Quality and Quantity of Return Flow as Influenced by Trickle and Surface Irrigation (Annual Report, Year 2)

INTRODUCTION

A. Purpose and Project Description

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 50, 75, 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.

To determine water loss by deep percolation, measurements are made of the water contents and the pressure gradients below the root zone of each plot. From the gradients and knowledge of the hydraulic conductivities and water contents below the root zone, the net fluxes in or out of the soil profiles are calculated, using Darcy’s equation. The quality of the water percolating below the root zone is determined by collecting samples from suction cups located below the root zone.

B. Summary of Current Year’s Work

This report presents results of the first cropping year of a three-year study on the quality and quantity of return flow as influenced by trickle and surface irrigation. Cotton yields were not significantly affected by the efficiency of the surface irrigation system. The differences in yield due to percent depletion were small, although there appeared to be a trend toward higher yields with increasing percent depletion. Cotton yields from the trickle plots were considerably higher than from the surface irrigated plots, and less water was used per unit of cotton produced.

Measurements of soil salinity in the surface plots showed no significant effect as a result of irrigation efficiency. It appears that even the 100 percent efficiency treatment had adequate leaching of salts out of the soil profile. Movement of salts around the trickle system emitters was monitored.

Measurements of the hydraulic gradients below the root zone also showed a downward gradient in all surface irrigated plots during the growing season, indicating seepage losses from all treatments. The quality of the water percolating to the subsoil varied greatly from plot to plot. The average salt content of the deep percolation water was about lOX as high as in the applied irrigation water. Measurements were made of the hydraulic properties of the subsoil below the surface irrigated plots.

Project No. 13030 GLM