The Perham study area includes about 350 square miles of surficial deposits of glacial outwash in the central part of Otter Tail County in west-central Minnesota. The aquifer characteristics have a wide range, as follows: Transmissivity values range from nearly 0 along the perimeter of the area to more than 100,000 gallons per day per foot in the central parts of the area; storage coefficient values range from 0.1 to 0.2; and the saturated thickness of the upper outwash material ranges from nearly 0 to more than 100 feet. Most of the aquifer material is fairly well sorted and is in the particle-size range of fine to coarse sand.
Wells penetrating the full thickness of the aquifer and developed to 100 percent efficiency can be expected to yield 1,200 gallons per minute for 30 days and to have drawdowns of less than two-thirds the aquifer thickness in much of the area; however, well yields vary widely within short distances. Yields of 300 gallons per minute or less can be expected from wells drilled near the edges of the area and in the general area east and southeast of Otter Tail Lake.
Results from the mathematical analyses show that the amount of streamflow leaving the area will not be depleted within the 10-year analysis period, if not more than 6 inches of water per year is used on all the irrigable land in the outwash area. If pumpage and its effects on the streams is assumed to be prorated proportionately along the full length of the streams within the area, then the levels of lakes along these streams generally will not be lowered appreciably. However, owing to the heterogeneity of the aquifer and the other variable factors involved, some reaches of the streams may cease to flow when full ground-water development is approached, which, in turn, would result in a decline in some lake levels. Lakes and ponds not connected to streams in the area are expected to be lowered considerably or to be dried up completely as pumping becomes more extensive in years to come.
If irrigation wells and other large-yield wells in the study area are spaced 1 mile or more away from streams and lakes, the effect of ground-water pumping on the streams will be small, and the lake levels will be affected very little. However, the lakes and ponds are expected to approach normal levels during periods of above-normal precipitation and during periods of no pumping.
