The role of stream network hydrologic turnover in modifying watershed runoff composition

Abstract

Stream networks can attenuate and modify hydrological, biogeochemical, and ecological signals generated in the terrestrial and in-stream portions of watersheds. Stream networks can modify watershed signals through spatially variable stream gains and losses to and from groundwater, described herein as hydrologic turnover. We measured hydrologic gain and loss at the reach scale using conservative tracer experiments throughout the Bull Trout Watershed in the Sawtooth Mountains of central Idaho. These experiments allowed us to track water moving into and out of groundwater from and to stream water. We extended these measured reach scale water balance components to the stream network using observed empirical relationships between 1) accumulated watershed area and stream discharge, and 2) stream discharge and percent discharge lost from the stream. We developed a watershed and stream network-scale model to simulate hydrologic turnover across stream networks to quantify its effects across watershed of varying structure and stream networks of varying geometry. These analyses elucidated the influence of watershed inputs to streams on downstream stream water composition. We determined that the magnitude of contributions to discharge from any upstream watershed input depended on the magnitude of the initial input, but also on the amount of hydrologic turnover downstream along the stream network. Downstream hydrologic turnover was a function of the intersection of watershed structure and stream network geometry. Our results suggest that a distributed representation of hydrologic turnover at the stream network scale is requisite for understanding how the stream network filters and modifies watershed inputs signals observed in streams or watershed outlets.