INVESTIGATING PREDICTED WATER RESERVES IN PORTIONS OF THE OGALLALA AQUIFER OF THE TEXAS PANHANDLE USING CONCEPTUAL MODELS
Green, Robert Glen
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The Ogallala aquifer is a vital source of water for the Texas High Plains. Irrigated crop production is primarily dependent upon water mined from the aquifer. State water policy requires establishing desired future conditions for groundwater management areas. In order to accurately estimate the water remaining in storage, groundwater models are used to monitor aquifer drawdown. Current models are based upon the Well Measurement Approach (WMA). This approach utilizes field measurements to determine the water surface elevation. Models are then constructed to match the surface elevations from year to year. Once calibration is complete, these models are then used to project future water remaining water levels. Observations of additional water in the aquifer using the Agronomic Water Mass Balance Approach (AWMBA) led to the hypothesis that additional water remains between wells after pumping. The additional water suggested by the AWMBA indicates that more water may be available than models based upon the WMA indicates. Further, this implies that new water surface elevations are not fully established when normal field measurements are taken. A conceptual model using two wells was developed to evaluate the output of the software. Two additional models using portions of both the Northern and Southern groundwater availability models were then studied to determine whether mounded water would be present between pumped wells. The wells used in the model were allowed to pump for 120 days per year and rebound for the remainder of the year. The models used in this study did not allow for recharge of the aquifer as this is not believed to be a viable source to explain the differences between the Well Measurement Approach and the Agronomic Water Mass Balance Approach. Results of the study indicate that mounded water does exist between pumped wells. This is due to the delay of the aquifer to return to a flat water table condition between pumping cycles. Complete well rebound takes as many as seven years. Well measurements do not allow for complete rebound and thus do not accurately measure the actual amount of water remaining in the aquifer. As a result of the proposed mounds in the Agronomic Mass Balance Approach, more water may be available in the Ogallala aquifer than previously reported. The scope of this study was not to provide an accurate account of the additional remaining water but the results do indicate further study along this line would provide a better estimate of the water remaining in storage.