Water resource planners use water-production functions, which are the relationships between yield and evapotranspiration, to determine the economic impact of various water allocation decisions. They also need to know the transferability of the functions within and among states.

Cotton (Gossypium hirsutum L.) and alfalfa (Medicago sativa L.) were irrigated for three years at Las Cruces, New Mexico, with a range of water levels using a sprinkler-line source to determine yield and evapotranspiration under deficit irrigation. Alfalfa was grown at five locations and cotton was grown at two locations in New Mexico using flood irrigated nonweighing lysimeters to measure the yield and evapotranspiration under nonlimiting soil-moisture conditions.

A linear water-production function was observed for alfalfa. This function appeared to be transferable to any location in New Mexico, based on the data from five locations within the state. The New Mexico relationship was statistically the same as alfalfa water-production functions for Nebraska and North Dakota. The New Mexico alfalfa water-production function had the same slope as the water production function for Nevada (P < 0.05) but had a different intercept.

A linear water-production function was also observed for cotton, but was applicable only for the two areas in southern New Mexico where the study was conducted. The water-production function for the third year of the study was different from that obtained from the first two years of the sprinkler-line source study and the lysimeter study. The cotton water-production function for New Mexico was statistically different from that reported in a similar study conducted in California. The cotton water-production function had a lower coefficient of determination than that of the alfalfa water production function since cotton was harvested for lint and seed rather than biomass. This study indicated that biomass production may require the same amount of evapotranspiration regardless of site and management differences, as in the case of alfalfa, but that lint or seed production per unit of water will vary from place to place, and from year to year.

Project Numbers: A-063-NMEX, 1423623, 1423620, 1423614