ESTIMATION OF EVAPOTRANSPIRATION AND IRRIGATION UNIFORMITY FROM SUBSOIL SALINITY (ARIZONA).

Abstract

Irrigation uniformity, efficiency, leaching fraction, salt and water ages, and evapotranspiration rate were estimated from subsoil salinity data for three cotton fields in Arizona. The estimation of these parameters was based on the assumption of steady-state water and salt flow through the crop root zone. The levels of salt concentration in the irrigation water were 21.3, 11.5, and 11.6 meq/L for Fields 1, 2, and 3, respectively. Two of these fields were furrow irrigated, and the third was subsurface drip irrigated. Each field was sampled for salt concentrations to a depth of 1.5 m at 10-15 sites. A total of 514 soil samples were collected. Significantly lower salt concentrations were observed in the soil profiles in Fields 1 and 2 compared to Field 3, but lower variations in the salt concentrations were observed in Field 3 compared with Fields 1 and 2. These variations in salt concentration could be due to restricted water movement within the soil profile caused by stratified soil. Since a soil-water extract model indicated little or no chemical precipitation of salt within the soil profile, there was no need to correct the data for chemical effects. The calculated irrigation uniformity was highest in Field 3 and lowest in Field 1. This may be related to more accurate land leveling in field 2 than Field 1. The irrigation efficiencies were 83.0%, 89.0%, and 80.0% for Fields 1, 2, and 3, respectively. The correlation coefficient between the ages of salt and water was 0.98, 0.99, and 0.97 for Fields 1, 2, and 3, respectively. Leaching fraction was highest in Field 3 and lowest in Field 2. Mean actual ET calculated from the Blaney-Criddle method were 372, 314, and 308 mm for Fields 1, 2, and 3, respectively. Mean ET calculated from the salinity data were 1,250, 1,590, and 1,140 mm for Fields 1, 2, and 3, respectively. Statistically significant correlation coefficients were, however, found between both methods of estimating ET. These values were 0.97, 0.86, and 0.93 for Fields 1, 2, and 3, respectively.