SALINITY AND TRACE ELEMENTS ASSOCIATED WITH WATER REUSE IN IRRIGATED SYSTEMS: PROCESSES, SAMPLING PROTOCOLS, AND SITE-SPECIFIC MANAGEMENT
Location: Water Reuse and Remediation
Title: Assessing the Suitability of Water for Irrigation
Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: December 5, 2010
Publication Date: February 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.
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 primarily for arid land soils only provide a general assessment.
Modeling Plant Response to Salinity
The UNSATCHEM computer model (Suarez and Simunek, 1997) and the more user friendly SWS model (Suarez and Vaughan, 2001) are utilized to assist in management decisions related to irrigation in arid regions. The models consider the chemical processes of precipitation and dissolution, cation exchange and adsorption of boron. These processes are coupled to a variably saturated water flow model water flow and a plant water uptake model that relates relative yield to water and salinity stress. Simulations using this model show that the traditional (Ayers and Westcot, 1985) calculation method for evaluating plant response to soil salinity overestimates the yield loss, especially at high salinity and low leaching fractions. The major effects are related to two factors: 1) Consideration that the plant responds to salinity of the water taken up by the plant and not average rootzone salinity as assumed and 2) Assumption that leaching fraction and crop ET are fixed inputs rather than crop responses to the stress experienced. These results (Suarez, 2010) suggest that with some relatively small losses in potential yield, we can irrigate crops with more saline water than previously considered, without the need for large quantities of leaching water. Modeling simulations also provide guidance for management options when using low quality waters. For example, Goldberg and Suarez (2006) determined using UNSATCHEM simulations that transient use of high B water is feasible and that the optimal leaching management was different for clay vs. sandy soils. Contrary to existing guidelines, for a single season use of high B waters, minimal water applications and leaching gave the lowest soil water B concentrations.
Ayers, R.S., and D.W. Westcot. 1985. Water Quality for Agriculture. FAO Irrigation and Drainage Paper 29 rev. 1 FAO Rome.
Goldberg, S. and D.L. Suarez. 2006. Prediction of anion adsorption and transport in soil systems using the constant capacitance model. In: Surface Complexation Modeling. J. Luetzenkirchen (ed.). Interface Science and Technology Series. Elsevier. 11:491-517.
Simunek, J., and D.L. Suarez. 1997. UNSATCHEM: Unsaturated water and solute transport with equilibrium and kinetic chemistry. Soil Sci. Soc. Am. J. 61(6):1633-1646.
Suarez, D.L. 2010. Irrigation water quality assessments. In: K.K. Tanji and W.W. Wallender (eds.) ASCE Manual and Reports on Engineering Practice No. 71. Agricultural Salinity Assessment and Management (2nd Edition). ASCE, NY. Chapter 11 (In press).
Suarez, D.L. 2010. Soil salinization and management options for sustainable crop production. In: M. Pessarakli (ed.) Handbook of Plant and Crop Stress. 3rd Edition. CRC Press. Boca Raton, FL. Chapter 3 pp: 41-54.
Suarez, D.L., Rhoades, J.D., Lavado, R. and C.M. Grieve. 1984. Effect of pH on saturated hydraulic conductivity and soil dispersion. Soil Sci. Soc. Am. J. 48(1):50-55.
Suarez, D.L. and P.J. Vaughan. 2001. FAO SWS Manual. George E. Brown Jr. Salinity Laboratory Research No. 147 pp: 1-113.
Suarez, D.L., Wood, J.D. and S.M. Lesch. 2006. Effect of SAR on water infiltration under a sequential rain-irrigation management system. Agric. Water Manage. 86:150-164.
Suarez, D.L., Wood, J.D. and S.M. Lesch. 2008. Infiltration into cropped soils: Effect of rain and sodium adsorption ratio – Impacted irrigation water. J. Environ. Qual. 37:S169-S179.