Spatiotemporal patterns in hydrologic connectivity within a semi-arid
montane headwater catchment in central Colorado
Abstract
Climate projections suggest that snowfall-dominated areas will decline
substantially in the coming decades. Such climate impacts are already
being observed in Colorado where the dominant source of annual peak
discharge is shifting from snowmelt to rainfall, altering the paths by
which water flows through a landscape and is ultimately delivered to
streams. Observed climate driven shifts in stream flow dynamics and
permanence highlight the increasing importance of understanding the
hydrologic connectivity of uplands to streams in lower elevation,
montane ecoregions. We collected geochemical and hydrometric data over
three years to quantify hydrologic connectivity of uplands to a montane
headwater stream at the Manitou Experimental Forest in central Colorado.
We use a combined approach of concentration-discharge relationships and
end-member mixing analysis, paired with high resolution measurements of
soil moisture, precipitation, and groundwater levels to characterize
source areas to the stream in 3-dimensions: longitudinal, lateral, and
vertical. Samples were collected and measurements were recorded along
the stream profile (longitudinal), from groundwater wells and soil
lysimeters installed with increasing distance from the stream (lateral),
and from shallow versus deep groundwater wells and soil moisture
measured at different depths (vertical). Results indicate distinct
differences in stream chemistry along the longitudinal stream profile,
with highest concentrations at the most upstream sites and lowest
concentrations at the most downstream sites. Stream solute
concentrations increased with decreasing stream discharge values from
spring to late summer. However, the stream remained chemostatic during
all recorded rain storms, suggesting a difference in flow pathways
during individual summer storm pulses. End member mixing analysis
suggests spatiotemporal differences in shallow and deep vertical source
areas, and between riparian and upland sources to the stream. These
results provide a promising step towards quantifying the expansion and
contraction of runoff source areas to a montane headwater stream.