he quality of the water in the Rio Grande deteriorates as the river flows through New Mexico from north to south. This quality degradation is partly a result of the return of poor quality drainage water into the river. Improved irrigation management could reduce the volume of drainage water and thereby improve the quality of the water in the river. This report deals with a field study in the Mesilla Valley in New Mexico on the effects of irrigation management on soil salinity, crop yields and return flow quality. Cotton yields and salinity were measured in plots irrigated by surface flooding and with a trickle system. The quality of the groundwater near the plot area and the quality of the water in a drain along the plot area were also measured.

In 1976 and 1977, irrigation treatments (efficiency and irrigation interval) had a significant effect on the electrical conductivity and chloride concentration of saturation extracts of samples taken from the root zone of surface irrigated plots. However, irrigation treatments did have no significant effect below 150 cm, indicating that improving irrigation efficiency is not expected to have an immediate effect on the quality of percolation water, and on the quality of return flow water. Surface irrigation treatments had no significant effect on cotton yields during the years 1972 through 1977, but trickle irrigated plots yielded six percent more lint cotton than the surface irrigated plots with 35 percent less water. Soil salinity in plots which were irrigated for five years with a trickle system increased by 25 percent for plots irrigated at a soil-watertension of 0.2 bar, and by 110 percent for plots receiving only three-fourth the amount of water applied to the 0.2 bar treatment. The electrical conductivity of the water in the Del Rio drain along the experimental site was nearly the same from 1971 to 1977 as between the years 1921 and 1936, indicating no major changes in return flow quality over the last forty to fifty years.

Project No. 3109-315