irrigation and water quality section f swes 316

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Irrigation and Water Quality Section F SWES 316 Slide 2 Slide 3 Slide 4 Salt Moves to the Terminus of the Wetting Front! Slide 5 Slide 6 Slide 7 Border/Flood Irrigation Furrow Irrigation Sprinkler Subsurface drip Slide 8 Slide 9 Why Water Quality is Important Effects on Soil Salinity Effects on Soil Sodicity Bicarbonate content Toxic ion content Effects on nutrients applied by fertigation Slide 10 Irrigation and Water Quality When soils are irrigated, the quality of the water used will eventually determine the salinity and sodicity of the soil: Soils will be at least 1.5 times more saline than the water used to irrigate them, unless very high (>20%) leaching fractions are used. The ESP (and SAR) of the soil will eventually equal the sodium adsorption ratio (SAR) of the water. Slide 11 Irrigation Water Quality Important Measurements: Salinity (measured by EC) Sodium (measured by SAR) Potential toxic ions (Na, Cl, B) Alkalinity or HCO 3 - SAR= Slide 12 Irrigation Water Hazards Interpreting Salinity Remember that even with excellent management, soil EC will be at least 1.5X that of the water used (except in very coarse-textured soils or very high LF). Crop tolerance varies, however Interpreting sodicity - depends on clay content and salinity Slide 13 Irrigation Water Hazards Concentrations of toxic ions: Cl - Na + B (H 3 BO 3 ) Bicarbonate - precipitates Ca 2+ from soils, makes Na problems worse. Slide 14 General Guidelines Degree of Restriction on Use Potential ProblemUnitsNoneSlight to ModerateSevere pHNormal range 6.5 8.4 Salinity ECdS/m3.0 TDSmg/L2,000 Infiltration SAR=0-3 and EC w =>0.70.7 0.21.21.2 0.31.91.9 0.52.92.9 1.35.05.0 2.9 Calcium Carbonate Precipitation 1. High bicarbonate Ca ++ + 2HCO 3 - > CaCO 3 + H 2 CO 3 >H 2 O + CO 2 2.Ammoniated water NH 3 + H 2 O > NH 4 OH NH 4 OH > NH 4 + OH OH- + HCO 3 > CO 3 -2 + H 2 O Ca ++ + CO 3 -2 > CaCO 3(s) Slide 31 Slide 32 Constant H 2 SO 4 injection keeps water pH low and prevents formation of CaCO 3 in the drip lines, and also dissolves some CaCO 3 in the soil, helping to maintain high exchangeable Ca 2+ and low exchangeable Na +. Slide 33 Slide 34 Water Quality and Nutrient Management (3) Many P fertilizers are not very soluble in water. Adding them to irrigation water high in soluble Ca will lead to precipitation of Ca-phosphates Loss of P Plugging of irrigation system

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