Abstract
Abstract. In this paper, simulations with the Soil Water Atmosphere Plant (SWAP) model are performed to quantify the spatial variability of both potential and actual evapotranspiration (ET), and soil moisture content (SMC) caused by topography-induced spatial wind and radiation differences. To obtain the spatially distributed ET/SMC patterns, the field scale SWAP model is applied in a distributed way for both pointwise and catchment wide simulations. An adapted radiation model from r.sun and the physically-based meso-scale wind model METRAS PC are applied to obtain the spatial radiation and wind patterns respectively, which show significant spatial variation and correlation with aspect and elevation respectively. Such topographic dependences and spatial variations further propagate to ET/SMC. A strong spatial, seasonal-dependent, scale-relevant intra-catchment variability in daily/annual ET and less variability in SMC can be observed from the numerical experiments. The study concludes that topography has a significant effect on ET/SMC in the humid region where ET is a energy limited rather than water availability limited process. It affects the spatial runoff generation through spatial radiation and wind, therefore should be applied to inform hydrological model development. In addition, the methodology used in the study can serve as a general method for physically-based ET estimation for data sparse regions.
Highlights
Evapotranspiration (ET) is a very important element in hydrological cycle, and it is adopted as the criteria for climate classification (Thornthwaite and Mather, 1955)
Correlation of radiation with aspect and slope are calculated to their functional values of sine and cosine respectively, given the known trignometrical relationship between radiation and topography
The dependence of wind on topographic is rather complicated, the correlation coefficients are derived based on the original values
Summary
Evapotranspiration (ET) is a very important element in hydrological cycle, and it is adopted as the criteria for climate classification (Thornthwaite and Mather, 1955). ET/SMC show high spatial heterogeneity at different scales (Bresnahan and Miller, 1997; Western et al, 2002), resulted from the vertical and/or lateral water transfer which are subjected to the interaction of local atmospheric factors (precipitation, radiation, temperature, humidity, pressure, etc.), soil characteristics and vegetation covers. Inferring ET/SMC with modeling approach using other remotely sensed parameters, such as land surface temperature (LST), vegetation index (NDVI/EVI), etc. This paper will apply the the Soil Water Atmosphere Plant (SWAP) model, which contains the Penman-Monteith approach as the core module for ET estimation, to avoid the complexity of surface temperature estimation associated typically with LSMs. A set of numerical experiments with the SWAP model are designed to investigate the spatial ET/SMC variability originating from spatial wind and radiation difference.
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