WATERSHED FLUSHING TIME: A LIQUID SIGNAL OF SOIL AS A CONTROLLER AND INTEGRATOR OF HYDROGEOLOGICAL PROCESSES?

Soil and soil processes control and integrate biospheric responses that affects water cycling into and out of watersheds. However, few techniques exist to quantify these controls at the watershed scale. Here we explore watershed flushing time as descriptor of soil-mediated hydrogeological processes We define watershed flushing time (often termed streamwater transit time) as the time that water takes water to flush through a watershed and replenish streamflow. Here we analyze watershed flushing time across 232 unique watersheds that cover 1.55 million km² (16% of U.S.A. land area) and range in watershed area from 5 to 20,000 km². To quantify the age of river waters we fit sin wave regressions to model intra-annual variability in δ¹⁸O and δ²H values for the 232 rivers and 130 precipitation stations. Next, the ratio of sin wave amplitudes for precipitation and river intra-annual variations are used to calculate the watershed flushing time: t = T[(A_river/A_{precipitation})^-2 – 1]^0.5, where t is the watershed flushing time, T is the period of the annual sin wave, and A_river and A_{precipitation} represent the amplitudes of the sin waves representing seasonal variability in river and precipitation δ¹⁸O or δ²H values. Human impacts from dam construction and land use vary between our study watersheds. Dam storages range from less than 5% of annual flow for 68% of watersheds to more than 50% of annual streamflow for 9% of watersheds. Our results show that the bulk of measured streamflow is regenerated by water taking 70 to 840 days to flush through watersheds. This is a considerably greater amount of time than water spends within stream networks, themselves (~30 days, globally) and considerably shorter than the mean age of groundwater at the continental scale (1,400 years, globally). We also calculate the depth of groundwater that actively contributes to streamflow and find that dynamic storage of less than 15 metres deep renews the majority of streamflow, a small fraction (~2%) of the total groundwater storage volume. Our findings highlight the importance of the critical zone on water mediated processes at the watershed scale.