During a normal growing year, the amount of irrigation required to achieve the maximum potential yield is about 17 inches in the southeast and 23 inches in the Panhandle. The balance of the water requirement is supplied by preseason soil water and effective rainfall during the growing season. The effective rainfall during a normal growing season decreases steadily as you travel across the state in a northwesterly direction. It ranges from a high of over 24 inches in the southeast down to less than 12 inches in the Panhandle. The rate at which alfalfa consumes water varies considerably, depending upon time of year and stage of growth.
Weather conditions have a great effect on water use rate of alfalfa.
Hot, dry, windy conditions necessitate considerably more water usage than cooler, humid, calm conditions for the same crop growth stage. Irrigation systems are normally sized to meet peak water use demand of the crop during the driest period of the growing season. Systems may be sized to supply the complete water needs of the crop, assuring maximum production even during sustained drought conditions. More commonly, they are sized to supplement normal growing-season rainfall and assure maximum potential production in approximately six years out of ten. An irrigation system designed to supplement normal rainfall has a smaller water supply and smaller equipment components than a system that can supply total water needs, and is normally less expensive.