SPATIAL AND TEMPORAL VARIABILITY OF GROUND-WATER RECHARGE IN MINNESOTA USING MULTIPLE METHODS

Local-scale estimates of ground-water recharge provide important information about recharge processes and factors, but commonly are not representative of regional rates. Regional estimates give broad perspectives of large-scale processes, but provide little insight about the recharge process or about factors that influence their variability. This study compares regional and local-scale estimates of recharge to unconfined sand and gravel aquifers in Minnesota. Four methods are being used: (1) water-table fluctuations (our approaches include using a computer program that calculates daily rise in water level, graphical, and a master recession curve), (2) age-dating of ground water, (3) unsaturated-zone water balance (utilizing soil moisture data), and (4) stream hydrograph separation, referred to in this study as the regional regression recharge (RRR) method. The first three methods yield point estimates of recharge, which represent spatial scales from about one to as much as one thousand square meters. The fourth method yields regional estimates at a scale of hundreds to thousands of square kilometers and, coupled with soils data from the STATSGO soils database, is used to produce a recharge map of the entire state of Minnesota. Recharge rates using the various methods are highly variable across the state ranging from less than 5 cm/yr in the northwest to greater than 20 cm/yr in sand-plain areas. The RRR recharge rates based on stream hydrograph separation correlate positively with precipitation rates, values of aquifer specific yield, and percentage of lakes in a given basin. At research sites, where data collection is intensive, the various recharge estimation methods compare quite favorably. Recharge rates based on the various methods generally represent 16-35 percent of average annual precipitation. Comparison of results gives improved understanding of the advantages and limitations of each method for recharge estimation.