SPATIAL VARIATION IN INFLOW TO THE HIGH PLAINS AQUIFER IN KANSAS DERIVED FROM A NEW WATER-BALANCE APPROACH

Inflow for stable groundwater levels (zero water-level change) has been estimated for the near term (one to two decades) in the High Plains aquifer (HPA) of Kansas using a recently developed water-balance method (Butler et al., GRL, 2016). The method is based on the relationship between average annual water-level change and annual water use for an area. Inflows have been determined for different areal scales, from regions (groundwater management districts [GMDs], thousands of km²), to counties, and to circles around individual monitoring wells (a few to several km²). Average inflows for the three GMDs in the Ogallala region of the HPA in western Kansas (semi-arid climate) range from 3.1 cm/yr to 7.8 cm/yr based on 2005-2017 data. Inflows for the two GMDs in the Quaternary region of south-central Kansas (sub-humid climate) range from 6.1 cm/yr to 6.3 cm/yr for the same period. Inflows in counties and around wells are either in the general range of the GMDs in which they occur or larger where water use density is greater. These inflow estimates are greater than predicted in existing numerical groundwater flow models. The reasons are that either the estimated components of the inflows in the models are too small or that they do not include all of the components. These components are precipitation recharge, irrigation return flow, enhanced precipitation recharge over irrigated areas, focused recharge over ephemeral stream valleys and playas, drainage from low-permeability units in the newly unsaturated zone, capture of groundwater discharge to streams, lateral groundwater inflow, and vertical groundwater flow from underlying bedrock aquifers where present.