It has been found that the production of one pound of dry plant substance on soils of average fertility, requires in humid districts not more than five hundred pounds of water, and in arid districts like Utah about seven hundred and fifty pounds. This indicates that the average rainfall of Utah, which is about twelve inches, if properly conserved in the soil, is sufficient to produce annually, without irrigation, from thirty to forty-five bushels of wheat to the acre, or corresponding yields of other crops. The realization of this truth has changed greatly our views of irrigation practices. The beginning of irrigation wisdom is now considered to be the conservation of the natural precipitation, which means that soil must be used as a storage reservoir for water. If the farmer succeeds in bringing the larger portion of the rain and snowfall into the soil, and in keeping it there until crops need it, some success of the crop is assured. Irrigation under such conditions is, as it always should be, supplementary only to the natural precipitation, for the amount of irrigation water needed decreases as the amount of water stored in the soil increases. As the value of the natural precipitation becomes better understood among irrigation farmers, the present supply of irrigation water may be made to cover a more extended area. The progress of irrigation depends very largely upon the degree to which the farmers will care for the natural supply of water that falls upon their farms in the form of rain and snow.
Widtsoe, John A., "Bulletin No. 104 - The Storage of Winter Precipitation in Soils" (1908). UAES Bulletins. Paper 55.