The hydrologically active rootzone in climate models

Climate models have difficulties in predicting the frequency and intensity of future droughts on regional scales, possibly caused by inadequate representation of land surface hydrological processes. Vegetation is controlling the Earth's water and energy balance by transporting water from the subsurface to the atmosphere, through its roots. The water storage capacity in the vegetation's rootzone is a key parameter in predicting evaporation fluxes in land surface models because its size determines how long into the dry season vegetation is able to evaporate. Ecosystems design the size of their rootzone water storage reservoir to optimally function and to overcome dry periods, based on climatic conditions. Whereas climate is the major driver of root development, the rootzone storage capacity in the HTESSEL land surface scheme is only dependent on soil type and modelled soil depth. Moreover, the model describes root parameters by tables based on scarce observations of individual plants that do not represent ecosystem scales. This research aims to analyse the effect of the climate-based mass balance method for estimating the maximum water storage capacity in the vegetation's rootzone on the representation of water and energy fluxes in the HTESSEL land surface model. Maximum rootzone water storage capacities are estimated for 15 river catchments in Australia based on catchment-scale water balances. These estimates are implemented in the current surface parameterisation of the HTESSEL land surface model and offline simulations are performed. The current model performance and the model performance with adapted rootzone water storage capacities are evaluated regarding simulation of water and energy fluxes. According to this study, the storage capacity in the vegetation’s rootzone represented in HTESSEL is larger than the mass-balance derived estimates. The model strongly overestimates evaporation fluxes and thereby underestimates river discharge, with larger relative simulation errors in the dry season than in the wet season. The climate-based mass balance total rootzone water storage capacities have small effects on the representation of water and energy fluxes by the model, but contribute to a consistent improvement in predicting these fluxes. Nash Sutcliffe Efficiencies of the modelled river flows improve on average from 0.44 in the base model to 0.51 in the model with mass balance rootzone water storage capacities. The results indicate that the inadequate rootzone representation is a source of modelling error. However, it is expected that other hydrological process are also inadequately represented by the model, as the modelling simulation errors remain large when implementing mass balance rootzone water storage capacities. Moreover, it was found that internal vegetation dependent model parameters strongly influence the simulated fluxes and could therefore be another source of model bias. This study shows that investigating uncertainties in the representation of the rootzone in the HTESSEL land surface model is paramount. More research on the representation of hydrological processes in land surface models could lead to significant improvements in climate model predictions.Civil Engineerin