Eight crops commonly grown in southern Alberta were irrigated when available moisture within the root zone had been depleted to 75, 50, and 25 per cent levels. Over a 3-year period the yields of alfalfa, wheat, barley, sugar beets, corn, peas, and sweet clover reached a maximum at or below the 50 per cent level of minimum available soil moisture. Potatoes produced highest yields at the 75 per cent level. The relationship between availability of water to the crop and the amount of water in the soil was shown to be somewhere between the concepts of constant availability and linear decrease and was not consistent among the various crops.