Does streamwater have an address? What flow dynamics tells us about flow origins

Abstract:

Stream levels rise and fall with rain events, and how quickly they respond is linked to rainfall amount, rainfall intensity, and soil moisture and groundwater levels prior to the event. In forested regions, vegetation water use during the growing season also plays a critical role in how quickly stream levels rise and fall through pumping of water from the subsurface. These factors contribute to nonlinear streamflow response to storm events that produce a break in slope between slowly increasing streamflows and rapidly increasing streamflows. These break in slopes, or thresholds, may reveal where in the landscape water is being produced. In our study, we use hourly measurements of soil moisture and groundwater to examine when streamflows originate from shallow soil moisture zones, deeper groundwater zones and surface flow, and how this varies across geologies. We utilized data from three forested, montane watersheds in the Coweeta Hydrologic Laboratory in southwest North Carolina and the Shale Hills Critical Zone Observatory in central Pennsylvania. Our study found that the relationship between soil moisture, groundwater, and streamflow formed three unique relationships between our catchments demonstrating a spectrum of streamflow sources ranging from almost entirely the shallow soil moisture zone to almost entirely the deeper groundwater zone. These measurements and analyses may reveal similar trade-offs and interactions between soil moisture versus groundwater processes that contribute to streamflow in other study catchments.