Ground Water, continued
Water for IrrigationThis discussion of the suitability of water for irrigation is adapted from Agriculture Handbook 60, U.S. Department of Agriculture (U.S. Salinity Laboratory Staff, 1954).
The development and maintenance of successful irrigation projects involve not only the supplying of irrigation water to the land, but also the control of the salinity and alkali of the soil. Irrigation practices, drainage conditions, and quality of irrigation water are all involved in salinity and alkali control. Soil that was originally non-saline and non-alkaline may become unproductive if excessive soluble salts or exchangeable sodium are allowed to accumulate because of improper irrigation and soil management or inadequate drainage.
In areas of sufficient rainfall and ideal soil conditions, the soluble salts originally present in the soil or added to the soil with water are carried downward by the water and ultimately reach the water table. This process of dissolving and transporting soluble salts by downward movement through the soil is called leaching. If the amount of water applied to the soil is not in excess of the amount needed by plants, there will be no downward percolation below the root zone, and mineral matter will accumulate at that level. Likewise, impermeable soil zones near the surface can retard the downward movement of water and cause waterlogging of the soil and deposition of salts. Unless drainage is adequate, attempts at leaching may not be successful, because leaching requires the free passage of water through and away from the root zone.
The characteristics of an irrigation water that seem to be most important in determining its quality are (1) total concentration of soluble salts, (2) relative proportion of sodium to other cations (magnesium, calcium, and potassium), (3) concentration of boron or other elements that may be toxic, and (4) under some conditions, the bicarbonate concentration as related to the concentration of calcium plus magnesium.
For purposes of diagnosis and classification, the total concentration of soluble salts in irrigation water can be adequately expressed in terms of electrical conductivity. Electric conductivity is the measure of the ability of the inorganic salts in solution to conduct an electric current and is usually expressed in terms of micromhos per centimeter. The electrical conductivity can be determined accurately in the laboratory, or an approximation of the electrical conductivity can be obtained by multiplying the total equivalents per million of calcium, sodium, magnesium, and potassium by 100, or by dividing the total dissolved solids in parts per million by 0.64. In general, water having electrical conductivity of less than 750 micromhos per centimeter is satisfactory for irrigation insofar as salt content is concerned, although salt-sensitive crops such as strawberries, green beans, and red clover may be adversely affected by water having an electrical conductivity in the range of 250 to 750 micromhos per centimeter. Water in the range of 750 to 2,250 micromhos per centimeter is widely used, and satisfactory crop growth is obtained under good management and favorable drainage conditions, but saline conditions will develop if leaching and drainage are inadequate. Use of water having a conductivity greater than 2,250 micromhos per centimeter is rare, and very few instances can be cited where such water has been used successfully.
In the past the relative proportion of sodium to other cations in irrigation water usually has been expressed simply as the percent sodium. According to the U.S. Department of Agriculture, however, the relative activity of sodium ions in exchange reactions with soil is a much better measure of the suitability of water for irrigation. The sodium-adsorption ratio (SAR) may be determined by the formula
where the ionic concentrations are expressed in equivalents per million. The sodium-adsorption ratio may be determined also by use of the nomogram shown in Figure 10.
Figure 10--Nomogram for determining value of sodium-adsorption ratio of irrigation water.
Figure 11--Diagram showing classification of typical waters of Sumner County for irrigation use. A larger version of this figure is available.
Water of low salinity (C1) can be used for irrigation of most crops on most soils with little likelihood that soil salinity will develop. Water of medium salinity (C2) can be used if a moderate amount of leaching occurs. Crops having moderate salt tolerances, such as potatoes, corn, wheat, oats, and alfalfa, can be irrigated with C2 water without special practices. Water of high salinity (C3) cannot be used on soils having restricted drainage. Water of very high salinity (C4) can be used only on certain crops and then only if special practices are followed.
Boron is essential to normal plant growth, although the quantity required is very small. Crops vary greatly in their boron tolerances, but in general it may be said that the ordinary field crops common to Kansas are not adversely affected by boron concentrations of less than 1 ppm.
Prolonged use, under adverse conditions, of water having a strong concentration of bicarbonate could have an undesirable effect upon the soil texture and plant growth.
Of the 67 samples of ground water from Sumner County that were classified as to suitability for irrigation use, 9 (Table 10) were of such poor quality that they could not be plotted on Figure 11. All samples plotted had a low sodium hazard, but one sample had a very high salinity hazard and could be used for irrigation only under special conditions; 23 samples had a high salinity hazard, but could be used for irrigating most field crops on soils having adequate drainage; 32 samples had a medium salinity hazard and could be used for irrigation with no special practices on most soils, and 2 samples had a low salinity hazard and could be used to irrigate any crop on all types of soil.
Table 10Classification of water in Sumner County for irrigation use.
Kansas Geological Survey, Sumner County Geohydrology|
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Web version January 2003. Original publication date August 1961.