RECHARGE IN THE ASSESSMENT OF GROUND-WATER RESOURCES: LINKING SCIENCE WITH A SOCIETAL NEED

In regions which rely on ground water for water supply, knowing recharge rates is a crucial part of determining ground-water budgets. The challenge to hydrogeologists is providing scientifically reasonable ground water information to planners in an understandable form and then getting it included in policy-making decisions. From a societal perspective, hydrogeologists need to be prepared to provide first-order estimates of recharge rates which: 1. are simultaneously conservative and reasonable, 2. can be made from readily available information, and 3. quantify the flux's non-uniformity and transience.

Recharge has been estimated in Wisconsin for use in a regional analysis of ground-water supplies and management options. Baseflow measured in low order streams has been used as a surrogate measure for recharge within the watershed. After normalization to precipitation, it has been correlated to topographic, geologic and land cover properties. The resulting relationship has then been used to calculate recharge in small watersheds across a region of over 6000 km². The values obtained show strong agreement with other estimation methods, which are sensitive at different scales: well hydrographs (<5 km²), distributed flow models ( 10 to 100 km²), and baseflow separation in watersheds from 100 to 500 km². Used as input to ground-water flow models at the community, county and regional scale, the recharge rates have substantially improved the calibration statistics.

The regional model is the basis for regional resource planning. Within this region of multiple, adjoining ground-water users, recharge is viewed as the primary sustainable supply available to any area. It is the only source over which occupants of an area have control. The complexities of ground-water budgets have been simplified into dimensionless ratios for presentation to planners and the public. Measures of the reduction of baseflow discharge to streams and pumping demand/recharge are the most universally understood and are being used to assess the viability of future management strategies. The next step will be getting policies in place to incorporate the water supply information. Success has been achieved at the community scale, but thusfar not beyond it.