BASEFLOW CONTRIBUTION TO INTERMITTENT STREAMS IN A SEMI-ARID HEADWATER NETWORK

Half the length of headwater stream networks is comprised of intermittent streams. These streams source water supplies, downstream sediment, solutes, and nutrients. Shallow groundwater storage is a key control on baseflow chemistry and streamflow presence, and its dynamics subsequently impact downstream water users. Many researchers have quantified the release of shallow groundwater into streams but have ignored how spatial and temporal heterogeneity in baseflow affects drying. Here, we quantify baseflow and water presence/absence at 15-minute intervals at multiple locations throughout two headwater streams to determine how baseflow fluxes vary both spatially and temporally.

We present preliminary field observations of baseflow in two headwater streams within the Reynolds Creek Critical Zone Observatory (RCCZO), the Murphy Creek sub-watershed (1.28 km², 1598 m mean elevation) and the eastern headwaters of Reynolds Creek in the Reynolds Mountain East (RME) sub-watershed (0.39 km², 2075 m mean elevation). Murphy Creek is instrumented with four baseflow monitoring locations that are interspersed between fourteen water presence-absence sensors along a 2-km channel while RME is instrumented with four baseflow monitoring locations that are interspersed between eight water presence-absence sensors along a 1-km channel that originates at the highest elevations in the larger Reynolds watershed. We quantified baseflow at multiple locations throughout the stream networks to determine whether groundwater enters streams through discrete pathways or as a spatially homogeneous flux along the channel length. Baseflow is calculated using hydrograph separation with 15-minute measurements of specific conductivity and discharge. Initial observations suggest that groundwater gains and losses from the stream network are spatially heterogeneous, with surface flows both increasing and decreasing longitudinally by up to an order of magnitude at any given moment. In addition, we correlated baseflow contributions to water presence or absence to determine how baseflow patterns are correlated to stream drying.