Runoff composition is insensitive to summer rain contributions in a montane headwater stream

Abstract

Study region

Manitou Experimental Forest, a montane, headwater catchment in the Upper South Platte Basin in Colorado, USA.

Study focus

In many parts of the western U.S., snowpack drives groundwater recharge that sustains streamflow. However, as climate change reduces snowpack, rainfall inputs may become increasingly crucial for maintaining streamflow. Rainfall inputs can be highly spatiotemporally variable due to differences in antecedent catchment moisture and topography, rainfall totals and intensities (e.g., summer monsoonal rains), and during wet versus dry years. We characterized rainfall and antecedent catchment moisture conditions during spring, summer, and fall seasons between a wet year and two dry years at upper and outlet catchment positions.

New hydrological insights for the region

Our findings indicate that rain events have minimal direct influence on the magnitude or composition of streamflow within this headwater montane stream, as groundwater overwhelmingly dominates streamflow regardless of rainfall characteristics or antecedent moisture conditions. Rain event runoff ratios and event water contributions (EV_Tot) were extremely low (max = 0.18, 13 %, respectively). Using multiple linear regression modeling, we found that antecedent baseflow was positively correlated with summer runoff ratios, and rainfall total was positively correlated with summer EV_Tot, whereas spring and fall controls on runoff response varied with catchment position. Our findings have implications for rainfall contributions to streamflow, impacting future water delivery in montane headwater streams.