Hydrological Spaces of Long-Term Catchment Water Balance

Land and water management often relies upon relationships describing the catchment-scale water balance using only a few parameters. The classic Budyko and Turc frameworks are examples of these relationships applicable to large catchments, where the effect of climatic variables on the water balance overshadows that of catchment characteristics, including the catchment ability to store water to supply evapotranspiration. To account for the latter in the list of variables driving evapotranspiration, here we introduce a new framework that includes Budyko and Turc as particular cases. The four variables in this framework are combined to form dimensionless groups, the choice of which leads to the definition of hydrological spaces highlighting different features of the long-term hydrological partitioning. In addition to the Budyko and Turc spaces, suitable for water- and energy-limited catchments, respectively, a new space ensues; this is especially apt to describe catchments where evapotranspiration is mainly controlled by catchment characteristics. An existing stochastic model for the soil water balance is used to specify the relationship between the variables in these different hydrological spaces. This framework, successfully tested here against about 400 catchments in the continental United States, provides a concise yet realistic description of long-term catchment-scale water balance and overcomes some limitations of current models for the estimation of long-term evapotranspiration and runoff in ungauged catchments.