Projecting the long-term effectiveness of groundwater conservation initiatives: A western Kansas case study

Abstract:

Regional aquifers that are essential for providing water for agricultural production and public consumption are being over-pumped, resulting in unsustainable decline of water tables, degradation of water quality, and loss of biodiversity and habitat. In an effort to mitigate these declines, groundwater conservation initiatives such as the Local Enhanced Management Area (LEMA) program in Kansas have been enacted. The first approved LEMA has done well; water table decline slowed and pumping volume reduced by 27% over the initial five-year (2013-2018) period. This initial success led to the approval of the LEMA for an additional five years, and the formation of a much larger LEMA encompassing northwest Kansas. Although early results are promising, it is imperative to evaluate how changes to the local water balance resulting from groundwater conservation may impact the long-term effectiveness of these initiatives. To investigate this, we develop a variably-saturated groundwater flow model to project rates of water table change to the year 2100 under a variety of scenarios. Reductions in pumping manifest as a decrease in irrigation water applied in excess of crop water needs, which reduces deep percolation and lessens groundwater recharge over years to decades due to a thick unsaturated zone. Thus, the impacts of pumping reductions on water levels are initially sizable, but the benefits of groundwater conservation initiatives diminish over time as the aquifer equilibrates with the recharge produced by the new pumping regime. The timescale to equilibration dictates the apparent effectiveness of these groundwater conservation initiatives. Here, we estimate the equilibration timescale under realistic conditions of the LEMA, and use a Latin hypercube sampling scheme to evaluate the sensitivity to several parameters of interest. Foremost, our results indicate that reductions in pumping can be an effective method for prolonging the usable lifetime of aquifers in heavily stressed areas. The interplay between vertical and horizontal flows ultimately dictates the time to equilibration of the system. These results also indicate that there can be large differences between the impact of groundwater conservation initiatives in the short and the long term, particularly where there is a thick unsaturated zone.