Irrigation water quality

Topical salt distributions in irrigated soil profiles are shown in Fig. 11.3. As the proportion of irrigated water passing through the root zone (the leaching fraction) increases, so does the depth of soil that has essentially the same salt concentration as the irrigation water. As the leaching fraction increases, salt accumulation is pushed down to lower depths. 

As the salt concentration of the soil increases, so does the potential for salinity effects on plant growth. Early appraisals of the salinity of irrigation waters were in terms of total dissolved solids (TDS). The TDS were determined by evaporating a known volume of water to dryness. The presence of hygroscopic water in the resultant salt mixtures made the values for TDS strongly dependent on the drying conditions. The concentration of salts in most irrigation waters is less than 1000 mg 

FIGURE 11.3. The steady-state profile of soil salinity, expressed as the electrical conductivity of the saturation extract, in lysimeters. The irrigation water has EC values of 2 (solid lines) and 4 (dashed lines) dS m 1. Numbers on the figure are the respective leaching fractions. 

More recently salinity has been measur ed in terms of tire electrical conductivity (EC) of a solution. In addition to overcoming some of the ambiguities of TDS measurements, the EC measurement is quicker and sufficiently accurate for most purposes. To determine the EC, the solution is placed between two electrodes of constant geometry, including constant distance of separation. When an electrical potential is imposed, the electrical current varies directly with the total concentration of dissolved salts. The current is inversely proportional to the solution s resistance and can be measured with a resistance bridge. Conductance is the reciprocal of resistance and has units of reciprocal ohms or siemens (formerly mhos). The EC of the saturation extract of the soil measures the salinity of the soil. 

The measured conductance is a result of the solution s salt concentration and the electrode geometry in the measuring cell. The effects of electrode geometry are embodied in the cell constant, which is related to the distance between electrodes and their cross-sectional area. The cel) constant is measured by calibration with KC1 solutions of known concentration. The conductivity of KC1 solutions is available in published tables. For example, calibration might yield a cell constant of 2.0 cm-1. A test solution that measures 2000 G resistance (conductance of 1/2000 S3-1 or 0.0005 siemens) in this cell has a conductivity of 1.0 dS m 1, or 1.0 mmho cm-1. 

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Repercussions of Soil Properties and Irrigation Water Quality on Crop Growth Under... 

Specific Examples of Implication of Irrigation Water Quality Under Water Scarcity 168... 

Irrigation has been practiced from the ancient time it is only in the twentieth century that the importance of the irrigation water quality was recognized [7]. The use of saline water may result in the reduction of crop yields. A high sodicity of water for irrigation may cause the deterioration in the physical properties of soils... 

With surface irrigation, water quality control procedures include careful land leveling and controlled water application to ensure adequate and rapid watering of... 

Mandal, U.K., Warrington, D.N., Bhardwaj, A.K., Bar-Tal, A., Kautsky, L. Minz, D., Levy, G.J. 2008. Evaluating impact of irrigation water quality on a calcareous clay soil using principal component analysis. Geoderma, 144, 189-197. 

Sodium Adsorption Ratio. Because of the swelling effects of Na+, the relative amount of sodium (sodicity) in the water quality especially in the irrigation water quality is an important measurement in soil science. Decreased permeability can interfer with the drainage required for normal salinity control and with the normal water supply and aeration required for plant growth. The relative sodium status of irrigation waters and soil solutions is often expressed by the sodium adsorption ratio (SAR)... 

Cass, A. and M. E. Sumner. 1982. Soil pore structural stability and irrigation water quality I-Empirical sodium stability model. Soil Sci. Soc. Am. J. 46 503-506. 

A serious limitation of such boron criteria is the lack of provision for difference in soils, irrigation management, and climate. Soils differ in their capacity for adsorbing boron, and irrigation and rainfall characteristics likewise exert an effect on the distribution of available boron in soils. Additional details are presented in discussions of criteria for irrigation water quality by Bernstein (41) and Rhoades (42). 

J. D., Vaughn, R. B., and Welsch, E. P. (1992). Results of Chemical Analysis for Sediments from Department of the Interior National Irrigation Water Quality Program Studies, 1988-1990. U.S. Geological Survey open-file report 92-443. 

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