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

Abstract

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.