HYDROACOUSTIC AND HEAT TRACER APPROACHES TO QUANTIFY GROUNDWATER RECHARGE FROM SELECTED IRRIGATION CANALS IN WESTERN NEBRASKA

The North Platte Natural Resources District is developing an integrated management plan to balance groundwater and surface-water supply and demand. An optimization model is being constructed to help decision-makers manage groundwater and surface-water use such that mandated flows of the North Platte River can be maintained. Leakage from unlined irrigation canals is the primary source of groundwater recharge and sustains base flow in the North Platte River. Accurately quantifying and spatially distributing canal leakage to the groundwater system is critical to the success of the optimization model. The geology of the project area consists of Quaternary-age deposits of eolian sands and alluvium overlying the relatively impermeable siltstone of the Tertiary-age Brule Formation of the White River Group. Capacitively coupled resistivity profiles identified a potentially high leakage reach, where the Tri-State Canal cuts through a section of alluvial sand and gravel, and a potentially low leakage reach, where the Interstate Canal cuts through several outcrops of locally fractured Brule siltstone. Reach-scale leakage rates were calculated from streamflow loss determined from acoustic sidelookers and manual acoustic Doppler current profiler measurements at upstream and downstream stations of both reaches. Continuous temperature and water-level data were collected from one location within each reach and inputs for the numerical model, VS2DH, which fit simulated temperatures to observed vertical temperature profiles to estimate the vertical hydraulic conductivity of canal-bed sediments. Streamflow records and manual hydroacoustic measurements indicate intraseasonal variability in canal leakage for both reaches. The use of heat as a tracer provided point-scale vertical hydraulic conductivity estimates, however, low diurnal variability in canal water temperature (1 to 2ºC) made precise estimation difficult. As a result, intraseasonal variability in canal leakage was not distinguished using the heat-tracer approach. Comparisons between point and reach-scale leakage estimates suggest that variability within the selected reaches is largely caused by differences in canal-bed sediment texture, depth to groundwater, and depth to bedrock.