Modeling seasonal surface runoff and base flow based on the generalized proportionality hypothesis

- Test the generalized proportionality hypothesis at the seasonal scale.
- Develop a seasonal model for surface runoff and base flow.
- Estimate initial wetting, wetting capacity, and initial evaporation.
- Evaluate the roles of rainfall variability, vegetation, soil properties and topography on seasonal runoff.

SummaryThe proportionality hypothesis, originating from the curve number method at the event scale, is extended for modeling runoff in the water-limited and energy-limited seasons, respectively. The proposed seasonal runoff model includes three parameters for two separate seasons, which are wetting capacity, initial wetting and initial evaporation. The parameters for 203 watersheds from the United States are estimated, and the empirical relationships between the parameters and watershed properties, which include duration of the season, duration, intensity and frequency of rainfall events, Normalized Difference Vegetation Index (NDVI) and soil saturated hydraulic properties, are obtained for applications in ungauged watersheds. These empirical equations present physical controls on runoff at the seasonal scale besides climate seasonality. The Nash-Sutcliffe Efficiency coefficient of the seasonal runoff simulation for total runoff is higher than 0.5 in 86% (77%) of the study watersheds; while the surface runoff is 38% (46%) and the base flow is 92% (78%) in the energy-limited seasons (water-limited seasons). This paper shows the potential application of the proportionality hypothesis for estimating seasonal runoff, which is valuable for water resources planning and management.