Quantifying daily versus annual contributions of snowmelt water to
streamflow using graphical and geochemical hydrograph separation
Abstract
In this study, we characterize the snowmelt hydrological response of
nine nested headwater watersheds in southeast Wyoming by separating
streamflow into three components using a combination of tracer and
graphical approaches. First, continuous records of specific conductance
(SC) from 2016 to 2018 were used to separate streamflow into direct
runoff and baseflow components. Then, diurnal streamflow cycles
occurring during the snowmelt season were used to graphically separate
direct runoff into quickflow, representing water with the shortest
residence time, and throughflow, representing water with longer
residence time in the soil column and/or regolith layers before becoming
streamflow. On average, annual streamflow was comprised of between 22%
to 46% baseflow, 7% to 14% quickflow, and 46% to 55% throughflow
across the watersheds. We then quantified hysteresis at both annual and
daily timescales by plotting SC versus discharge. Annually, most
watersheds showed negative, concave, anti-clockwise hysteretic direction
suggesting faster flow pathways dominate streamflow on the rising limb
of the annual hydrograph relative to the falling limb. At the daily
timescale during snowmelt-induced diurnal streamflow cycles, hysteresis
was negative, but with a clockwise direction implying that quickflow
peaks generated from the concurrent daily snowmelt, with shorter
residence times and lower specific conductance, arrive after throughflow
peaks and preferentially contribute on the falling limb of diurnal
cycles. Slope aspect and surficial geology were highly correlated with
the partitioning of streamflow components. South-facing watersheds were
more susceptible to early season snowmelt at slower rates, resulting in
less direct runoff and more baseflow contribution. Conversely,
north-facing watersheds had longer snow persistence and larger
proportions of direct runoff and quickflow. Watersheds with surficial
and bedrock geologies dominated by glacial deposits had a lower
proportion of quickflow compared to watersheds with large percentages of
metasedimentary rocks and glaciated bedrock.