CLIMATE CHANGE IMPACTS ON RECHARGE IN A SEMI-ARID IRRIGATED WATERSHED

Agricultural regions in semi-arid regions of the United States rely on irrigation to satisfy crop water demands. Irrigation methods used in the region commonly allow about half the applied water to percolate below the root zone. This recharges shallow aquifers and supplies water to nearby streams as return flow. For a study area in northeast Colorado, temperature and precipitation changes that have been predicted by global general circulation modcls (GCMs) will result in an increase in potential evapotranspiration and a shift in peak streamflow of one to two months earlier in the growing season. For both wet and dry end-member GCM results for the region, crop water supply-and-demand ratios decrease during the growing season. A Modflow model of the study area showed resulting decreases in recharge and thus return flow, leading to significant declines in downstream water available for irrigation and agricultural productivity. KEY TERMS: climate change; evapotranspiration; recharge; return flow.