THE PEDANTIC AND THE FUNDAMENTAL: REFINING THE QUANTIFICATION OF STREAM-DEPLETION TIMING FOR CONJUNCTIVE USE AND WATER-RIGHTS MANAGEMENT

For the conjunctive use of surface water and groundwater resources governed under the “prior appropriation” system, a central issue is quantifying the timing of transient stream impacts such as depletion from pumping wells and augmentation from managed recharge operations. Increasing water demand, policy shifts, and aquifer water-level changes have brought closer scrutiny to stream-depletion estimation methods used in common practice. Direct field measurement of stream impacts has practical limits at many relevant scales, so we rely on indirect tests evaluated and then applied through analytical and numerical models. This presentation will review observations from field testing and numerical modeling of stream depletion, and it will include recent work on the influence of regional-scale aquifer heterogeneity. It is observed that the appropriate value of “unit conductance” or “leakance” in the modeled stream boundary is influenced by the model’s horizontal and vertical discretization to a degree not commonly accounted for in practice. The influence of aquifer heterogeneity on depletion timing will be presented in terms of “effective transmissivity” values computed via a power mean. In numerical experiments, the effective power-mean values are observed to be intermediate of the classical means and to vary with the spatial correlation structure of the aquifer.