A study on implementing catchment-scale rootzone water storage capacities, derived from climatic parameters, in the HTESSEL land surface scheme

Evaporation is a key flux in both Earth’s water and energy balances. It is largely controlled by the transport of water from the subsurface to the atmosphere, through the roots of vegetation. The water storage capacity in the rootzone is a key parameter in predicting evaporation fluxes in land surface models. Drought predictions are of particular interest because the size of the rootzone-storage-reservoir determines how long into the dry season vegetation is able to evaporate. Whereas climate is the major driver of root development, the storage in the rootzone in the HTESSEL land surface scheme is only dependent on soil type and model soil depth. Moreover, the model describes root parameters by tables based on observations of individual plants that do not represent ecosystem scales. This research analyses the effect of implementing rootzone water storage capacities estimated with catchment-scale mass balances, in the land surface model HTESSEL on water and energy fluxes for 15 Australian river catchments.

This study found that the storage capacity in the vegetation’s rootzone represented in HTESSEL is larger than the mass-balance derived estimates. This leads to an underestimation of river discharge and overestimation of evaporation fluxes by the model, with significantly larger errors in the dry season. Implementation of the climate-based rootzone storage estimates in the current HTESSEL scheme leads to small model improvements regarding long term mean discharge predictions, but larger improvements in dry season discharge predictions. Transpiration fluxes in the dry season are directly linked to the size of rootzone water storage reservoir. The results indicate that inadequate rootzone representation could result in errors in modelled discharge and evaporation fluxes in the land surface model HTESSEL.

This study shows that investigating uncertainties in the representation of the rootzone in the HTESSEL land surface model is paramount. Future research is required to improve the representation of the rootzone in climate models.