Making sense of the interactions between groundwater and streamflow: lessons for water masters and adjudicators

The effects of groundwater pumping on streamflow are hydrologic issues that often are poorly understood in law. In some states, as illustrated with an Arizona example, different laws govern surface-and groundwater. In reality, however, surface-and groundwater form one continuum and conflicts can arise when different parties use both. How groundwater pumping affects streams depends on the depth to groundwater and whether or not the stream bottom is covered with fine sediment or organic deposits that control seepage. Using the hydrologic concept of capture, we present the basis quantitative aspects of stream-aquifer interactions in four case examples. More quantitative refinements and regional aspects can be achieved with computer models that can closely simulate specific regional or basin-wide systems. Once the broader concepts are sufficiently understood, integrated water management schemes can be developed that best resolve conflicts between the users of surface-and groundwater while protecting third parties, such as public and environmental interests. An underlying principle of these schemes should be the balance between the deisre for good scientific results and the economic reality of securing such results. We recommend steps to achieve the balance.