A leaching model is a hydrological
model by which the leaching with irrigation
water of dissolved substances, notably salt, in the soil is described depending
on the hydrological regime and the soil's properties.
The model may describe the process (1) in time and (2) as a function of amount of water applied.
Leaching is often done to reclaim saline soil or to conserve a favorable salt content of the soil of irrigated land as all irrigation water contains salts.
The model may describe the process (1) in time and (2) as a function of amount of water applied.
Leaching is often done to reclaim saline soil or to conserve a favorable salt content of the soil of irrigated land as all irrigation water contains salts.
Leaching
curves
The leaching
process in a salty soil to be reclaimed is illustrated in the
leaching curves of figure 1, derived from data of the Chacupe pilot area, Peru. It shows the soil
salinity in terms of electrical conductivity (EC) of the soil
solution with respect its initial value (ECi) as a function of amount of water
percolating through the soil. The top-soil leaches quickly. The salinity of the
deeper soil first increases due to the salts leached from the top-soil, but
later it also decreases.
Leaching
efficiency
Owing to
irregular distribution of salt in the soil or to irregularity of the soil
structure (figure 2), the leaching efficiency (EL) can be
different from unity.
Soils with a low
leaching efficiency are difficult to reclaim. In the Tagus delta, Portugal, the
leaching efficiency of the dense clay soil was found as low as 0.10 to 0.15. The soil could not
be developed for intensive agriculture and was used for rearing of bulls in coarse
natural pasture.
The clay soil in the Nile delta, Egypt, on the other
hand has a much better leaching efficiency of 0.7 to 0.8. In figure 3, leaching
curves are shown for different leaching efficiencies, as assumed in the leaching
model SaltMod
with data from the Mashtul pilot area. The observed values of soil
salinity correspond best to a leaching efficiency of about 0.75. The figure
illustrates the calibration process of leaching efficiency, which parameter is
difficult to measure directly.
Leaching
requirement
- The total amount of water required to bring the soil salinity from an initially high value down to an acceptable value in accordance with the salt tolerance of the crops to be grown. From figure 1 it is seen that 800 mm of water (or 8000 m3/ha) is required to bring the soil salinity down to 60% of its original value in the soil layer at 40 to 60 cm depth. When the salinity must be less than 60%, extrapolation of the leaching curve, the use of a leaching equation (see below) or a leaching model like SaltMod is necessary to obtain a reliable estimate of the additional leaching requirement.
- The annual amount of percolation water (i.e. the extra amount of irrigation water on top of the crop consumptive use) required to conserve an acceptable salt balance of the soil in accordance with the salt tolerance of the crops to be grown. The ratio
FL =
Perc/Irr, where Perc = amount of required percolation water, and Irr = total
amount of irrigation water,
is called leaching
fraction,see also below.
Leaching
equation
The downward limb
of the leaching curves, as in figure 3, can be described with the leaching
equation:
- Ct = Ci + (Co - Ci) exp (-EL.T.Qp/Ws)
where C = salt
concentration, Ct = C in the soil at time T, Co = C in the soil at time T=0, Ci
= C of the irrigation water, EL = leaching efficiency, Qp = average
percolation rate through the soil, and Ws = water stored in the soil at field
saturation.
Leaching
fraction
To conserve an
acceptable salt balance of the soil in accordance with
the salt tolerance of the crops to be grown, the leaching fraction must be at
least:
- FL = Ci/Cs
where Ci = salt
concentration of the irrigation water, and Cs is the acceptable salt
concentration of the soil moisture at field
capacity in accordance with the salt tolerance of the crops to be grown.
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