Shah Nirjhar, Nachabe Mahmood, Ross Mark
Center for Modeling Hydrologic and Aquatic Systems, Department of Civil and Environmental Engineering, University of South Florida, Tampa, FL 33620, USA.
Ground Water. 2007 May-Jun;45(3):329-38. doi: 10.1111/j.1745-6584.2007.00302.x.
In many landscapes, vegetation extracts water from both the unsaturated and the saturated zones. The partitioning of evapotranspiration (ET) into vadose zone evapotranspiration and ground water evapotranspiration (GWET) is complex because it depends on land cover and subsurface characteristics. Traditionally, the GWET fraction is assumed to decay with increasing depth to the water table (DTWT), attaining a value of 0 at what is termed the extinction depth. A simple assumption of linear decay with depth is often used but has never been rigorously examined using unsaturated-saturated flow simulations. Furthermore, it is not well understood how to relate extinction depths to characteristics of land cover and soil texture. In this work, variable saturation flow theory is used to simulate GWET for three land covers and a range of soil properties under drying soil conditions. For a water table within half a meter of the land surface, nearly all ET is extracted from ground water due to the close hydraulic connection between the unsaturated and the saturated zones. For deep-rooted vegetation, the decoupling of ground water and vadose zone was found to begin at water table depths between 30 and 100 cm, depending on the soil texture. The decline of ET with DTWT is better simulated by an exponential decay function than the commonly used linear decay. A comparison with field data is consistent with the findings of this study. Tables are provided to vary the extinction depth for heterogeneous landscapes with different vegetation cover and soil properties.
在许多地貌中，植被会从不饱和区和饱和区提取水分。蒸散（ET）在包气带蒸散和地下水蒸散（GWET）之间的分配很复杂，因为它取决于土地覆盖和地下特征。传统上，假定GWET占比会随着地下水位深度（DTWT）的增加而衰减，在所谓的灭绝深度处达到0值。通常采用随深度线性衰减的简单假设，但从未使用非饱和-饱和流模拟进行严格检验。此外，对于如何将灭绝深度与土地覆盖和土壤质地的特征联系起来，人们还了解得不够透彻。在这项研究中，利用可变饱和流理论模拟了三种土地覆盖以及一系列土壤性质在土壤干燥条件下的GWET。对于地下水位在地表半米范围内的情况，由于非饱和区和饱和区之间紧密的水力联系，几乎所有的ET都来自地下水。对于深根植被，发现地下水与包气带的解耦始于地下水位深度在30至100厘米之间，这取决于土壤质地。与常用的线性衰减相比，用指数衰减函数能更好地模拟ET随DTWT的下降情况。与实地数据的比较与本研究结果一致。文中提供了表格，用于针对具有不同植被覆盖和土壤性质的异质景观改变灭绝深度。
