Seasonal water-level perturbations beneath the high plains of the Llano Estacado

Authors: Stout, John

Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/10/2019
Publication Date: 5/26/2018
Citation: Stout, J.E. 2018. Seasonal water-level perturbations beneath the high plains of the Llano Estacado. Journal of Hydrology. 18:1-14. https://doi.org/10.1016/j.ejrh.2018.04.009.
DOI: https://doi.org/10.1016/j.ejrh.2018.04.009

Interpretive Summary: Beneath the Llano Estacado of western Texas and eastern New Mexico, a vast elevated plain located at the southern end of the Great Plains of North America, lies the Ogallala Aquifer, which supplies groundwater for a highly productive and profitable irrigated agricultural system. In Texas, land owners operate under the rule of capture, which grants landowners the right to capture groundwater beneath their property, which has led to a significant depletion of groundwater resources. In general, water levels decline during the growing season and gradually recover as irrigation wells are switched off at the end of each growing season. However, this process of drawn down and recovery are not fully understand. In an attempt to better understand this process, ARS scientist from Lubbock, Texas developed a theoretical framework to compute the average or changes in water table using data collected at a given location over periods of many years. The resulting curves can be interpreted and compared to provide insight into seasonal water availability patterns.

Technical Abstract: The Llano Estacado of North America is a vast elevated plain favorably situated above a large portion of the Ogallala Aquifer. The Ogallala Aquifer provides an essential source of freshwater for a highly productive irrigated agricultural system. Each year, groundwater is pumped from the aquifer to the surface to irrigate crops during the growing season, which gradually reduces the remaining volume of stored water in the aquifer. Recorder wells, maintained by the Texas Water Development Board, routinely track these seasonal and long-term changes and provide a wealth of hydrologic data. Here, an attempt has been made to closely examine data collected by three recorder wells located at three locations across the Llano Estacado. Of special interest here are seasonal water-level perturbations that are induced by overlapping cones of depression that extend outward from nearby actively pumped wells. Water levels are observed to generally decline during the growing season and gradually recover as irrigation wells are switched off at the end of each growing season. In an attempt to better quantify this process, a method was developed to compute the average or normal perturbation of the water table. Results suggest that perturbation curves computed at all three locations tend to follow a similar overall pattern of high water levels during the winter and early spring and low levels during the summer growing season. In areas with limited groundwater, farmers may try to conserve water by shutting off irrigation systems during periods when soil moisture is adequate or when the supply of available water becomes critically low. As a result, one can often detect hydrographic aberrations associated with irrigation breaks when irrigation systems are shut down and the water table is allowed time to partially recover. Such hydrographic deviations appear to correlate closely with periods of plentiful rainfall, which typically occurs in late May and early June on the Llano Estacado. In areas where groundwater supplies are more plentiful, these late-spring irrigation breaks are less evident. This suggests that farmers with adequate groundwater may be less inclined to shutdown irrigation systems to conserve water despite adequate rainfall or it may indicate that farmers are growing water-intensive crops that may require more continuous irrigation. Further analysis of the derived perturbation curves could provide additional insight into the dynamic nature of seasonal withdrawals of groundwater from the Ogallala Aquifer.