Quantifying irrigation recharge sources using groundwater modeling

Abstract

Irrigated agriculture has created artificially high groundwater levels that are now considered the normal condition in some Montana (USA) valleys. Rural housing developments that use wells completed in these shallow aquifers depend upon the annual recharge from irrigation practices and infrastructure. However, water conservation efforts aiming to reduce the volume of irrigation water diverted from streams, by either converting fields from flood irrigation to sprinkler irrigation or lining irrigation canals, may reduce recharge to the underlying alluvial aquifers. For resource managers looking to both preserve the groundwater resource for domestic well use and reduce the volume of water diverted from rivers, knowing the primary source of groundwater recharge is key to choosing the most appropriate water-saving method.To address this question, a groundwater model of an irrigated valley was constructed to quantify the relative importance of groundwater recharge from infiltration of flood-applied irrigation water versus leakage from irrigation canals. In contrast to using assumed infiltration rates based upon irrigation method efficiency, applied irrigation recharge rates were approximated through the model by matching the magnitude and timing of measured hydrographs.Model results suggest that 1. canal leakage is the primary irrigation-related recharge source in this flood-irrigated valley, and 2. assumed applied-irrigation recharge rates based upon irrigation method can greatly overestimate recharge. Flood irrigation’s low efficiency (a high percentage of applied water is not evapotranspired) may result in groundwater recharge studies that find applied irrigation is the more important groundwater recharge pathway. Our work shows that water budgets and groundwater models that use inputs based upon irrigation method efficiency may overstate the importance of irrigation application to groundwater recharge.At this site, irrigation-water conservation efforts that also aim to maintain domestic groundwater supplies should focus on improving the efficiency of irrigation method rather than reducing leakage from the canals.