Modeling the Water and Energy Balance of Vegetated Areas with Snow Accumula on

1013 W in the soil–plant–atmosphere system are important at the local, regional, and global scales. Direct measurement of these fl uxes is generally limited to a few locations and to relatively short periods. Many computer simulation models have been developed to study the spatial and temporal patterns in water and energy fl uxes. Th e models facilitate studying larger spatial domains and longer time periods than would be practical using measurements alone. Th ese models are used in agricultural, ecosystem, and climate research (e.g., Flerchinger et al., 1998; van Dam et al., 2008; Oleson et al., 2008). Th e number of incorporated processes as well as process detail varies considerably among existing models. Th is is not surprising given the complex nature of soil–plant–atmosphere water and energy fl uxes, which results in many interacting factors. Modeling of water and energy fl uxes in snow-dominated mountainous terrain is particularly challenging. Th e presence of snow modifi es the land surface energy balance considerably. Fresh new snow in particular has a high albedo and a low thermal conductivity, which limits daytime soil warming and nighttime soil cooling. Snow is a complicated medium due to continuously changing properties such as grain size, density, and height. Snow modeling concepts vary from relatively simple single-layer representations (e.g., UEB, Tarboton and Luce, 1996; COUP, Jansson and Karlberg, 2004), to more advanced two-layer representations (e.g., Marks et al., 1998; Koivusalo et al., 2001), to sophisticated multilayer numerical approaches (Anderson, 1976; SNTHERM, Jordan, 1991; Lehning et al., 2006). Soil freeze–thaw may have an important impact on the water and energy fl uxes in mountainous terrain. Th is is especially true during periods in which the snow cover is limited so that the soil is exposed to the atmosphere. Freezing of soil water produces heat, keeping the soil close to 0°C. In contrast, the melting of soil ice requires energy, which delays soil warm-up during spring. Most current soil freeze–thaw algorithms are based on the Clausius– Clapeyron equation, which is used to relate the freezing point of soil water to soil water potential (Fuchs et al., 1978; Spaans and Baker, 1996; Koren et al., 1999; Niu and Yang, 2006). Snow can be included in vadose zone models using simple degree day concepts (e.g., HYDRUS, Simunek et al., 2005). More physically based methods for modeling snow accumulation involve calculating the surface energy balance. Th e most sophisticated approaches calculate both the canopy energy balance and the ground surface energy balance (e.g., SHAW, Flerchinger, Modeling the Water and Energy Balance of Vegetated Areas with Snow Accumula on