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

Abstract. Water levels in streams and aquifers often exhibit daily
cycles during rainless periods, reflecting daytime extraction of shallow
groundwater by evapotranspiration (ET) and, during snowmelt, daytime
additions of meltwater. These cycles can aid in understanding the mechanisms
that couple solar forcing of ET and snowmelt to changes in streamflow. Here
we analyze 3 years of 30 min 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.
Despite their sharply contrasting geological settings (most of the
Independence basin is glacially scoured granodiorite, whereas Sagehen is
underlain by hundreds of meters of volcanic and volcaniclastic deposits that
host an extensive groundwater aquifer), both streams respond similarly to
snowmelt and ET forcing. 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 at night. These cycles are reversed during snowmelt,
with stream stages and riparian groundwater levels rising during the day in
response to snowmelt inputs and falling at night as the riparian aquifer
drains. Streamflow and groundwater maxima and minima (during snowmelt- and
ET-dominated periods, respectively) lag the midday peak in solar flux by
several hours. A simple conceptual model explains this lag: 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. 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 mostly 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 response time of the
riparian aquifer can be determined. As the snowmelt season progresses, snowmelt forcing of groundwater and
streamflow weakens and evapotranspiration forcing strengthens. The relative
dominance of snowmelt vs. ET can be quantified by the diel cycle index,
which measures the correlation between the solar flux and the rate of rise
or fall in streamflow or groundwater. When the snowpack melts out at an
individual location, the local groundwater shifts abruptly from
snowmelt-dominated cycles to ET-dominated cycles. Melt-out and the
corresponding shift in the diel cycle index occur earlier at lower
altitudes and on south-facing slopes, and streamflow integrates these
transitions over the drainage network. Thus the diel cycle index in
streamflow shifts gradually, beginning when the snowpack melts out near the
gauging station and ending, 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, snowmelt signals generated in the upper
basin are gradually overprinted by ET signals generated lower down in the
basin. The gradual springtime transition in the diel cycle index is mirrored in
sequences of Landsat images showing the springtime retreat of the snowpack
to higher elevations and the corresponding advance of photosynthetic
activity across the basin. Trends in the catchment-averaged MODIS enhanced
vegetation index (EVI2) also correlate closely with the late springtime
shift from snowmelt to ET cycles and with the autumn shift back toward
snowmelt cycles. Seasonal changes in streamflow cycles therefore reflect
catchment-scale shifts in snowpack and vegetation activity that can be seen
from Earth orbit. 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.