Environmental Drivers of Streamflow Change in the Upper Rio Grande

The purpose of this study was to assess changes in streamflow in the Upper Rio Grande (URG) basin, as it exits the San Luis Valley (SLV) at the Lobatos gauge station, in relation to changes in local environmental drivers. Irrigation-dependent agriculture accounts for more than 85% of surface and ground water withdrawals in the SLV. Inflows of the Rio Grande and the Conejos and Los Pios rivers were aggregated into a single inflow into the SLV. Streamflow data were taken from gauges above all major diversions. Results of the analysis indicated annual streamflow at Lobatos declined by 400 hm(3) after 1924, coinciding with increases in surface water extractions. Additional reductions of about 50 hm(3) in annual streamflow, not accounted for by inflow reductions during the period 1925-1964, coincided with increases in groundwater extractions. In contrast, an increase of 12.5 hm(3) in annual streamflow occurred during 1965-2007. The increases coincided with several changes, but were primarily related to extreme peak flow years during the period 1985-1987 and increased water deliveries in compliance with the Rio Grande Compact.