PREDICTING IRRIGATION PUMPING REDUCTIONS TO ACHIEVE SUSTAINABILITY AT DIFFERENT SCALES IN THE HIGH PLAINS AQUIFER, KANSAS

A recently developed water-balance method (Butler et al., GRL, 2016) has been applied to estimate the reductions in irrigation pumping needed to achieve sustainability (zero water-level change) for the near term in the High Plains aquifer (HPA) of Kansas. The method is based on the relationship between average annual water-level change and water use for a particular area. Application of the approach to different areal scales indicates that pumping reductions are generally consistent for similar water-use densities in the same aquifer region but differ substantially across regions depending on the climate. Scales examined range from the largest groundwater management district (GMD) in Kansas (21,600 km²) to circles around individual monitoring wells (down to 2.6 km²). Pumping reductions required for stable water levels (based on 2005–2016 data) for the three GMDs in the Ogallala region of the HPA in western Kansas (semi-arid climate) range from 27% to 33%. Pumping reductions needed in the two GMDs in the Quaternary region of south-central Kansas (sub-humid climate) range from <1% to 2%. These low values are dependent on very wet years; the reductions increase a few to several percent if one or two wet years are excluded. Pumping reductions for sustainability generally decrease for county areas from west to east across the HPA, concomitant with the eastward increase in average rainfall. In subareas of greater use density within counties, the pumping reductions for stable water levels can exceed 40% for the Ogallala region; reductions for subareas in the easternmost GMD in the Quaternary region can exceed 15% where low permeability sediments limit recharge and stream-aquifer interactions. Pumping reductions needed for stable water levels based on water-level change at individual monitoring wells and water-use within circles around the wells (radii 1.6–8 km) are in the same general range as for the counties and GMD areas within which they are located, if the water-use densities are similar. Our conclusion is that practically feasible reductions can significantly impact decline rates across the Kansas HPA at all scales. These pumping reduction estimates are smaller than predicted from existing numerical models primarily because those models assumed higher specific yields than obtained from the water-balance approach.