PREFLO: A WATER MANAGEMENT MODEL CAPABLE OF SIMULATING PREFERENTIAL FLOW

Abstract

Preferential flow through large continuous pores affects the distribution of water in the soil profile by reducing runoff and increasing total infiltration. In this study, a water management model (PREFLO) was developed which could be used to simulate unsaturated and saturated movement of water in a soil profile in which preferential flow might occur. PREFLO is based on a one-dimensional finite difference solution to the Richards equation with a nonuniform grid spacing. Large pores are described on a macroscopic scale with vertical movement of water computed from the equation for flow in a capillary tube. Water moving from the large pores into the soil matrix via horizontal infiltration is added to the sink term in the Richards equation. Example simulations indicated that PREFLO can be a useful tool in simulating the timing, frequency, and volume of preferential flow in a soil profile. Use of hourly rainfall data and the Richards equation to simulate soil water conditions allowed the PREFLO model to predict preferential flow when infiltration into the soil matrix was limiting. Simulated water table response in soils that contain preferential flow channels was shown to be dependent on hydraulic conductivity and the number of large pores. For a simulated rainfall event on a soil with a hydraulic conductivity of 0.15 cm/h, PREFLO predicted a rapid rise of the water table caused by water ponding in the large pores. Simulations using soils with hydraulic conductivities of 0.5 cm/h and 0.8 cm/h resulted in less rapid movement of the water table. The wetting front was shown to move slowly through the soil profile for the low conductivity soil which deviates from the drained to equilibrium state assumed in DRAINMOD.