#37 Variable soil thickness in ALM

Abstract

Currently, the ACME Land Model (ALM), like the Community Land Model, has a constant depth (~3.8 m) that is hydrologically active with a poorly-defined unconfined aquifer somewhere below this. Recently, we developed a global estimate of bedrock depth at a horizontal resolution of ~30 arcsec (or about 1 km) (Pelletier et al. 2016) that allowed us to implement variable soil thickness into the Community Land Model version 4.5 (CLM4.5) (Brunke et al. 2016). Such an implementation allowed the unconfined aquifer to be removed, as the aquifer would be physically represented within the soil column. In CLM4.5, baseflow was the most impacted, while other fluxes such as surface runoff and latent heat flux were minimally impacted. Soil temperature was also substantially impacted due to changes in the thermal properties assumed and to the presence of water.

We are currently working with ACME developers on implementing variable soil thickness in ACMEv2. Model code changes for the inclusion of variable soil thickness is relatively straightforward, as we have our CLM4.5 code changes as a template. The number of hydrologically active soil layers is determined based upon the mean bedrock depths for each subgrid tiles in a grid box from the 30 arcsec resolution data. The impact on the 0.25 degree ALM is expected to be more substantial than on CLM4.5 which is on a ~1 degree grid. The impact would be even greater when the elevation-based subgrid tiles are implemented in ALMv3. The variable soil thickness will also facilitate the implementation of our hybrid three-dimensional (h3D) hydrological model in ACMEv3.