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Runoff and erosion in semiarid woodlands is highly dependent on
ground cover. We view these systems as a set of canopy and
intercanopy patches, which differ in many properties, including
runoff and erosion. At a finer spatial scale, within intercanopy
patches we can differentiate between bare and vegetated patches.
We conducted
field studies to test these hypotheses, in which runoff and
erosion was measured for isolated and connected patches.
Our results
document that most of the runoff is generated in intercanopy
rather than canopy patches, and that within intercanopy patches
most of the runoff is generated in bare patches and redistributed
to canopy patches (Reid 1997; Reid et al. submitted). We are
working on relating these measurements to other data we have
collected at larger spatial scales (Wilcox 1994, Wilcox et al.
1996a, 1996b).
Viewing the
system as a grid of these three patch types (canopy, intercanopy-bare,
and intercanopy-vegetated), we developed a conceptual model based
on percolation theory that explains how patch-scale runoff relates
to larger spatial scales (Davenport et al. 1998). In our model, we
assume that runoff is generated only on bare patches, runs off
only directly downhill or to the side (but not uphill or
diagonally), and is completely absorbed if in encounters a canopy
or intercanopy-vegetated patch.
When the
proportion of intercanopy-bare patches is small, most of the
runoff is redeposited within hillslope, resulting in little
hillslope runoff (low connectivity). However, at a threshold value
the proportion of intercanopy-bare patches is sufficiently high
that the hillslope becomes highly connected and a much greater
proportion of runoff leaves the hillslope.
Consequently,
at some critical value for % intercanopy-bare cover, an erosion
threshold is crossed and the scaling relationship between
small-scale and hillslope scale runoff change dramatically. This
apparently happened as vegetation patterns shifted in response to
a severe drought (Allen and Breshears 1998). These observations
are consistent with a catastrophe model and predictions of the
Universal Soil Loss Equation:
These
results have implications from local to global scales. Locally,
they are relevant to contaminant transport, landfill-cover
integrity, and the protection of cultural resources (e.g.,
archeological sites). Globally, they illustrate the potential
ecosystem consequences that could result from climate variability
and particularly from climate change.
References
Allen, C.
D., and D. D. Breshears. 1998. Drought-induced shift of a
forest/woodland ecotone: rapid landscape response to climate
variation. Proceedings of the National Academy of Science (USA)
95:14839-14842.
Davenport,
D. W., D. D. Breshears, B. P. Wilcox, and C. D. Allen. 1998.
Viewpoint: Sustainability of piņon-juniper woodlands: a unifying
perspective of soil erosion thresholds. Journal of Range
Management 51: 231-240.
Reid, K. D.
1997. Runoff and sediment yield in a semiarid piņon-juniper
woodland, New Mexico. M.S. Thesis. Colorado State University, Fort
Collins.
Wilcox, B.
P., 1994. Runoff and erosion in intercanopy zones of piņon-juniper
woodlands. Journal of Range Management 47:285-295.
Wilcox, B.
P., C. D. Allen, B. D. Newman, K. D. Reid, D. Brandes, J. Pitlick,
and D. W. Davenport. 1996a. Runoff and erosion on the Pajarito
Plateau: observations from the field. Pages 433-439 in New Mexico
Geological Society Guidebook, 47th Field Conference, Jemez
Mountains Region, 1996. New Mexico Geological Society, Soccorro.
Wilcox, B.
P., J. Pitlick, C. D. Allen, and D. W. Davenport. 1996b. Runoff
and erosion from a rapidly eroding pinyon-juniper hillslope. In:
Advances in Hillslope Processes, British Geomorphological Research
Group, 20-22 Sept. 1996.
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