QUANTITATIVE MAPPING OF RECHARGE/DISCHARGE FOR THE PLANNING OF GROUND-WATER SUSTAINABILITY IN MINNESOTA

The basis of this approach is to acquire the streamflow characteristics from stream discharge records, which are then associated by statistical analysis with the hydrologic landscape as watershed characteristics. These characteristics reflect deep symmetry in structural organization of patterns of climate, vegetation, geomorphology, geology and hydrology. The hydrologic landscape comprises the atmospheric, surface and subsurface portions of hydrologic system as linked freshwater reservoirs, and values of recharge/discharge are delineated into zones or regions of the hydrogeologic reservoir where they are constant. The derived recharge/discharge values are time-and space-independent, and are used for ground-water sustainability mapping. The investigation proceeds from a regional or basin level to a local level, with the greater detail available at the lower levels adding refinement to the description of the hydrologic landscape-hydrogeological unit.

The principal quantitative characteristic used in this process is the minimal monthly stream runoff or the minimal monthly base flow. The results of the method represent standardized, conservative values of minimal ground-water recharge/discharge that are associated with February low-flow characteristics and the areal extend of hierarchical hydrogeological units. Hierarchical hydrogeological units are derived from bedrock and Quaternary hydrogeologic maps based on permeability and porosity of material.

In Minnesota, the ground-water sustainability mapping was applied in three stages. First, the spatio-temporal structure of stream runoff as well as recharge and discharge rates were estimated for the entire state. As a further areal refinement, the results derived from water-resources analysis for the state were then used to quantify and map the recharge/discharge for East Central Minnesota (ECM), where rigorous quantitative analysis was done. Finally, the ground-water recharge/discharge map for the Minneapolis-St. Paul Metropolitan Area (TCMA), depicting minimal annual ground-water recharge, was compiled based on the recharge values determined for the ECM map.

The quantitative maps of this type could be used for direct comparison with water use data and for the planning of ground-water sustainability in Minnesota.