| dc.description.abstract |
<p>Watersheds are complex, three dimensional structures that partition water between
the components of the water balance and multiple storage pools within the watershed.
This central function, however, remains poorly understood in a broadly transferable
way despite decades of research. Perhaps one reason for this is the disciplinary bias
towards studying pristine, mountainous watersheds with steep terrain and shallow soil.
Although the relative simplicity of such systems has made them ideal hydrologic laboratories,
understanding how watersheds function globally will require the incorporation of new
types of landscapes into the studies of hillslope and watershed hydrology. </p><p>The
Southern Piedmont region of the United States is situated between the Appalachian
mountains and Atlantic coastal plains and stretches from Alabama to Maryland. It’s
generally rolling terrain if underlain by deeply weathered and highly stratified soil
characterized by relatively shallow argillic Bt horizons while weathered saprolite
can extend tens of meters deep. Although it is a low-relief landscape, headwaters
are often highly dissected with steep narrow valleys containing temporary streams
surrounded by diverse topography. The region represents an ideal opportunity to incorporate
more diverse landscapes into our studies of watershed hydrology.</p><p>As part of
the NSF funded Calhoun Critical Zone Observatory, we intensively instrumented a 6.9
ha headwater (watershed 4, WS4), along with other targeted sensor locations including
discharge in the 322 ha watershed that contains it (Holcombe’s Branch, HLCM), a nearby
meteorological station, a deep groundwater well on a relatively flat interfluve, and
a small network of wells in the buried floodplain. Sensors were continuously monitored
for over 3 years while logging at 5 minute intervals. This sensor network allowed
us to quantify the timing and magnitude of runoff, precipitation, deep and shallow
groundwater levels distributed across a watershed, and soil moisture at multiple depths
and hillslope positions. By doing so we were able to 1) describe the interactions
between water balance components in WS4, 2) compare these watershed-scale measurements
to internal hydrologic dynamics to determine what parts of the watershed are responsible
for distinct watershed functions, and 3) explore how headwaters connect to higher
order streams.</p><p>Using the monthly water balance in WS4, we calculated changes
in integrated watershed storage and then derived a cumulative monthly storage time
series from its running integral. We found that storage changes within the year by
hundreds of millimeters (~25% of annual precipitation) in conjunction with seasonal
peaks in evapotranspiration. Additionally, of all the potential variables that correlated
to runoff magnitudes at the watershed scale, we found storage to be the best, particularly
above a threshold value which remained remarkably consistent across all three years
even with substantial differences in precipitation. </p><p>However, despite the storage
threshold dependence of runoff, when we calculated daily storage we found that while
runoff increased primarily in response to major precipitation events and then decreased
again shortly thereafter, storage primarily wet up once from its low point at the
end of the growing season and then drained starting at the growing season and continuing
through the summer. Similarly, individual measurements of internal watershed hydrology
like soil moisture or water table level displayed either seasonal or event-scale changes.
We determined that measurements taken at watershed positions with more convergent
hillslopes, or farther from the watershed divide, or installed deeper in the soil
are more likely to display seasonal changes, and vice versa for event-scale changes.
These three gradients are essentially proxies for vertical, lateral, and longitudinal
distances, and so it appeared that the underlying gradient being measured was actually
contributing volume. We determined the functions of different landscape components
based on this analysis, and came to understand that storage-linked sites wet up first
and then stay consistently so, making conditions for runoff. Subsequently, when runoff-linked
sites wet-up, they mobilize significant runoff fluxes either by hydraulic displacement,
or interflow, or a transmissivity feedback, or likely some combination of them all.
During these times a substantial portion of the watershed is connected before drying
down again with the exception of more storage-linked locations.</p><p>The result of
this threshold setting followed by large runoff events is extremely flashy outputs
from WS4. In contrast, we found HLCM to be far less flashy and relatively less sensitive
to year to year fluctuations in precipitation. Further, we observed that except in
the most extreme storms, surface flow from WS4 across the former floodplain in between
it and HLCM always fully infiltrates into the sandy, legacy sediments deposited along
the entire former floodplain. These sediments are the legacy of centuries of intensive
and poorly managed agriculture across the Southern Piedmont. Wells in these sediments
revealed a highly dynamic water table that was very responsive to outflow from WS4.
A simple geometric simplification of the shape of these sediments and an estimate
of their porosity revealed that these sediments had ~900 m3 of available storage space,
space that was constantly filling and draining. Interestingly, that available storage
volume level was sufficient to absorb discharge from WS4 on 97% of the days we measured.
Through most of WS4 flow states, this storage served to buffer HLCM from flashier
runoff coming from WS4, and then subsequently releasing it much more slowly and drawn
out as shallow subsurfaceflow. However, when it reached volumes within 15% of maximum,
usually in conjunction with large fluxes coming from WS4, runoff in HLCM reacted closely
with WS4. So the storage volume in legacy sediments serves as an effective buffer
from flashy upstream hydrology, but when the reach or approach saturation they become
effective at transmitting surface flow, likely via saturation excess. Although we
observed this phenomenon in only one alluvial fan, we have reason to think that such
features are quite common locally and regionally, and represent a heretofore underappreciated
legacy of historic agriculture. </p><p>Taken together, these findings describe a hydrologic
system that is much more dynamic than its abundant rainfall and surface water resources
would suggest. Further, they indicate that even a century or more after agricultural
land abandonment and forest regrowth, legacies of the 18th and 19th century remain
in the landforms and soils of the region. We feel that these findings are strong support
for continued and expanded hydrologic study at the CCZO and in the Southern Piedmont
in general.</p>
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