Management of Salt-Affected Soils - Practices of Irrigation and On-farm Water Management - page 305

Considering a field water application efficiency, E f , the above equation becomes

ET(1 + LF)

AW = (8.9)

E f

This equation is more straightforward and versatile to calculate total irrigation
depth (or water to be applied both for crop water demand and leaching). For practical
purpose, the net irrigation demand or ET demand is calculated as the water required
to bring the root zone soil to field capacity.
Equation proposed by Ayers and Westcot ( 1985 ) to calculate total depth of water
for a particular season or period is

ET

AW = (8.10)

1 − LR

where

AW = depth of applied water, cm/season

ET = total crop water demand, cm/season

LR = leaching requirement, expressed as fraction (leaching fraction)

The Eq. (8.9) has several merits over the Eq. (8.10):

• it has strong theoretical basis

• it is straightforward

• it is logical that the total applied depth will vary directly with the leaching fraction

• it includes field application efficiency, which is a logical demand

The leaching fraction can be calculated based on the salinity of applied water
using the following equation (Rhoades, 1974 ) :

EC w
5EC t −

LF = (8.11)

EC w

where EC w is salinity of the applied irrigation water, and EC t is the threshold salinity
(average soil salinity tolerated by the crop).
A leaching fraction can be calculated for sprinkler irrigated fields using the
equation:

EC water

=

EC max ×

% Leaching fraction 100 (8.12)

2 ×

In this equation, EC max is the maximum soil EC wanted in the root zone.
In general, leaching fractions of 10-20% can be used depending on the degree of
existing soil salinity and salinity of irrigation water.