Quantifying hydrologic fluxes in an irrigated region characterized by groundwater return flows

Abstract

In flood irrigation systems in which water is diverted from a river system, the return of recharge water to the river via groundwater discharge can play a key role in sustaining streamflow during irrigation and post-irrigation months. In this study, we use a combination of field data analysis and numerical hydrologic modeling to quantify the spatio-temporal hydrologic fluxes in a flood irrigated canal-field-aquifer-river system. To accomplish this objective, we develop a new irrigation package for MODFLOW that includes all major hydrologic features and fluxes: precipitation; canal diversions; irrigation type (sprinkler, drip, flood); runoff capture by downgradient canals; seepage from irrigation canals; and a soil water balance for each field, soil unit, and natural area that simulates crop ET and recharge. The model is applied to the White River Valley in the Meeker, Colorado (USA) region (180 km²), noted for extensive flood irrigation practices. From results, we conclude that of the water diverted from the White River for irrigation, approximately 75 % returns to the river. The 25 % irrigation efficiency is extremely low but, through extensive groundwater recharge, creates conditions conducive to groundwater return flow to the White River. The aquifer therefore acts as a slow-release reservoir of diverted river water to maintain streamflow and its ecosystem function during post-irrigation months. A holistic, basin-scale approach should be taken when considering conversion from flood irrigation to sprinkler irrigation, as benefits in conserving water at the farm scale likely will result in a decrease in groundwater return flows and therefore late season streamflow.