Northwestern Kansas, an area of 8,050 square miles (21,000 square kilometres), is a flat to gently rolling plain that is dissected by the Smoky Hill and Republican Rivers. Loessial soils underlying the plain are ideal for cultivation.
The climate is semiarid with the mean annual precipitation ranging from 16 to 21 inches (41 to 53 centimetres). Precipitation occurring mainly as thunderstorms during the 6-month growing season is four to five times less than the potential evaporation.
Principal aquifers are the Dakota Formation, which yields a few gallons per minute of water to domestic and stock wells; the Ogallala Formation, which commonly yields 500 to 1,200 gallons per minute (32 to 76 litres per second) to irrigation wells; and the alluvium, which locally yields as much as 1,500 gallons per minute (95 litres per second) to irrigation wells.
Irrigation by ground water from wells has developed principally since the early 1950's. Development has increased from about 100 wells irrigating 10,000 acres (4,000 hectares) in 1950 to about 2,200 wells irrigating 300,000 acres (120,000 hectares) in 1972. The rate of withdrawal by irrigation, municipal, and industrial wells in 1972 was about 500,000 acre-feet (620 x 106 cubic metres) of water per year, of which about 99 percent was for irrigation.
A comparison of water levels measured in 1950 and in 1973 indicates that declines greater than 10 feet (3 metres) are common in much of the area. Declines greater than 30 feet (9 metres) have occurred near Goodland and Colby. The areas of greatest irrigation-well density correspond to the areas of greatest water-level decline.
More ground water could be withdrawn annually for irrigation, but additional water-level declines in some areas may make the wells uneconomical to pump. The ground-water reservoir can be managed to prolong the irrigation economy of the area.
