Soluble salts that accumulate in soils must be leached below the crop root zone to maintain productivity. Leaching is the basic management tool for controlling salinity. Water is applied in excess of the total amount used by the crop and lost to evaporation. The strategy is to keep the salts in solution and flush them below the root zone. The amount of water needed is referred to as the leaching requirement or the leaching fraction.

Excess water may be applied with every irrigation to provide the water needed for leaching. However, the time interval between leachings does not appear to be critical provided that crop tolerances are not exceed. Hence, leaching can be accomplished with each irrigation, every few irrigations, once yearly, or even longer depending on the severity on the salinity problem and salt tolerance of the crop. An occasional or annual leaching event where water is ponded on the surface is an easy and effective method for controlling soil salinity. In some areas, normal rainfall provides adequate leaching.

Determining Required Leaching Fraction

The leaching fraction is commonly calculated using the following relationships:

(1)

where

LF = leaching fraction - the fraction of applied irrigation water that must be leached through the root zone

EW_iw = electric conductivity of the irrigation water

EW_c = electric conductivity of the soil in the root zone

Equation (1) can be used to determine the leaching fraction necessary to maintain the root zone at a targeted salinity level. If the amount of water available for leaching is fixed, then the equation can be used to calculate the salinity level that will be maintained in the root zone with that amount of leaching. Please note that equation (1) simplifies a complicated soil water process. EC_e should be checked periodically and the amount of leaching adjusted accordingly.

Based on this equation, Table 13 lists the amount of leaching needed for different classes of irrigation waters to maintain the soil salinity in the root zone at a desired level. However, additional water must be supplied because of the inefficiencies of irrigation systems (Table 14), as well as to remove the existing salts in the soil.

Table 13. Leaching requirement* as related to the electrical conductivities of the irrigation and drainage water.
 

Electrical conductivity of irrigationw ater (mmhos/cm)	Leaching requirement based on the indicated maximum conductivities of the irrigation and drainage water
	4 mmhos/cm  Percent	8 mmhos/cm  Percent	12 mmhos/cm  Percent	16 mmhos/cm  Percent
0.75  1.00  1.25  1.50  2.00  2.50  3.00  5.00	13.3  25.0  31.3  37.5  50.0  62.5  75.0  ---	9.4  12.5  15.6  18.7  25.0  31.3  37.5  62.5	6.3  8.3  10.4  12.5  16.7  20.8  25.0  41.7	4.7  6.3  7.8  9.4  12.5  15.6  18.7  31.2

 

Table 14. Typical overall on-farm efficiencies for various types of irrigation systems.

System	Overall efficiency (%)
Surface  a. average  b. land leveling and delivery pipeline meeting design  standards  c. tailwater recovery with (b)  d. surge  Sprinkler (moving and fixed systems)  LEPA (low pressure precision application)  Drip	50 - 80 50 70   80 60 - 90* 55 - 85 95 - 98 80 - 90 **

*Surge has been found to increase efficiencies 8 to 28% over non-surge furrow systems

**Drip systems are typically designed at 90% efficiency, short laterals (100 feet) or systems with pressure

compensating emitters may have higher efficiencies.