Salinity leaching
Salinity leaching
From an irrigation standpoint, the dissolved salts or salinity of irrigation water, commonly measured by electrical conductivity (EC), has received the most attention (Butler et al., 1985). The key to salinity control and to irrigation sustainability is leaching. Leaching is the net downward movement of water and controls the salt distribution in the profile.
When the crop root zone has become too saline for the existing plants, it requires extra water to leach out the accumulated salts by increasing LF. Maintenance leaching is necessary to keep the average rootzone salinity below the plant threshold EC levels. The leaching fraction (LF) is defined as
LF = Ddw/Diw = Ciw/Cdw = ECiw/ECdw
and it is customary to substitute EC for C since the EC is much easier to measure than concentration and EC is linearly related to the total salt concentration,
where Ddw= the equivalent depth of water passing below the rootzone
Diw = the equivalent depth of water applied (irrigation + rainfall)
Ciw (ECiw) = Salt concentration (EC) of the applied water
Cdw (ECdw) = Salt concentration (EC) of the drainage water
Leaching requirement (LR) is the minimum LF that a plant can endure without salinity tress.
LR = Ciw/C*dw = ECiw/EC*dw
where C*dw (EC*dw ) is the maximum permissible salt concentration (EC) of the drainage water. Using Rhoades equation (1974), the LR is related to ECa by
LR = ECiw / (5ECt -ECa)
where ECt is equal to the plant salt tolerance or threshold EC (see the tables in "Crop salinity tolerance and yield" section.
Reclamation leaching is the application of water until the soil salts are leached out of the intended root zone. If the land being developed for a landscape is already salt-affected and requires reclamation leaching before crop establishment. Hoffman proposed the the following empirical equation for the salt transport efficiency under 1-dimensional leaching:
C/Co = k/(D/Ds)
where C = desired salt concentration in the soil to be reclaimed, Co = the initial salt concentration before reclamation, D = depth of applied leaching water, Ds = depth of the soil to be leached, and k is an empirical coefficient ranging from 01. (sandy loam) to 0.3 (clay).