The High Plains Aquifer (HPA) is a vital water resource for the agricultural production of the United States. Most of the aquifer is being used at an unsustainable rate, creating an irrigation economy that relies on resource exhaustion. In the very long term, much of the aquifer will not be usable for irrigated agriculture, but this leaves a number of questions: How quickly will the aquifer run dry? What will the aquifer’s senescence look like? What are the primary drivers of decline, and how are patterns of decline changing? What are the implications for land use change and management? This project uses two approaches to address these questions. The first is statistical: through spatial statistics and regression analysis, I investigate the patterns and relationships in the water table data. The second approach is process-based modeling, which uses the physical framework of the water cycle and the landscape to examine the distribution of water in the aquifer. These approaches are highly complementary. Statistical methods offer an empirical estimate of the water table in space and time. These methods suggest relationships between parameters used in modeling, such as the lag between precipitation and water table change, that might be difficult to incorporate a priori into a process-based model due to the difficulty in observing the relevant processes. Process-based models are grounded in the physics and geography of the system, and so they are able to accommodate scenarios that do not strictly follow the historical course of events. This counterfactual modeling is helpful in teasing apart the roles of the many processes that make up the water cycle, and in understanding the likely results of different management scenarios. Both process-based and statistical treatments demonstrate that the aquifer is going dry in many places. The Southern High Plains (SHP) region in Texas and New Mexico is at greatest risk. In the SHP, recharge is low, and groundwater is virtually the sole source for irrigation. The rate of groundwater decline is linked with the climate and with crop prices, but these factors only account for 25-40% of the total rate of depletion or recovery across the aquifer. In the SHP, conversion from dryland farming to perennial grassland, as through the Conservation Reserve Program (CRP), does not confer protection to the water table. Reductions in irrigation water can increase the lifespan of the aquifer. Model results indicate that in the most vulnerable part of the SHP, a 50% reduction in irrigation could have slowed the increase in the depleted area of the HPA by a factor of six between 2001 and 2014, from about 3.3% to about 0.6% of land area drying out in that timeframe. This holds whether the decrease is at the intensive margin (less irrigation per acre) or extensive margin (fewer acres irrigated). Meanwhile, with no irrigation during that 15-year period, the water table would have recovered by about 0.93 m, enough saturated thickness to provide water for about two irrigation seasons.
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