A generalized estimate of natural ground-water-recharge rates was developed by analysis of streamflow, precipitation, and basin-characteristics data. Streamflow data were analyzed to determine the ground-water-discharge component of gaged areas. Long-term precipitation data were used to adjust ground-water-discharge data to reflect long-term average recharge characteristics. Basin-characteristics data were used to aid in the interpolation of recharge characteristics within gaged and ungaged areas. The generalized estimate provides a consistent method for approximating recharge rates in the Lower Peninsula of Michigan.

Ground-water-discharge and surface-water-runoff components were determined by use of a hydrograph-separation-analysis technique known as streamflow partitioning. Data were analyzed from 114 selected basins (Table of Basin Recharge Rates) having a total of 3,456 station-years of daily streamflow record. Annual ground-water discharge ranged from 0.19 to 22.3 inches per year. The average ground-water discharge was 8.41 inches per year. The average annual percentage of streamflow identified as ground-water discharge ranged from 29.6 to 97.0 percent.

Basinwide average ground-water discharges were adjusted to provide consistent estimates of recharge from streamflow data collected during different intervals of time. A set of 114 dynamic regression equations relate annual precipitation to annual ground-water discharge in each basin. The equations explained from 18.6 to 75.8 percent of the variation in annual ground-water discharges among the selected basins. Normal basin recharge rates were computed by use of these equations and the normal precipitation for 1951-80 for each selected basin.

A multiple-regression equation was developed to estimate the spatial variation of natural recharge within the Lower Peninsula of Michigan. Selected explanatory variables include (1) latitude and longitude of the basin centroid, (2) surficial geologic material classifications of outwash sand and coarse-textured till, and (3) land-use classifications of deciduous forests and coniferous forests. The equation accounts for 71 percent of the variability of normal basin recharge rates.

Coefficients of the regression equation, which were computed by use of a generalized least-squares procedure, indicate that recharge generally increases from north to south and from east to west throughout the Lower Peninsula. This geographic variation is thought to be associated with climatic factors. The regression coefficients associated with surficial geologic materials classified as outwash sand or coarse-textured till and with the forest land-use classifications were positive. The positive coefficients are thought to be associated with infiltration capacities of soils associated with the selected surficial materials and land-use classifications. Maps showing the generalized estimate of recharge and the relative uncertainty of the generalized estimate were developed.