Soil water lost to the atmosphere through ET contains little or no salt. Therefore, as water evaporates or is captured by plants, most of the salt present in the soil water is left behind in the root zone. The resultant salt concentration of the soil water increases approximately in proportion to the extent of ET. How much of this salt remains in the root zone is regulated by the leaching fraction (LF).
Note: When evaluating and interpreting salinity, either for irrigation water or for soil water, electrical conductivity (EC) is used as the main salinity parameter. ECiw is EC of the irrigation water, expressed in deciSiemens per meter (dS/m). ECsw and ECdw are the EC of the soil water and the drainage water, respectively, also expressed in dS/m.
Assume that the EC of the recycled water for irrigation of Kentucky bluegrass is 0.8 dS/m. Using the LF of 0.2 calculated in a preceding example, one can estimate the EC of the drainage water at the bottom of the root zone as:
The average root zone salinity (ECsw) will be the average of the ECsw at the soil surface (ECiw) and the EC at the bottom of the root zone (ECdw). For the stated conditions, that works out to be: (0.8 dS/m + 4 dS/m)/2, or 2.4 dS/m. Thus, the average salinity of the root zone is, for an LF of 0.2, three times greater than the salinity of the irrigation water, all because of evaporative concentration resulting from ET. The drainage water exiting the bottom of the root zone is even more concentrated; its salinity, at 4 dS/m, as noted above, is five times greater than for the irrigation water.
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