IDENTIFYING FLOW BARRIERS AND SUBSURFACE FLOW PATHS AFFECTING RETURN FLOW FROM FLOOD IRRIGATION USING TIME-LAPSE BOREHOLE NMR AND ERT

摘要

Flood irrigation is one of the most common irrigation techniques in the Intermountain West, accounting for 42 percent of all of the irrigated land in the US in 2006. With water resources available for irrigation under threat of overuse, sustainable management of these resources is important. An important factor when managing water resources is quantifying the amount of applied water that returns back to the stream - return flow - and the timing at which this occurs. Current practice is to assume return flow estimates as a straight percentage of applied irrigation. Local soils and subsurface flow dynamics are rarely taken into account. Breakthrough curves, staining techniques and soil structure models have been used to predict infiltration rates and the effect of preferential flow paths during irrigation in the field; however, these conventional techniques lack spatial or temporal resolution to address the detail with which infiltration through preferential flow paths happen. We monitored the subsurface using time-lapse ERT, in combination with time-lapse borehole NMR and soil moisture measurements under wetting and drying experiments at field scale, following regular field irrigation schedules. Combining these techniques we are able to track spatial and temporal heterogeneous changes in soil moisture down to 10 m. We observed subsurface processes related to heterogeneity and preferential flow paths cause irregular wetting of the subsurface both deeper below the surface and at faster rates than would be predicted under assumption of homogeneous piston flow. The application of borehole NMR during irrigation field experiments, merged with more temporal dense time-lapse resistivity observations at a larger spatial extent, enables direct quantification of dynamic redistribution of soil moisture in the vadose zone at field scale. Incorporation of this knowledge into larger scale irrigation analysis will improve accuracy of predictions of water use dynamics and return flow under flood irrigation.