Spatial Dynamic Optimization of Groundwater Use with Ecological Standards for Instream Flow

We examine how variations in the spatial distribution and timing of groundwater management actions, as well as the physical characteristics of a stream aquifer system, affect regulators' ability to meet instream goals in a stream-aquifer system. We propose an optimization model that distributes restrictions on groundwater pumping among a set of irrigators in a way that minimizes costs, subject to meeting instream flow requirements. Our model is distinguished by two features: (1) instream flow requirements must be met daily and (2) we incorporate a hydrologic model of stream-aquifer interaction that allows the time lags of the stream depletion impact that vary across space. Our results show that there is a tradeoff between the daily magnitude and duration of the stream-depletion effect: more distant wells have a smaller, but longer-lived, impact on streamflow. We find that there exist important cases where in drought years where wells located closer to the stream should be allocated more water than wells farther from the stream, despite having a smaller overall effect on stream depletion. Our results show the importance of considering the lag between the time at which pumping occurs and the time at which stream depletion related to that pumping occurs.