in the previous EMFX model, making it possible to predict the hydraulic conductivity from the soil water retention curve over the complete moisture range. Model performance is evaluated with published data of soils in a broad texture range from sand to clay. The testing results indicate that 1) the modified EMFX model (namely the EMFX-K model), incorporating both capillary and adsorption forces, provides good agreement with the conductivity data over the entire moisture range; 2) a value of 0.5 for the tortuosity factor in the EMFX-K model as that in the Mualem’s model gives comparable estimation of the relative conductivity associated with the capillary force; and 3) a value of −1.0 × 10 −20 J for the Hamaker constant, rather than the commonly used value of −6.0 × 10 −20 J, appears to be more appropriate to represent solely the effect of the van der Waals forces and to predict the film conductivity. In comparison with the commonly used van Genuchten-Mualem model, the EMFX-K model significantly improves the prediction of hydraulic conductivity under dry conditions. The sensitivity analysis result suggests that the uncertainty in the film thickness estimation is important in explaining the model underestimation of hydraulic conductivity for the soils with fine texture, in addition to the uncertainties from the measurements and the model structure. High quality data that cover the complete moisture range for a variety of soil textures are required to further test the method.

Following the scheme of the groundwater-soil-plant-atmosphere continuum (GSPAC), a simple process-based model (SiTH; Simple Terrestrial Hydrosphere) was developed to estimate the dynamics of terrestrial evapotranspiration (ET) at the daily step. The input data include net radiation, air temperature, precipitation, leaf area index, vegetation type and soil data, most of which are readily available. Locally, the model performed well in simulating the dynamics of ET and soil moisture over selected FLUXNET sites. Globally, the daily 0.25° ET and groundwater table depth estimations in year 2005 were determined using available gridded datasets. The spatial pattern was reasonable and the range of values corresponded well with other global ET and groundwater table depth products. In future studies, we will produce a long-term daily 0.25° global ET and groundwater table depth products that spans from 1984 to present by using different input datasets.

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 3 m 2 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.

Summary We intensively investigated the composition and distribution of several major ions and stable isotopes (δ2H, δ18O) in precipitation in 2008 and 2009 at three sites in the Shiyang River Basin of northwestern China. The goal was to provide basic data that would help us to understand the geochemical evolution and recharge sources of groundwater in the basin’s Quaternary aquifer. The δ2H and δ18O values for 75 precipitation samples ranged from +35.7‰ to −170.9‰ and from +4.6‰ to −23.3‰, respectively. The relationship between δ2H and δ18O defined a well-constrained local meteoric line, which was nearly identical to the meteoric water line for northern China. The evaporation process in this dry region of northwestern China obviously altered the original relationship between rainfall δ2H and δ18O, resulting in d-excess values SO 4 2 - , NO 3 - , Cl−, NH 4 + , Ca2+, Mg2+, Na+, and K+ in mountain and desert areas were lower than those in most of China’s cities. The majority of the rainfall samples had a Cl− concentration of 1.5–2.5 mg L−1, and the excess of Na+ over Cl−, combined with a strong excess of non-marine SO 4 2 - and the overall precipitation chemistry, indicates that some solutes were contributed from terrestrial sources during the air mass trajectory over land. These values will provide reliable rainfall input information that can be used in future groundwater recharge calculations in the study area.

A hydrochemical investigation was conducted in the Minqin Basin to identify the groundwater evolution and recharge in the aquifer. The mBr/Cl ratio is strongly depleted (average 0.000451) compared with sea water (0.0035), indicating an evaporite origin. The ionic ration plot, saturation index (SI), and chloro, alkaline indices (CAI) suggest that the dissolution of halite, the glauberite, gypsum, dolomite and calcite determine Na+, Cl-, Ca2+, Mg2+, SO42-, and HCO3- chemistry, but other processes, such as Na+ exchange for Ca2+ and Mg2+, and calcite precipitation also contribute to the water composition. The 6180 and 62 H in precipitation near the study area are linearly correlated, similar to that for the world meteoric water line (WMWL), with an equation of delta H-2 = 7.49 delta O-18 + 5.11 (r(2) = 0.97). According to radiocarbon residence time estimates, the deep groundwater is approximatety 40 ka old, and was recharged during a period when the climate was wetter and colder. The radiocarbon content of shallow groundwater shows a clear evolution along the groundwater flow path. From the beginning of the groundwater flow path to similar to 31 km the radiocarbon values are > 73.6 pmc, whereas beyond this point the values are < 42.9 pmc. Based on radiocarbon content, the shallow groundwater is older than 1 ka, and represents pataeowaters mixed with a limited quality of modern recharge. The rain-fed groundwater direct recharge was estimated by chloride mass balance (CMB) method to range from 1.55 to 1.64 mm yr(-1), with a mean value of 1.6 mm yr(-1). This value represents about 1.5% of local rainfall. The direct recharge volumes is about 0.666 x 10(8) m(3) yr(-1). Indirect recharge volumes by the surface water is about 0.945 x 10(8) m(3) yr(-1). The total natural recharge in the Minqin Basin is 1.6 x 10(8) m(3) yr(-1), whereas the groundwater abstraction has reached 11.6 x 10(8) m(3) yr(-1), far exceeding the groundwater natural recharge. (c) 2006 Elsevier B.V. All rights reserved.