Quality and Quantity of Return Flow as Influenced by Trickle and Surface Irrigation (Annual Report, Year 3)

Deterioration of the quality of the water in the Rio Grande is a major problem for the water users in Texas and New Mexico. The usual practice of irrigation in arid and semi-arid areas involves the use of heavy applications of water, in addition to the water used by the plants, for the purpose of removing accumulated salts or preventing an excessive increase of salts in the soil. The subsequent transport of this excess irrigation water to the groundwater causes pollution of the groundwater and of the irrigation return flow. The objectives of the present study, initiated in July of 1971, are to determine the quality and quantity of return flow as influenced by two irrigation systems: i.e., trickle and surface irrigation.

The effects of amount and frequency of water applications on water and solute movement within the soil are being studied for both irrigation systems. Twenty-seven field plots, each 20 x 20 feet, have been surrounded with plastic to a depth of three feet for the surface irrigation studies. The main treatment effects on these plots are frequency of irrigation and application efficiency. The plots are irrigated when 25, 50, or 75 percent of the available water is depleted. Field water application efficiencies of 80, 90, and 100 percent are used. The 100 percent efficiency treatment is irrigated to prevent any loss of moisture to the subsoil. Each treatment is block randomized with three replications per treatment. Six 20 x 60 feet plots were established to study the effects of trickle irrigation on return flow. The trickle plots are irrigated to maintain a soil water tension of or below .2 and .6 bars, respectively, for the two treatments measured at a depth of six inches. The quality of the water percolating below the root zone is determined by collecting samples from suction cups located below the root zone.

Project #13030 GLM