An appraisal of ground water for irrigation in the Wadena area, central Minnesota

Abstract

The Wadena area is part of a large sandy plain in central Minnesota whose soils have low water-holding capacity. Drought conditions which adversely affect plant growth frequently occur in the summer when moisture is most needed. To reduce the risk of crop failure in the area supplemental irrigation is on the increase. This study was made to evaluate the ground-water resources of the area and to determine possible effects of development on them. About half the area's approximately 102,000 acres is considered irrigable at the present time. In 1967, about 1,100 acres were under irrigation. Outwash sand and gravel, which forms the water-table aquifer, is the main source of water presently known. Saturated thickness ranges from 0 to 70 feet and averages about 36 feet. Sandy till underlies the outwash. Within the till are sand and gravel lenses whose distribution and water-yielding characteristics were not determined. Average annual precipitation at the U.S. Weather Bureau station in Wadena from 1934 to 1967 was 26.4 inches, of which about 22.5 inches was lost by evapotranspiration, and the balance of 3.9 inches was surface runoff. Even in wet years, evapotranspiration during the .summer months exceeds precipitation, and a moisture deficiency for optimum plant growth occurs. In 1967, about 8 inches of the total precipitation of 19.3 inches reached the water table. Recharge to the water table in 1967 was about 70,000 acre-feet. Result of field aquifer (pumping) tests were used to estimate transmissivity values at test-hole sites. Information gained by auger test drilling was the basis for estimating transmissivity values elsewhere. Transmissivity of the watertable aquifer in most of the Wadena area ranges from 15,000 to 120,000 gallons per day per foot. A map was prepared to show the maximum yield, in gallons per minute, which might be obtained from individual wells completed in the water-table aquifer. The map indicates that in about 60 percent of the area, individual wells can be pumped at rates greater than 300 gallons per minute for a 30-day period if drawdown in the pumped well is two-thirds the saturated thickness after correction for dewatering. Quality of both ground and surface waters is such that they are well suited for irrigation. Locally, nitrate concentrations in ground water, in excess of the U.S. Public Health Service's drinking water standards, might be related to a local source of organic pollution or to the increased use of fertilizers which accompanies irrigation. An electric analog model of the water-table aquifer in the Wadena area was built and used to analyze possible effects of ground-water development of the hydrologic system. The model was designed to .simulate existing hydrologic conditions and used to predict changes in the system which might result from development. The withdrawal of large quantities of ground water would lower the water table, thereby reducing evapotranspiration losses and making more water available for beneficial use. Additional water would be salvaged when normal ground-water discharge to streams is intercepted by pumping from wells. Analyses were made to determine effects of development on ground-water levels under different development schemes both after a single irrigation season and after 5 and 20 successive years of irrigation. Where development is concentrated, some interference between wells can be expected. Although water levels recover rapidly when pumps are shut off, recovery will not be complete prior to the next irrigation season in heavily developed areas. After several years of watertable lowering, yields from wells will decrease because of deceased saturated thickness, unless climatic changes result in abnormally high amounts of recharge.