On the water balance representation in urban land surface models

Urban Land Surface Models (ULSM) are developed to simulate the urban climate and vary in their complexity. The need for this complexity was assessed by two successive systematic intercomparison projects. Both projects focused on the energy balance and found the latent heat flux to be the most challenging flux to model. However, these projects did not address the closure of the water balance, although the energy balance is directly linked to the water balance. This study aims to assess the representation and dynamics of the water balance in 14 ULSMs from the Urban-PLUMBER project each ran for 20 sites. The water balance could not be evaluated by straightforwardly comparing the model results against measurements since most water balance fluxes are not measured. Therefore, the water storage dynamics were derived from the modelled water balance fluxes. We examined the inter-model variation in both the storage dynamics and the separate fluxes and developed seven indicators of a well-captured water balance. The variation in both the fluxes and the storage dynamics is in the same order of magnitude as the size of the fluxes themselves. The indicators show that no ULSM in this study can consistently reproduce a physically realistic water balance regardless of the model’s complexity. As the water balance is linked to the energy balance, the poor water balance representation may explain the poor performance for the latent heat flux. The linked balances illustrate model evaluations and comparisons should extend beyond the target variables of the model to all processes that directly influence these variables.