Improving Soil-Vegetation Dynamics in the Soil and Water Assessment Tool (SWAT)

A non-iterative 1D Richard's equation model is developed and implemented in the Soil and Water Assessment Tool (SWAT) to improve the physical representation of soil-water-vegetation dynamics. SWAT’s improved version (UN-SWAT) explicitly represents infiltration, soil evaporation, unsaturated water flow, root water update, and lateral drainage. Water-exchanges across the surface-subsurface and unsaturated-saturated zone interfaces are defined as the system’s dependent top and bottom boundaries of the soil profile, respectively. In the continuum from the land surface to the aquifer, the top boundary of the soil profile accounts for non-ponding or ponding infiltration, as well as atmosphere-controlled or soil-controlled evaporation. Vegetation’s root water update and lateral drainage are represented as sink terms in each soil layer. The soil profile is discretized by a variable number of computational nodes of the soil profile, whose bottom position is determined based on the groundwater table. UN-SWAT validation is performed by a single-HRU and a multi-HRU simulations in the Little Washita River Experimental Watershed in Oklahoma. Results prove that UN-SWAT’s performance simulating the soil water movement in both space and time under complex conditions agree observed soil moisture and stream discharge data. UN-SWAT represents an improvement over other hydrologic models by providing a more accurate solution to the soil-water-vegetation model and accounting for the dynamics of climate and groundwater conditions.