Hydrological response of a headwater catchment in Southeast Brazil-Threshold patterns of stormflow response

Headwater catchments are essential water sources for sustaining low flow in downstream rivers. The montane region of Serra da Mantiqueira in Southeast Brazil is the largest water supplier for the densely populated area of Sao Paulo, where we investigated the hydrological response of a subtropical catchment and four of its sub-catchments. We discussed the relationships between rainfall, streamflow, baseflow, stormflow, soil moisture, and water table depth using high-frequency field observations over 4 years. Baseflow accounted for most of the streamflow during the dry season and for half of streamflow in the wet season. In contrast, the variations in soil moisture were dampened, with increases in the early wet season preceding the increase in streamflow and decreases in the late wet season following the decrease in streamflow. The mean runoff coefficient and baseflow index varied from 23% to 37% and from 62% to 75% across the catchments, respectively. At the event scale, all catchments had a pronounced threshold behaviour, where stormflow increased sharply when event precipitation reached similar or equal to 10 mm, soil moisture reached similar or equal to 51% and water table depth decreased to similar or equal to 135 cm. The event stormflow coefficient was highly variable, with a maximum of 25%. The temporally integrated stormflow coefficient was low for conditions below a soil moisture threshold (similar or equal to 3%-10%) and high above a soil moisture threshold (similar or equal to 8%-15%). Our estimates of flows and threshold behaviours were consistent across the various study catchments, improving our understanding of runoff generation at a catchment scale. This information should help improve the management of water resources focused on water and food security in a context of climate change.

Automated summary

The hydrological response of a subtropical catchment in Serra da Mantiqueira, Brazil, and its four sub-catchments were investigated. Baseflow played a vital role in sustaining streamflow during the dry and wet seasons. Soil moisture did not synchronize with streamflow, showing increases in early wet season followed by streamflow increase and decreases in late wet season followed by streamflow decrease. The study provided consistent estimates of flows and threshold behaviors, contributing to understanding runoff generation at the catchment scale.