HYDROLOGIC TRADEOFFS IN CONJUNCTIVE USE MANAGEMENT

An aquifer, in a stream/aquifer system, acts as a storage reservoir for groundwater. As wells in the aquifer are moved away from the stream, the aquifer acts to filter out annual fluctuations in pumping; with distance the stream depletion tends to become the average rate of pumping spread through the year, with only a small fluctuation. This is true for both a single well, and an ensemble of wells pumping from the/aquifer. A typical growing season in much of the western U.S. is three to four months. An ensemble of irrigation wells, spread more or less uniformly across an aquifer several miles or more wide, pumping during the growing season will deplete the stream by approximately one third of the total amount of water pumped during the growing season. The remaining stream depletion occurs outside the growing season. Furthermore, it takes more than a decade of pumping for an ensemble of wells to reach a steady state condition in which the impact on the stream is the same in succeeding years. After a decade or more of pumping, the depletion is nearly constant through the year, with only a small seasonal fluctuation: +/- 10%. Conversely, the impacts on a stream of shutting down the pumping from an ensemble of wells take more than a decade for the stream depletion to fully recover from the impacts of prior pumping. Managing a conjunctive groundwater and surface water system requires integrating the entire system into a single management institution