The upper Black Earth Creek drainage basin has an area of 46 square miles and is in Dane County in south-central Wisconsin. The oldest rock exposed in the valley walls is the sandstone of Late Cambrian age. Dolomite of the Prairie du Chien Group of Ordovician age overlies the sandstone and forms the. resistant cap on the hills. The St. Peter Sandstone, Platteville and Decorah Formations, and Galena Dolomite, all Ordovician in age, form a narrow belt along the southern boundary of the area. Outwash and alluvium of Pleistocene and Recent age fill the valleys. The eastern half of the area was glaciated and is covered with till. The sandstone of Late Cambrian age and the sand and gravel of the outwash deposits are hydraulically connected. Ground water occurs under unconfined (water-table) conditions in the western unglaciated part of the basin and under artesian conditions beneath the till locally in the eastern part. The source of most of the ground water is direct infiltration of precipitation; however, some ground water enters the area as underflow from the south. About 7 inches of the 30 inches of average annual precipitation recharges the ground-water reservoir. The ground water generally moves toward Black Earth Creek where it is discharged. Some ground water moves out of the basin as underflow beneath the valley of Black Earth Creek, and some is discharged by evapotranspiration or is withdrawn by pumping from wells. Water levels in shallow nonartesian wells respond rapidly to precipitation. The effect of precipitation on water levels in artesian wells is slower and more subdued. Water levels are generally highest in spring and lowest in fall and winter. The flow of upper Black Earth Creek is derived mostly from ground-water discharge, except during short periods of and immediately after precipitation when most of the flow is derived from surface runoff. The runoff from upper Black Earth Creek basin decreased from an average of 8.72 inches per square mile of drainage area in 1955 to 5.55 inches in 1958; the decrease reflects the generally decreasing precipitation and declining water levels in the basin during that period. On July 10, 1958, the discharge from the basin was 0.367 cubic feet per second per square mile, and the greatest discharge was 0.84 cubic feet per second per square mile from the southwest subbasin. The ground water has an average temperature of about 50°F. It is a calcium magnesium bicarbonate type water and is very hard.
