Soil Salinity Control - Soil Leaching

Soil Leaching

The unsaturated zone or vadose zone of the soil below the soil surface and the watertable is subject to four main hydrological inflow and outflow factors :

• Infiltration of rain and irrigation water (Irr) into the soil through the soil surface (Inf) :
• Inf = Rain + Irr
• Evaporation of soil water through plants and directly into the air through the soil surface (Evap)
• Percolation of water from the unsaturated zone soil into the groundwater through the watertable (Perc)
• Capillary rise of groundwater moving by capillary suction forces into the unsaturated zone(Cap)

In steady state (i.e. the amount of water stored in the unsaturated zone does not change in the long run) the water balance of the unsaturated zone reads: Inflow = Outflow, thus:

• Inf + Cap = Evap + Perc or :
• Irr + Rain + Cap = Evap + Perc

and the salt balance is

• Irr.Ci + Cap.Cc = Evap.Fc.Ce + Perc.Cp + Ss

where Ci is the salt concentration of the irrigation water, Cc is the salt concentration of the capillary rise, equal to the salt concentration of the upper part of the groundwater body, Fc is the fraction of the total evaporation transpired by plants, Ce is the salt concentration of the water taken up by the plant roots, Cp is the salt concentration of the percolation water, and Ss is the increase of salt storage in the unsaturated soil. This assumes that the rainfall contains no salts. Only along the coast this may not be true. Further it is assumed that no runoff or surface drainage occurs.
The amount of salts removed by plants (Evap.Fc.Ce) is usually negligibly small: Evap.Fc.Ce = 0

The salt concentration Cp can be taken as a part of the salt concentration of the soil in the unsaturated zone (Cu) giving: Cp=Le.Cu, where Le is the leaching efficiency. The leaching efficiency is often in the order of 0.7 to 0.8, but in poorly structured, heavy clay soils it may be less. In the Leziria Grande polder in the delta of the Tagus river in Portugal it was found that the leaching efficiency was only 0.15.
Assuming that one wishes to avoid the soil salinity to increase and maintain the soil salinity Cu at a desired level Cd we have:
Ss = 0, Cu = Cd and Cp = Le.Cd. Hence the salt balance can be simplified to:

• Perc.Le.Cd = Irr.Ci + Cap.Cc

Setting the amount percolation water required to fulfill this salt balance equal to Lr (the leaching requirement) it is found that:

• Lr = (Irr.Ci + Cap.Cc) / Le.Cd .

Substituting herein Irr = Evap + Perc − Rain − Cap and re-arranging gives :

• Lr = / (Le.Cd − Ci)

With this the irrigation and drainage requirements for salinity control can be computed too.
In irrigation projects in (semi)arid zones and climates it is important to check the leaching requirement, whereby the field irrigation efficiency (indicating the fraction of irrigation water percolating to the underground) is to be taken into account.
The desired soil salinity level Cd depends on the crop tolerance to salt. The University of Wyoming, USA, and the Government of Alberta, Canada, report crop tolerance data.