The pulse of a montane ecosystem: coupled daily cycles in solar flux, snowmelt, transpiration, groundwater, and streamflow at Sagehen and Independence Creeks, Sierra Nevada, USA
Authors: Kirchner James W. , Godsey Sarah E. , Osterhuber Randall , McConnell Joseph R. , Penna Daniele 
Published: February 2020

Abstract. Water levels in streams and aquifers often exhibit daily cycles during rainless periods, reflecting diurnal extraction of shallow groundwater by evapotranspiration (ET) and, during snowmelt, diurnal additions of meltwater. These cycles can potentially aid in understanding the mechanisms that couple solar forcing of ET and snowmelt to variations in streamflow. Here we analyze three years of 30-minute solar flux, sap flow, stream stage, and groundwater level measurements at Sagehen Creek and Independence Creek, two snow-dominated headwater catchments in California's Sierra Nevada mountains. During snow-free summer periods, daily cycles in solar flux are tightly correlated with variations in sap flow, and with the rates of water level rise and fall in streams and riparian aquifers. During these periods, stream stages and riparian groundwater levels decline during the day and rebound during the night. During snowmelt, daily cycles in solar flux have the opposite effect, with stream stages and riparian groundwater levels rising during the day in response to snowmelt inputs, and declining at night as the riparian aquifer drains. The mid-day peak in solar flux coincides with the fastest rates of water level rise and decline (during snowmelt and ET-dominated periods, respectively), not with the maxima or minima in water levels themselves. A simple conceptual model explains these temporal patterns: streamflows depend on riparian aquifer water levels, which integrate snowmelt inputs and ET losses over time, and thus will be phase-shifted relative to the peaks in snowmelt and evapotranspiration rates. The highest and lowest riparian water levels (for snowmelt and ET cycles, respectively) will not occur at mid-day when the solar forcing is strongest, but rather in the late afternoon when the solar forcing declines enough that the riparian aquifer transiently achieves mass balance. Thus, although the lag between solar forcing and water level cycles is often interpreted as a travel-time lag, our analysis shows that it is predominantly a dynamical phase lag, at least in small catchments. Furthermore, although daily cycles in streamflow have often been used to estimate ET fluxes, our simple conceptual model demonstrates that this is infeasible unless the time constant of the riparian aquifer can be determined. As the snowmelt season progresses, snowmelt forcing of groundwater and streamflow weakens and evapotranspiration forcing strengthens. Because these two forcings have opposite phases, groundwater and stream level variations reflect the balance between them. The relative dominance of snowmelt vs. ET can be quantified by the diel cycle index, the correlation coefficient between the solar flux and the rate of rise or fall in streamflow or groundwater, which will be close to +1 and 1 when water level cycles are dominated by snowmelt and ET, respectively. When the snowpack melts out at an individual location, the diel cycle index in the local groundwater shifts abruptly from snowmelt-dominated cycles to ET-dominated cycles. Streamflow, however, integrates these transitions over the drainage network. Thus the transition in the streamflow diel cycle index begins when the snowpack melts out near the gauging station, and ends, months later, when the snowpack melts out at the top of the basin and the entire drainage network becomes dominated by ET cycles. During this long transition, Sagehen Creek's upper reaches exhibit snowmelt cycles at the same time that its lower reaches exhibit ET cycles, implying that snowmelt signals generated in the upper basin are overprinted by ET signals generated lower down in the basin. Sequences of Landsat images show that the gradual springtime transition in the diel cycle index mirrors the springtime retreat of the snowpack to higher and higher elevations, and the corresponding advance of photosynthetic activity across the basin. Furthermore, trends in the catchment-averaged MODIS enhanced vegetation index (EVI2) correlate closely with both the late springtime shift from snowmelt to ET cycles and the autumn shift back toward snowmelt cycles. The data and analyses presented here illustrate how streams can act as mirrors of the landscape, integrating physical and ecohydrological signals across their contributing drainage networks.