Thus streamflow volume is usually expressed in acre-feet or in inches of runoff. One inch of runoff represents the volume of water required to cover the entire drainage area of a stream to a depth of one inch. Because of high variabilities of streamflows, many municipalities and industries in the eastern third of Kansas depend on water stored in major reservoirs to meet current and future needs. Without the benefit of impoundments, flows in all but the largest Kansas streams would be almost zero for 30-day periods during moderate drought conditions.
Surface water is distributed unevenly across Kansas mainly because of the state's climate.
With few exceptions, western Kansas has little surface water. Ground water is the principal source of freshwater in most of this area. Average annual rainfall ranges from 15 to 18 inches per year in far western Kansas to more than 40 inches per year in southeastern Kansas. Although this climatically controlled rainfall variation is significant, average annual runoff across the state varies much more than the precipitation. Measured streamflow entering Kansas averages 1.7 million acre-feet annually.