CONCEPTUAL MODEL OF IRRIGATION AND LAND USE CHANGE EFFECTS ON STREAMFLOW IN SEMI-ARID CONDITIONS

With proliferation of various numerical models, most water budget studies resort to numerical techniques. However, availability and uncertainty in input data limits advantages of this approach. Often, analytical models capture the major traits of the watersheds and can assimilate important data. We develop a model for baseflow dominated watersheds for relatively rapid assessment of irrigation and land use effects on streamflow and apply it to the Frenchman Creek Basin in semi-arid southwestern Nebraska. In this agriculture-dominated area water levels in the aquifer and streamflow data at several gages along Frenchman Creek declined since the 1950s. The cause of this reduction is a combination of groundwater abstraction for irrigation, terrace construction, and other land use changes. However, the relative influence of each factor has not been well quantified. The objective of this study is to develop a physically-based analytical model of streamflow changes on Frenchman Creek from 1941 to 2009, including the effects of pumping, terracing, and other land use changes. Analytical stream depletion calculations show as much as 60 percent of the pumped water originates from Frenchman Creek. Over the last 10 years, pumping from the 465 irrigation wells in the basin consumed 70 to 99 percent of the total predevelopment groundwater discharge to the stream. The model includes three coefficients that serve as calibration parameters and quantify pumping, terracing, land use change effects. The first parameter signifies the fraction of water pumped that returns to the aquifer. The second parameter accounts for the change in recharge via terracing through addition or subtraction from streamflow. The third parameter accounts for the effect on recharge via other land use changes through addition or subtraction from streamflow. Model output is streamflow at the outlet of Frenchman Creek. Recharge coefficients in the model for irrigated land, terracing, and other land uses are identified for specific time periods. Modeled results are favorably compared with observed streamflow reductions at the outlet.