Da Cd

where

LR = Leaching fraction (unitless). Dd = Drainage depth (m).

Ca = Concentration of salt applied (dS/m).

Cd = Concentration of salt in drainage (dS/m).

If Equation 8.10 is solved for Cd, the salt concentration of the drainage is equal to the concentration of the salt applied divided by the leaching fraction:

a LR

All terms are as described above. The leaching requirement is determined based on the crop sensitivity. The average root zone salts are calculated based on solving the continuity equation for salt throughout the root zone (Hoffman and van Genuchten, 1983):

C = Mean root zone salt concentration (dS/m).

Ca = Salt concentration of applied water (dS/m).

8 = Empirical constant = 0.2Z.

LR = Leaching fraction as defined in Equation 8.10.

To determine the desired EC value of drainage, both the crop sensitivity to salinity and the groundwater quality should be reviewed. The groundwater uses, quality, and flux beneath the site should be reviewed to determine the impact of the leachate of groundwater. High EC values can be offset by small leaching depths resulting in insignificant loading to the groundwater. Also, precipitation of minerals continues to occur below the root zone reducing the loading to groundwater.

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