Groundwater Recharge Response to Reduced Irrigation Pumping in Western Nebraska

Given current and continued investment in irrigation scheduling technologies, a need exists to better estimate the longevity and magnitude of water savings at watershed level to avoid the paradox of irrigation efficiency. This paradox occurs within a watershed as not all irrigation inefficiencies lead to the system losing water. For example, irrigation pumping rates in excess of crop water demand may lead to enhanced groundwater recharge or surface runoff that migrates to a stream. Thus, increases in efficiency may not lead to similar magnitudes of water savings. I hypothesize that water savings longevities are short given previous work demonstrating rapid responses of groundwater recharge rates to changing surface conditions. To test this hypothesis, I used numerical modeling and hydrogeological field techniques. This work provides localized ranges of: weather, management, soil variability, depth to groundwater, and water fluxes. In chapter two, utilizing a crop modeling and numerical modeling of soil moisture redistribution, I found that irrigation practices within the study area could be reduced by 120 mm yr–1 with impact on yield less than 3% when compared to a long-term dataset of irrigation pumping rates for ~50 fields within the study area. From work in chapter three, I found that sampling locations informed via repeat hydrogeophysical surveys, required only five cores to reduce the cross-validation root mean squared error by an average of 64% as compared to soil parameters predicted by a commonly used benchmark, SSURGO and ROSETTA. This work then informed an intermediate core sampling framework in chapter four to constrain how soil hydraulic fluxes vary on subfield scale. In chapter four, I compared deep drainage outputs of a numerical model parameterized with localized measurements to a chemical tracer analysis and find agreement within 80% despite a wide range of fluxes observed (135–515mm yr–1). Scenario testing informed using the parameterized numerical model and the irrigation reduction potential from chapter two indicated that a 120mm yr–1 reduction of pumping leads to modest water savings (1–3 years; 50–200mm over 10 years). However, when applied over a number of fields, similar irrigation efficiency programs may be competitive with other water resource management programs