Climatic Control on Spatial Distribution of Water Storage at the Catchment Scale: A Framework for Unifying Saturation Excess Runoff Models.

This paper aims to investigate the connection between TOPography‐based hydrological model (TOPMODEL) and Variable Infiltration Capacity (VIC) model through virtual experiments from the perspective of water table and storage at the catchment scale. A simple finite‐difference groundwater flow model was built for a hypothetical catchment forced by a sequence of recharges. A steady‐state water table under a low recharge rate is used as the climatic lower limit, above which the pore space is considered as the maximum storage capacity. When the water table is shallow, the land surface is a good proxy of the water table as assumed in the original TOPMODEL, and the underlying water storage distribution curve is similar as the maximum storage capacity distribution curve. When the water table is deep, the climatic lower limit is a good proxy of water table, and the storage is approximately spatially uniform over the unsaturated area as assumed in the VIC model. The systematic variation of water table and storage distribution potentially provides a framework for unifying the TOPMODEL and VIC model. Plain Language Summary: The TOPography‐based hydrological model (TOPMODEL) and Variable Infiltration Capacity (VIC) model are two widely used saturation excess runoff models in the field of hydrology. They have different conceptualizations on the rainfall‐runoff process, but how to select between them when solving realistic problems is not clear. This paper revisits their assumptions in terms of water table and storage distribution by comparing the simulations from a groundwater flow model for a hypothetical catchment. Results show that the water table and storage distribution assumed by the TOPMODEL and VIC model are two extreme cases under very humid and arid conditions, respectively. This finding suggests that these two different saturation excess runoff models are possibly unified into a single general hydrological model in which the assumptions of water table and storage distribution could be considered as functions of recharge or water table depth. Key Points: The storage in unsaturated area is uniform in the Variable Infiltration Capacity (VIC) model but follows the maximum storage capacity distribution in TOPography‐based hydrological model (TOPMODEL)The water table is topography‐controlled in the TOPMODEL but climatic lower limit‐controlled in the VIC modelThe systematic variation of water table and storage distribution potentially provides a framework to unify the TOPMODEL and VIC model [ABSTRACT FROM AUTHOR]