Quantifying groundwater resilience through conjunctive use for irrigated agriculture in a constrained aquifer system

The Rincon Valley in arid, south-central New Mexico, is especially impacted by reduced surface water supply because the contribution of groundwater is limited by aquifer constraints. Consecutive surface water allotment shortages in the Elephant Butte Irrigation District (EBID) have reduced recharge. The effects are compounded by farmers continuing to extract groundwater to meet crop requirements. Conjunctive use assumes aquifer resilience (i.e., ability to absorb pumping stress), but not necessarily in drought. This study further develops the water table fluctuation method by analyzing data from the EBID's groundwater monitoring program to reveal conjunctive use controls over the spatial and interannual variability of net storage changes from 2009 to 2016 in the Valley and introduces the term groundwater-surface water ratio of application (GSRA), that has potential for characterizing system resilience in conjunctive use settings. Regression modeling shows that variation in the annual EBID surface water allotment correlates strongly with year-end water table elevations, even more strongly than total annual groundwater extractions for irrigation, suggesting that variable surface water allotments are a primary driver of this system. Dewatering of the aquifer as of 2011 significantly altered the system hydrology such that from 2011 to 2016, net change in storage correlates strongly with the annual surface water allotment, corresponding to large river losses for the same period, but resulting in net gains in storage from 2014 to 2016. Rapid storage loss and rebound in this constrained aquifer system allowed quantification of aquifer resilience, enabling the development of a GSRA as a potential planning metric.