Threshold controlling runoff generation mechanisms in Mediterranean headwater catchments

Abstract

Runoff generation thresholds and pathways are critical to understanding catchment-scale hydrological processes. The variability in catchment storage, soil moisture dynamics, and rainfall characteristics influence the spatial and temporal variability of runoff thresholds and pathways. This study used six years of hourly hydro-meteorological datasets of two independent (P301 and P303) and one nested (P300) headwater catchments located in the southern Sierra Nevada region of California, USA, to understand the spatial variability in runoff thresholds and runoff pathways. Further, we identified the spatio-temporal variability in preferential flow pathways, changes in soil moisture, and vertical wetting front velocity along with their controlling factors. From the analysis of 129 rainfall-runoff events, it was found that the catchment runoff was activated when the catchment wetness (i.e., storage plus rainfall) was greater than 212 mm, 238 mm, and 250 mm in P301, P303, and P300, respectively. The catchment storage was higher at the lower slope, resulting in higher runoff thresholds, compared to upslope locations inside P301 and P303. Further, we observed dominance of preferential flow (> 60%) at 60 cm and 90 cm depth at the lower and upper slope locations, respectively, indicating a domination of subsurface runoff generation mechanism in these catchments. The preferential flows were more frequent during the drought years (2012–2015) than those in the normal years (2010–2011). Furthermore, we observed higher vertical wetting front velocity at deeper depths, but no consistent pattern was observed in the change in soil moisture along the soil profile. This study will be useful for field hydrologists and modelers to design an experimental setup and to develop a more realistic hydrological model considering the spatio-temporal variation in runoff thresholds and preferential flow pathways in forested montane headwater catchments of the Mediterranean region.