Preferential transport of nitrate to a tile drain in an intermittent-flood-irrigated field: Model development and experimental evaluation
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abstract
A comprehensive field experiment was conducted near Las Nutrias, New Mexico, to study fieldscale flow and transport in the vadose zone. The field data were analyzed in terms of a twodimensional numerical model based on the Richards equation for variably saturated water flow, convectiondispersion equations with firstorder chemical decay chains for solute transport, and bimodal piecewisecontinuous unsaturated hydraulic functions to account for preferential flow of water and nitratenitrogen (NO3N; loosely used as NO3) following flood irrigation events at the experimental site. The model was tested against measured NO3 flux concentrations in a subsurface tile drain, several monitoring wells and nested piezometers, and against resident NO3 concentrations in the soil profile (obtained at 52 spatial locations and four depths along a transect). NO3 transport at the field site could be described better with the bimodal hydraulic functions than using the conventional approach with unimodal van GenuchtenMualem type hydraulic functions. Average resident nitrate concentrations measured across the soil profile were predicted reasonably well. However, NO3 flux concentrations in the subsurface tile drain and piezometers at the field site were occasionally underestimated or overestimated depending upon the irrigation sequence in three field benches, probably reflecting unrepresented threedimensional regional flow/transport processes. Limiting the capture zone to a region closer to the tile drain did lead to a better match with observed sharp increases and decreases in predicted NO3 flux concentrations during the irrigation events. On the basis of this result we inferred that the preferential flow intercepted by the tile drain was generated in close proximity of the drain and essentially oriented vertically. In summary, our study suggests that irrigation scheduling in adjacent field plots, drainage design (e.g., spacing between tiles, drain depth, drain diameter) and effectiveness (e.g., drain blockage), preferential flow in (horizontal) surfaceopened shallow cracks and (vertical) macropores, and transient regional groundwater flow can add significant uncertainty to the predictions of (localscale) flow and transport to a tile drain.
