IMPACTS OF LOCAL HYDROGEOLOGY AND CLIMATE ON WATER LEVELS IN THE HIGH PLAINS AQUIFER IN KANSAS

Groundwater levels have been generally declining across the High Plains aquifer in western Kansas (Ogallala HPA) for many decades. Although water levels measured in most wells respond in a largely similar manner, spatial variations in local hydrography, hydrogeology, and climate can cause significantly different responses in some wells. Upland field studies have typically found recharge to the Ogallala HPA, where depths to water are usually >30 m, to be very low (<1 cm/yr). However, net inflow estimates based on the water balance approach of Butler et al. (GRL-2016) indicate that regional recharge must be higher than these estimates. A major reason is likely focused recharge in features such as ephemeral stream valleys and playas. The Index Well Program of the Kansas Geological Survey has revealed the heterogeneity of water-level responses in the HPA. Examples of focused recharge to the Ogallala HPA observed by index wells are a small closed basin at the end of an ephemeral stream valley and a small low-head dam trapping water in an ephemeral drainage after extended wet periods. During recent wet years, annual water levels have significantly risen in these areas while levels elsewhere in the Ogallala HPA have either remained relatively steady or slowly declined.

Spatial variability in climate during a year can be appreciable across the Ogallala HPA. Highly localized rainstorms can cause differences in pumping for irrigation and thus, groundwater levels. However, an even greater impact was recently observed as the result of a hailstorm that severely damaged irrigated crops such that irrigation was discontinued. A substantial rise in the groundwater level monitored by an index well in the affected area demonstrated the impact of this hail-induced pumping reduction. The annual water use versus radar precipitation correlation, which is highly statistically significant for the area around the well, shows the hailstorm year as an anomaly. However, the annual water-level change and water use correlation, also statistically significant, indicates that the water-level change for that year responded in a linear fashion as all other years, i.e. the net inflow was essentially the same as that for the previous decade. Such heterogeneity in water-level responses can provide important insights about aquifer conditions and processes.