Agroforestry Drainage Management Model. I: 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.