Modeling and evaluating marginal pumping fees in groundwater commons: do varying scarcity levels matter?

Price-based irrigation water-conservation policies are often designed as fixed per unit fees. In groundwater commons, however, this approach presupposes that irrigators assign the same value to each unit of water withdrawn, irrespective of the scarcity levels they individually face. This ignores spatial interdependencies in groundwater commons. In this paper, I examine the effect this possible tax structure misspecification has in measuring the performance of such Pigouvian taxes. I model the price of irrigation water as a non-constant marginal cost function dependent on the constant per unit fee and a variable cost-metric measure of scarcity, namely depth-to-water. Using a difference-in-difference econometric framework with irrigation data from San Luis Valley, results show that irrigators’ response to the constant marginal fee significantly depends on the scarcity levels individual irrigators face. More importantly, the results suggest that models that overlook the spatial element of scarcity would overestimate irrigators’ response to such pumping fee—which can misguide policy decisions.