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

Abstract

Study region: The Llano Estacado is a vast elevated plain located at the southern end of the Great Plains of North America. The Llano Estacado region is favorably situated above the southernmost extension of the Ogallala Aquifer, which provides groundwater for a highly productive irrigated agricultural system. Study focus: A method was developed to compute the average deviation of the groundwater level from a nonstationary annual-average water level. New hydrological insights for the region: Groundwater is pumped from the Ogallala Aquifer during each growing season, which induces seasonal and long-term changes in observed groundwater levels. Recorder wells, maintained by the Texas Water Development Board, track these changes and provide a wealth of hydrologic data. Of special interest here are seasonal water-level perturbations induced by nearby actively pumped wells. Groundwater levels are observed to follow a regular pattern of declining water levels during the growing season followed by a recovery after irrigation systems are shut down. In areas with limited groundwater, farmers may shut off irrigation systems when soil moisture is adequate or when the supply of available water becomes critically low. As a result, one can often detect periods during the growing season when irrigation is paused and the water table is allowed time to partially recover. Such hydrographic deviations appear to correlate closely with periods of abundant rainfall. In areas where groundwater supplies are more plentiful, 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. Keywords: Groundwater, Depth to water, Water table, Saturated thickness, Ogallala Aquifer