AGROFORESTRY DRAINAGE MANAGEMENT MODEL .1. THEORY AND VALIDATION

The cultivation of eucalyptus trees is being recommended to lower the shallow ground-water level and to reuse saline subsurface drainage waters from tile-drained cropland in drainage-impacted areas in California's San Joaquin Valley. To assess the efficacy of trees in using these saline-problem waters, both field and modeling studies are being carried out. The present paper presents the theory and validation of a dynamic 2D numerical model (D-HYSAM) to simulate root-water extraction in a variably saturated soil. The Galerkin-type finite-element method is used to solve the governing nonlinear-water-flow and salt-transport equations using three approaches for root-uptake sink mechanisms. Field data obtained from the 1989 growing season in an irrigated, tile-drained eucalyptus plantation was used for model validation. The simulated patterns on field-soil water pressure, volumetric soil moisture, and salinity suggest that the macro root-extraction approach for the sink term gives the best fit.