Aquifer management zones, based on simulated surface-water response functions

Conjunctive management of surface and ground water requires an understanding of the degree to which surface water resources will be impacted by aquifer stresses at different locations. Response functions quantify the rate of depletion or accretion of a surface-water body relative to a unit stress on an aquifer under simplified, but often realistic, conditions. Response functions for four reaches of the Snake River have been determined for each cell of the eastern Snake River Plain aquifer model grid in southern Idaho. Mapping the response functions for each reach creates a visual image of how effects of aquifer stresses at different locations are distributed among the four hydraulically connected river reaches. The mapped distribution of response functions reflects aquifer properties such as the distribution of aquifer transmissivity. Cluster analysis of the response functions for each cell and each river reach has proven useful for subdividing the aquifer into zones for the conjunctive management of ground water and surface water. The response function variance within each zone is minimized in this procedure. In most situations, zone boundaries defined through cluster analysis will likely be modified to better conform to existing political and administrative units. The selection of the number of zones, and the degree to which boundaries based on response functions are modified, reflect a balance between the exactness provided by the science and administrative convenience and efficiency. The approach of managing an aquifer through response function based zones has gained initial acceptance in Idaho, as evidenced by the citation of these zones in Idaho's draft Water Management Rules. How management zones based on response functions will ultimately be used in Idaho water policy and procedures has not yet been determined.