Accounting for soil structure in pedo-transfer functions: swelling vs non swelling clays

Tropical clay Oxisols have a strong granular structure, as opposed to other clay-textured soils, such as Vertisols. Their highly aggregated structure favors infiltration and drainage, while Vertisols favor runoff, due to their high content of swelling clays. Most global scale pedotransfer functions (PTFs) are derived from soils of temperate regions where Oxisols are absent, which does not allow to represent this type of tropical soils.

In ORCHIDEE, which is a state-of-the-art land surface model (LSM), soil hydraulic properties are derived from a soil texture map, using PTFs of temperate regions (Carsel and Parrish, 1988), which does not allow to represent tropical clay Oxisols. This has been shown to induce large negative evapotranspiration biases in areas effectively covered by clay Oxisols, and mapped as clay (Tafasca et al., 2020). In order to distinguish the two types of clays of different behavior, we introduce a new soil class to represent clay Oxisols. This is equivalent to splitting the clay soil texture in two sub-classes: clay Oxisols and swelling clay. First, we modify the Reynolds soil texture map (Reynolds et al., 2000) such as to represent clay Oxisols based on the FAO Soil Order Map of the World. Then we obtain the corresponding Van Genuchten parameters based on previous studies from the litterature. We evaluate the new PTFs and the modified soil texture map using the ORCHIDEE LSM.

Using the new soil texture mapping increases evapotranspiration by 11%, allowing to correct important negative bias in tropical areas. In these zones, the mean annual river discharge decreases, consistently with the observations.