NEW PIECEWISE-CONTINUOUS HYDRAULIC FUNCTIONS FOR MODELING PREFERENTIAL FLOW IN AN INTERMITTENT-FLOOD-IRRIGATED FIELD

Modeling water flow in macroporous field soils near saturation has bee n a major challenge in vadose zone hydrology. Using in situ and labora tory measurements, we developed new piecewise-continuous soil water re tention and hydraulic conductivity functions to describe preferential flow in tile drains under a flood-irrigated agricultural field in Las Nutrias, New Mexico. After incorporation into a two-dimensional numeri cal flow code, CHAIN 2D, the performance of the new piecewise-continuo us hydraulic functions was compared with that of the unimodal van Genu chten-Mualem model and with measured tile-flow data at the field site during a number of irrigation events. Model parameters were collected/ estimated by site characterization (e.g., soil texture, surface/subsur face saturated/unsaturated soil hydraulic property measurements), as w ell as by local and regional-scale hydrologic monitoring (including th e use of groundwater monitoring wells, piezometers, and different surf ace-irrigation and subsurface-drainage measurement systems). Compariso n of numerical simulation results with the observed tile flow indicate d that the new piecewise-continuous hydraulic functions generally pred icted preferential flow in the tile drain reasonably well following al l irrigation events at the field site. Also, the new bimodal soil wate r retention and hydraulic conductivity functions performed better than the unimodal van Genuchten-Mualem functions in terms of describing th e observed flow regime at the held site.