Management of Salt-Affected Soils - Practices of Irrigation and On-farm Water Management - page 304

The term “Leaching fraction” and “Leaching requirement” is interchangeably
used in the literature. Leaching requirement (LR) is the absolute amount (depth) of
water required to leach out the salts below the crop root zone. But in practice, it is
expressed as a fraction of net irrigation demand (or ET) of the crop.

Derivation of Equation for Total Irrigation Depth in Leaching Purpose

From the principle of mass balance of salt, we can write

V 1 C 1 = V 2 C 2 = ...

where V 1 , V 2 are the volume of solution (water) containing salt for state 1 and 2
respectively, and C 1 , C 2 are the concentration of salt in the solutions, respectively.
Then we can write

V 1
V 2 =

C 2
C 1

For a fixed surface area, we can write

C 2
C 1
where D 1 and D 2 are the depths of the solutions (cm). If we express the salt
concentration in terms of EC (electrical conductivity), then

D 1
D 2 =

D 1
D 2 =

EC 2
EC 1

(8.7)

For leaching purpose, the total (or gross) depth of irrigation (AW) consists of net
irrigation amount, D i (or evapo-transpirative demand, ET) plus drainage or leaching
amount, D d . That is,

AW = ET (or D i )+ D d

Or, D d = AW − D i

Thus, the Eq. (8.6) can be rewritten as

D d
D i =

AW − D i AW

D i −

LF = = 1 (8.8)

D i

Or, AW

D i =

1 + LF

Or, AW = D i (1 + LF) = ET(1 + LF) [since D i = ET]