DETERMINATION OF HEAT UNIT BASED CROP COEFFICIENT FOR ALFALFA IN WESTERN ARIZONA

Alfalfa is the second largest production crop grown in Arizona with an estimated 150,000 acres of production in 1993. Water requirements for alfafa have been estimated at 6 acre-feet per year. These two facts together make it imperative that data be gathered to help growers apply their irrigation water efficiently while maintaining their yields. In 1994, a study was initiated in the Parker Valley region of La Paz County, Arizona, to measure daily water use in alfalfa. Two sites were chosen for measurement: one site was a surface irrigated field located at the Quail Mesa Farm; the second site was a surface irrigated field located on the Colorado River Farm. The two sites were chosen to give a contrasting schedule of irrigation and allow for data collection at differing times throughout the year. Nine neutron probes were installed in each field, three 113 in from the head, three in the center, and three 113 in from the tail on one field at each location. Neutron probe readings were taken at 18 ", 30 ", 42 ", and 54" depths below the soil surface to measure soil moisturefrom 1 '-2 ' , 2 '-3 ' , 3'-4', 4 '-5 '. The soil moisture for the top foot was determined by gravimetric sampling. The data gathered by this study was used to compare with data used in AZSCHED, a computerized irrigation scheduling program. With this data, determination of alfalfa water used based on heat units after cutting was made to help growers better use their irrigation water