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Publication Type: Abstract Only Publication Acceptance Date: 12/5/2010 Publication Date: 2/1/2011 Citation: Suarez, D.L. 2011. Assessing the Suitability of Water for Irrigation (abstract). American Society of Agronony, California Chapter Conference Proceedings. Agricultural Certification Programs - Opportunities and Challenges. Fresno, CA. February 1 -2, 2011. p. 62-64. Interpretive Summary: Technical Abstract: Introduction Water quality assessment to evaluate the suitability of an irrigation water has traditionally (Ayers and Westcot, 1985) considered only salinity and SAR (sodium adsorption ratio). The criteria have been developed from a combination of field observations by experts and short duration column experiments with continuous, saturated water flow. Considering only the effects on soil physical properties, there are a large number of additional variables that need to be considered when evaluating irrigation water. Among these factors are clay mineralogy, oxide content, organic matter content, tillage practices, mode of irrigation water applications, rain, pH and Ca/Mg ratio of irrigation water. In most instances we understand the qualitative impact on soil stability but we lack quantitative data on their impacts and have almost no information on their interactions. Water Quality Assessment We have conducted a series of infiltration studies each of season long duration, examining the effects of salinity, SAR, pH, rain, rain interacting with water composition and cover crop. These outdoor container studies include wetting and drying cycles, attempting to simulate a field condition. We have determined that there is greater sensitivity of infiltration rates to SAR than previously considered (Suarez et al., 2006). Decreases in infiltration were observed with any increase in SAR above 0, thus there was no threshold SAR where infiltration first started to be reduced. This is in contrast to existing recommendations and laboratory studies on soil flocculation (in test tubes) where there is a relatively sharp break in the SAR, dependent on salinity, above which a soil does not flocculate in a test tube. Typically, current water quality criteria consider waters below SAR 5 or in some references waters below SAR 15 to be safe from infiltration loss. We have determined that the reduction in infiltration and thus sensitivity to SAR was greater in the experiments where we cycled between rain events (using a rain simulator) and surface irrigation of water of SAR greater than 0 (treatments in this case were SAR 2, 4, 6, 8 and 10). We also determined that with high rainfall intensity almost the same relative reductions in infiltration with varying SAR occurred in the presence of a cover crop (alfalfa) as with uncropped soil (Suarez et al., 2008). In both of these studies (Suarez et al., 2006; 2008) we observed approximately the same relative decrease in infiltration for a coarse - textured and fine textured soil. Although the relative decreases were comparable, the impact of these decreases is clearly more significant for the finer textured soil, as in arid regions with high evapotranspiration demands water infiltration may already be a limiting consideration for optimal crop production. Additional experiments have demonstrated that even small increases in pH (pH 7 vs. pH 6) of the irrigation water (with constant SAR and EC) result in decreases in infiltration, and that the greater the increase in pH the greater the decrease in infiltration. (Suarez and Gonzalez, in preparation). These studies are consistent with earlier laboratory studies (Suarez et al., 1984) in which hydraulic conductivity increased with increasing pH in short term saturated flow column experiments. Thus pH, independent of the effect of SAR, is important to predict changes in soil physical properties of arid land soils. Reductions in infiltration increased with time over the course of the experiments, with a greater separation among the infiltration rates of the various treatments, indicating greater sensitivity to SAR as compared to the short term laboratory column experiments. Based on these experiments, we developed alternative criteria for evaluating the impact of salinity, SAR and also pH on infiltration (Suarez, in press). These criteria are primaril |
