ANALYTICAL MODELING OF IRRIGATION AND LAND USE EFFECTS ON STREAMFLOW IN SEMI-ARID CONDITIONS: FRENCHMAN CREEK, NE

With proliferation of various numerical models, water budget studies commonly resort to numerical techniques. However, availability and uncertainty in input data limit 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 and apply it to Frenchman Creek in southwestern Nebraska. Frenchman Creek has experienced large streamflow reductions since the 1960s. The cause of these reductions is a combination of irrigation, terrace construction, and other land use changes. However, the 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 these factors. Analytical stream depletion rate calculations show that Frenchman Creek is a major source of water for irrigation wells. Over the last 10 years, pumping from the 462 irrigation wells in the basin consumed most of the total groundwater discharge to the stream. Calculations show that currently, baseflow is largley supplied by returnflow. Stream depletion and canal diversions are the major components that cause streamflow reductions. Three coefficients serve as calibration parameters and quantify pumping, terracing, and land use change effects, respectively. The model output generates streamflow at the outlet of Frenchman Creek. Modeled results compare favorably with observed streamflow reductions at the outlet which indicates viability of analytical modeling for less studied watersheds.