IRRIGATION WATER QUALITY STANDARDS AND SALINITY MANAGEMENT

Nearly all waters contain dissolved salts and trace elements, many of which result from the natural weathering of the earth's surface. In addition, drainage waters from irrigated lands and effluent from city sewage and industrial waste water can impact water quality. In most irrigation situations, the primary water quality concern is salinity levels, since salts can affect both the soil structure and crop yield. However, a number of trace elements are found in water which can limit its use for irrigation.

Generally, "salt" is thought of as ordinary table salt (sodium chloride). However, many types of salts exist and are commonly found in Texas waters (Table 1). Most salinity problems in agriculture result directly from the salts carried in the irrigation water. The process at work is illustrated in Figure 1, which shows a beaker of water containing a salt concentration of 1 percent. As water evaporates, the dissolved salts remain, resulting in a solution with a higher concentration of salt. The same process occurs in soils. Salts as well as other dissolved substances begin to accumulate as water evaporates from the surface and as crops withdraw water.

Table 1. Kinds of salts normally found in irrigation waters, with chemical symbols and approximate proportions of each salt¹. (Longnecker and Lyerly, 1994)

¹Waters vary greatly in amounts and kinds of dissolved salts. This water typifies many used for irrigation in Texas.