Land Surface Hydrololgy Model

Our modeling activities are based on VIC-2L, the two-layer Variable Infiltration Capacity model. It is distinguished from other soil-vegetation-atmosphere-transer-schemes (SVATS) in that it uses a spatially varying infiltration capacity to represent subgrid-scale heterogeneity in soil properties.

Each grid cell can have partial spatial coverage by a number of vegetation types (n), as well as bare soil. Vegetation characteristics are assigned to each vegetation, such as leaf area index (LAI), minimal stomatal resistance, and roughness length; all of which may be time-varying. The total actual evapotranspiration is calculated as the sum of canopy evaporation (Ec), evaporation from bare soil (E1), and transpiration (Et). Moisture can be extracted from either or both soil layers by evapotranspiration, depending upon the prescribed fraction of roots in each zone. The subsurface scheme consists of two soil layers. The upper layer receives moisture from precipitation (P) by infiltration. The drainage of base flow from the lower soil layer (Qb) is represented as a non-linear recession.

By using a probability distribution of the Beta distribution form, the model represents spatial heterogeneity at scales smaller than the application scale, without assigning specific infiltration capacity values to specific subgrid-scale locations. instead the shape of the spatial probability function is determined by a single parameter (bi), which characterizes the amount of available infiltration capacity (i) as a function of relative saturated grid cell area. Precipitation in excess of infiltration becomes surface runoff (Qd).