Advances of modeling water flow in variably saturated soils with SWAP

The Soil Water Atmosphere Plant (SWAP) model simulates transport of water, solutes, and heat in the vadose zone in interaction with vegetation development. Special features of the model are generic crop growth, versatile top boundary conditions, macroporous flow, and interaction of soil water with groundwater and surface water. We discuss typical model applications that have appeared in recent scientific literature. New model developments are explained with respect to the numerical solution of Richards' equation, macroporous flow, evapotranspiration, and interactions with groundwater and surface water. We describe case studies on agricultural water productivity, regional nutrient management, and groundwater conservation by surface water management. Finally we envision model developments with respect to SWAP for the coming 5 to 10 yr.