Parametrization of lakes water dynamics in the ISBA-CTRIP land
surface system (SURFEX v8.1)

Abstract. Lakes are of fundamental importance in the Earth system as they support essential environmental and economic services such as freshwater supply. They also modify the local hydro-meteorological continuum as a lower boundary of the atmosphere. Sentinels of climate change and anthropization, these open water bodies are facing disruptions of their equilibrium generally leading to a notable reduction of their levels worldwide. Stream-flow variability and temporal evolution are impacted by the presence of lakes in the river network, therefore any change in the lake state can induce a modification of the regional hydrological regime. Despite the importance of the impact of lakes on hydrological fluxes and the water balance, a representation of the mass budget is generally not included in climate models and global scale hydrological modeling platforms. The goal of this study is to introduce a new lake mass module, MLake (Mass-Lake model), into the river routing model CTRIP to resolve the specific mass-balance of open water bodies. Based on the inherent CTRIP parameters, the development of the non-calibrated MLake model was introduced to examine the influence of such hydrological buffer areas on the global scale river routing performances. In the current study, an off-line evaluation was performed for four river networks using a set of state-of-the-art quality atmospheric forcings and a combination of in situ and satellite measurements for river discharge and lake level observations. The results reveal a general improvement in CTRIP simulated discharge and its variability, while also generating realistic lake level variations. MLake produces more realistic streamflows both in terms of daily and seasonal correlation. Excluding the specific case of Lake Victoria having low performances, the mean skill score of Kling-Gupta Efficiency (KGE) is 0.41 while the Normalized Information Contribution (NIC) shows a mean improvement of 0.56 (ranging from 0.15 to 0.94). Streamflow results are spatially scale-dependent, with better scores associated with larger lakes, and increased sensitivity to the width of the lake outlet. Regarding lake levels variations, results indicate a good agreement between observations and simulations with a mean correlation of 0.56 (ranging from 0.07 to 0.92) which is linked to the capability of the model to retrieve seasonal variations. Discrepancies in the results are mainly explained by the anthropization of the selected lakes which introduces high-frequency variations in both streamflows and lake levels that degraded the scores. Anthropization effects are prevalent in most of the lakes studied, but they are predominant for Lake Victoria and are the main cause for relatively low statistical scores for this river. However, results on the Angara and the Neva rivers also depend on the inherent gap of ISBA-CTRIP processes representation which relies on further development such as the partitioned energy budget between the snow and the canopy over a Boreal zone. The study is a first step towards a global coupled land system that will help to qualitatively assess the evolution of future global water resources leading to improvements in flood risk and drought forecasting.