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

The Frenchman Creek Basin in semi-arid southwestern Nebraska is an area dominated by agriculture where water levels in the aquifer have declined significantly since predevelopment. Analysis of streamflow data at several gages along Frenchman Creek indicates a streamflow reduction since the 1950s. The cause of this reduction is a combination of groundwater abstraction for irrigation, terracing, and other land use changes. However, the relative influence for each factor is not well quantified. The objective of this study is to develop a physically-based analytical model that characterizes the effects of pumping, terracing, and other land use changes on streamflow of Frenchman Creek from predevelopment to today. The model utilizes data on predevelopment streamflow in the 1930s and accounts for stream depletion via pumping using existing analytical methods. Three coefficients characterize the effect of pumping, terracing, and other land use changes. The first coefficient denotes the fraction of water pumped that returns to the aquifer and compensates for changes in stream length over the pumping season. The second coefficient accounts for the change in recharge via terracing through addition or subtraction from predevelopment streamflow. The third coefficient accounts for the effect on recharge via other land use changes through addition or subtraction from predevelopment streamflow. The model output is a change in baseflow which constitutes the majority of streamflow. These results are compared with observed streamflow reductions, and recharge coefficients in the model for irrigated land, terracing, and other land uses are identified for a specific time period. This model can be utilized in other baseflow dominated watersheds for relatively rapid assessment of irrigation and land use effects on streamflow.