Redefinition of new Soil Physical Groups
Determine SWAP with CERU32
The starting point for the calculation of Soil Water Available for Plants (SWAP) is the CERU32 program of INRA (ArcInfo AML and Fortran) which calculates the SWAP for each STU.
First, the soil profile is schematized in three layers (Le Bas et al., 1997) (see Figure 5-1):
1) a worked surface layer, defined by ploughing depth. The available water for this layer corresponds to the total available water (water volume between –1500 kPa suction (wilting point) and –5 kPa (field capacity)) estimated by rules using the topsoil input variables.
2) a subsurface layer, between the ploughing depth and the EAW (Easily Available Water) depth. The latter is user defined. The available water for this layer corresponds to the total available water (water volume between –1500 kPa suction (wilting point) and –5 kPa (field capacity)) estimated by rules valid for the subsoil, using the subsoil input variables. But if the soil layer is less than the depth to textural change the rules for subsoil uses topsoil input variables.
3) a deeper layer, between the EAW depth and the maximum rooting depth. The available water for this layer corresponds to the easily available water (water volume between –200 kPa suction and –5 kPa (field capacity)) estimated by rules valid for the subsoil, using the subsoil input variables. But if the soil layer is less than the depth to textural change the rules for subsoil uses topsoil input variables.
The main rule estimates the AWC and EAWC using texture class and packing density class (see rule 9 and 10 in Annex 4). Other rules are used to estimate input variables like topsoil and subsoil texture class, topsoil and subsoil structure class and topsoil and subsoil packing density, depth of soil and depth to textural change (see rules 1 to 8 in Annex 4).
Figure 5-1: Schematization of the soil profile in the AWC calculation of the CERU32 program (Le Bas et al., 1997)
The total SWAP is obtained by summing the available water for each layer taking into account the worked surface layer, the depth to textural change and the maximum rooting depth. If gravel or stones are present, a percentage of water is removed from the calculated SWAP (see rule 11 in Annex 4). In the same manner, for taking account of capillary rises, an amount of water is added to the SWAP for particular substrata such as loess or chalk (see rule 12 in Annex 4).
The rules are summarized below.
PTR01: This rule mainly converts the soil parameter ‘depth class to a textural change’ on the basis of soil name and the difference between texture of top and subsoil into depth of textural change (cm).
PTR02: This rule determines rooting depth
PTR03: This rule corrects the topsoil texture class on the basis of soil name, parent material and subsoil texture class. The texture classification is changed by adding classes 7 for rocks and 8 for peat.
PTR04: This rule corrects the subsoil texture class on the basis of topsoil texture class (PTR03), secondary subsoil texture class, rooting depth, depth to textural change (PTR01) and parent material. As in PTR03 the texture classification is changed by adding classes 7 for rocks and 8 for peat.
PTR05: This rule determines the topsoil structure class on the basis of topsoil texture class and soil name.
PTR06: This rule determines the subsoil structure class on the basis of subsoil texture class and soil name.
PTR07: This rule determines the topsoil packing density class on the basis of the topsoil texture class and the topsoil structure class.
PTR08: This rule determines the subsoil packing density class on the basis of the subsoil texture class, subsoil structure class and soil name.
PTR09: This rule determines (easy) available water capacity of the topsoil on the basis of topsoil texture class and topsoil packing density class.
PTR10: This rule determines (easy) available water capacity of the subsoil on the basis of subsoil texture class and subsoil packing density class.
PTR11: This rule determines a correction factor to decrease the SWAP due to agricultural constraints. The factor is based on parent material and agricultural limiting phase.
PTR12: This rule determines a correction factor to increase the SWAP due to capillary rise originating from retained water stored in the parent material. This factor is based on parent material.
