Towards a comprehensive regional water policy model for the Texas High Plains
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In Texas, irrigation groundwater withdrawals account for 90% of all Southern Ogallaia aquifer withdrawals. The reliance on groundwater to satisfy water demand in the Texas High Plains (THP) is attributable to limited surface supplies and the relatively high cost of developing surface water storage facilities. Beginning in the 1940's and extending into the early 1980's, improved irrigation technology facilitated the conversion of dryland acreage to irrigated acreage in the THP that resulted in the rapid depletion of Southern Ogallala aquifer's groundwater reserves, the only major groundwater source for the region. By the early 1990's, declining groundwater reserves in the Southern Ogallala aquifer had become a statewide concern for Texas policy makers and contributed to recent conservation legislation. Recent Texas legislation (Senate Bills 1 and 2) explicitly recognize the growing scarcity of Texas's groundwater supplies and requires the state to develop a comprehensive statewide water plan that incorporates locally developed regional water plans. In this dissertation, an integrated regional water policy model was developed by linking a spatially disaggregated hydrology model with a dynamic optimization model for 19 THP counties. The baseline scenario was first developed that simulated, over a 50 year planning horizon, the distribution of aquifer declines, crop pattern changes, and the net present value agricultural producers receive under current management practices. The effectiveness of two conservation policies, a groundwater extraction tax and quota restriction were then examined in terms of economic cost and volume of groundwater conserved relative to the baseline. The effectiveness of groundwater conservation tax under two alternate crop market price scenarios was then examined. Finally, the economic and hydrologic impacts of non-homogeneous land use within a county were assessed. Results indicated that current water conservation policies being considered for the THP did not significantly inhibit producer agricultural groundwater use over a 50 year planning horizon. In 13 of the 19 study region counties, water use was not binding. Both the Optimal Tax policy and the Quota policy conserved approximately the same amount of groundwater, but the total regional cost of the Optimal Tax policy to production agriculture was 8.56 times larger than for the Quota policy. Under both the price scenarios, the optimal tax was ineffective in curbing groundwater use. At the regional level, for each acre foot of groundwater conserved, the Optimal Tax policy cost $333 compared to the Quota policy cost of $40. Further, in addition to water use variability, due to the significant variation in land use within a county, the current groundwater conservation policies were not effective within all parts of a county and policy costs varied greatly over a county.