PREFERENTIAL TRANSPORT OF NITRATE TO A TILE DRAIN IN AN INTERMITTENT-FLOOD-IRRIGATED FIELD - MODEL DEVELOPMENT AND EXPERIMENTAL EVALUATION

A comprehensive field experiment was conducted near Las Nutrias, New M exico, to study field-scale how and transport in the vadose zone. The field data were analyzed in terms of a two-dimensional numerical model based on the Richards equation for variably saturated water flow, con vection-dispersion equations with first-order chemical decay chains fo r solute transport, and bimodal piecewise-continuous unsaturated hydra ulic functions to account for preferential flow of water and nitrate-n itrogen (NO3-N; loosely used as NO3-) following flood irrigation event s at the experimental site. The model was tested against measured NO3- flux concentrations in a subsurface tile drain, several monitoring we lls and nested piezometers, and against resident NO3- concentrations i n the soil profile (obtained at 52 spatial locations and four depths a long a transect). NO3- transport at the field site could be described better with the bimodal hydraulic functions than using the conventiona l approach with unimodal van Genuchten-Mualem type hydraulic functions . Average resident nitrate concentrations measured across the soil pro file were predicted reasonably well. However, NO3- flux concentrations in the subsurface tile drain and piezometers at the field site were o ccasionally underestimated or overestimated depending upon the irrigat ion sequence in three field benches, probably reflecting unrepresented three-dimensional regional flow/transport processes. Limiting the cap ture zone to a region closer to the tile drain did lead to a better ma tch with observed sharp increases and decreases in predicted NO3- flux concentrations during the irrigation; events. On the basis of this re sult we inferred that the preferential flow intercepted by the tile dr ain was generated in close proximity of the drain and essentially orie nted vertically. In summary, our study suggests that irrigation schedu ling in adjacent field plots, drainage design (e.g., spacing between t iles, drain depth, drain diameter) and effectiveness (e.g., drain bloc kage), preferential flow in (horizontal) surface-opened Shallow cracks and (vertical) macropores, and transient regional groundwater flow ca n add significant uncertainty to the predictions of (local-scale) flow and transport to a tile drain.