The objective of this report was to evaluate the economic and water use effects of alternative water management systems for the Mesilla Valley, New Mexico, which could result in reduced irrigation return flows to the Rio Grande. The economic effects of four alternative water management systems for improving irrigation water applications were evaluated. Three periods of time (1967, 1973, and 1976) were included in each alternative that represented below-average (1967), average (1973), and above average (1976) surface water irrigation supplies. The alternatives evaluated were (1) the implementation of Irrigation Scheduling Service (ISS) into the agricultural sector of the Mesilla Valley for the three periods, (2) the addition of sprinkler irrigation for seed emergence of vegetable crops and incorporation with ISS, (3) trickle irrigation on orchard crops in conJunction with ISS, and (4) the combination of ISS, sprinkler irrigation of vegetable crops, and trickle irrigation of orchard crops

For the base model the average irrigation water application rate per acre during 1967 was 46 acre-inches (60 percent supplied from groundwater), for 1973 was 47.9 acre-inches (50 percent supplied from groundwater). and for 1976 was 46.9 acre-inches per acre (40 percent supplied from groundwater).

A linear programming model (LP) was designed to reflect existing Mesilla Valley agricultural water management practices for each of the time periods.

The quantity of irrigation return flow (drain flow for the base model was determined by incorporating the irrigation water quantities derived from the LP model into the hydrology model. The irrigation efficiencies for the hydrology model were assumed to be 65 percent for the base period. The drain flows for the base water supply periods were 187,411 acre-feet, 207,593 acre-feet in 1973, and 209,960 acre-feet in 1976. The salt load to the Rio Grande from the irrigation return flows was estimated to be 142.949 tons in 1967. 149.772 tons in 1973. and 149.211 tons in 1976.

The base model net return to land and risk to the Mesilla Valley for 1967 was estimated at approximately $4.3 million, for 1973 at approximately $5.7 million, and for 1976 at approximately $14.9 million.

Economic analysis of alternative water management systems for the Mesilla Valley indicated that Irrigation Scheduling Service (ISS) reduced irrigation water use by 17 to 24 percent while net returns to land and risk increased by 5 to 10 percent. A combination of ISS and sprinkler-irrigation of vegetable crops for seed germination resulted in the highest net returns to land and risk with only additional minor savings in irrigation water use. ISS and trickle irrigation management practices only added marginal water savings above ISS, but net returns would be reduced drastically.

ISS could reduce irrigation return flows to the Rio Grande from 30 to 36 percent and the salt load to the river by 42,600 to 46,700 tons annually. ISS plus sprinkler could reduce irrigation return flows by an additional 3 to 5 percent and the salt load by an additional 4,000 to 7,000 tons annually. ISS and trickle-irrigation could reduce return flows 7 to 16 percent below the ISS alternative and the salt load in the river by 14,000 to 30,700 tons annually.

Grant Number: S803565