Leaching Requirements

Leaching is required to wash out accumulated salts from the soil. The total amount of leaching (as a percentage of annual NIR) depends on the type of crop and the salinity class of the irrigation water (soil type is not considered in the irrigation module at all).

Table LEACHING_REQUIREMENTS from The ORACLE database is containing for each of the 180 different crops and each of the 4 different salinity classes the leaching requirement in percent of total annual regard (without leaching).

Table LEACHING_REQUIREMENTS (top rows only) - requirements in Percent

CROP_GROUP CLASS REQUIREMENT COMMENTS DATE_LAST_UPDATED USER_NAME
110 1 10 BBeans_Peas_Aut1 Salinity class 1 (.7mmhos/cm) 13/11/1999 12:54:29 GAMMOH
110 2 33 BBeans_Peas_Aut1 Salinity class 2 (2mmhos/cm) 13/11/1999 12:54:29 GAMMOH
110 3 78 BBeans_Peas_Aut1 Salinity class 3 (3.5mmhos/cm) 13/11/1999 12:54:29 GAMMOH
110 4 500 BBeans_Peas_Aut1 Salinity class 4 (40mmhos/cm) 13/11/1999 12:54:29 GAMMOH
120 1 10 BBeans_Peas_Aut2 Salinity class 1 (.7mmhos/cm) 13/11/1999 12:54:29 GAMMOH
120 2 33 BBeans_Peas_Aut2 Salinity class 2 (2mmhos/cm) 13/11/1999 12:54:29 GAMMOH
120 3 78 BBeans_Peas_Aut2 Salinity class 3 (3.5mmhos/cm) 13/11/1999 12:54:29 GAMMOH
120 4 500 BBeans_Peas_Aut2 Salinity class 4 (40mmhos/cm) 13/11/1999 12:54:29 GAMMOH

The calculation of leaching is in several steps:

In the first step, the total annual leaching requirement for a crop group is calculated. Thereby, the application efficiency is considered, because the additional amount of water "wasted" by less effective irrigation methods contribute already to leaching.

The calculation in the application is by queries, which are using the following function:

Function LRev(NIRA1, NIRA2, NIRA3, Q1, Q2, Q3, LR) As Integer

  ' LRev = NIR + Leaching Requirements in mm
  ' NIR = Net Irrigation Requirement in mm
  '      (for the 3 irrigation methods)
  ' LR = Leaching Requirements in %
  ' Q = Gross Irrigation (GIR) without leaching
  '      (for the 3 irrigation methods)
  ' WAL = Weighted Application Losses in %
  '     = NIR/Q
  ' LR_r = revised Leaching Requirement in %

  Dim t, NIR, FG As Single

  IR = NIRA1 + NIRA2 + NIRA3
  FG = Q1 + Q2 + Q3

  If FG = 0 Then
      LRev = 0
    Else
      t = NIR / FG * 25
      If LR > t Then
          LRev = LR - t
        Else
          LRev = 0
      End If
      If LRev < 0 Then LRev = 0
  End If

End Function


In the second step, monthly leaching is calculated by equally distributing the annual total over those months where leaching will take place.