Vertical Unsaturated Flow Governs Storm Runoff Responses: Observational and Theoretical Foundations

The old water paradox is difficult to resolve based on the traditional runoff mechanism consisting of a downslope flow system that includes overland and subsurface flows. Our observations and theoretical studies revealed that rainfall‐runoff responses are governed by vertical unsaturated flow augmenting shallow groundwater that is subsequentially drained by rapid preferential flow. This mechanism explains the physical basis for runoff‐storage relationship models and enables the evaluation of the sensitivity of peak runoff rates to soil properties and soil‐layer depth. This commentary discusses the development of preferential flow pathways from a geomorphologic perspective and provides information for developing calibration‐free, physically‐based runoff models in which parameters can be estimated from measurable field indicators.