Geology Reference
In-Depth Information
Orographic Precipitation and River Profiles
+
-
temperature drops with altitude,
decreasing degree of saturation
hydrometeors are delivered to
the land surface downwind
of their site of formation
delivery of moisture-laden air
at a rate
U with degree of saturation e
lift of air, w, governed by slope
of topography
A
4
Precipitation Pattern
3
V=2 m/s
2
uniform precipitation
1
B
0
temperature only
River Profile
3
C
2
1
0
5
4
3
2
1
0
Distance downstream (km)
Fig. 11.23
One-dimensional models of orographic precipitation.
Changes in bedrock steady-state stream profiles caused by orographic feedbacks. A. Competing effects of slope and
elevation on precipitation. Slope of the topography forces rise of the air mass, promoting precipitation. The
temperature drop with elevation results in reduction of the degree of saturation. Precipitation falls to the surface
downwind of where it condenses. B. Resulting change in precipitation pattern from the far-field rate for temperature
effect only, and for both temperature and slope effects: the latter dominates. Uniform precipitation case shown for
comparison (dashed). C. Resulting steady-state channel profiles for temperature-only and full-feedback model. Rock
uplift rates are spatially steady and are equal for each simulation. The local channel slope needed to accomplish
incision of rising rock mass is everywhere lower in the full model, resulting in a gentler profile and lower total relief
in the channel network. Modified after Roe
et al.
(2002).
the range to a site on the river whose incision
rate we wish to know. Roe
et al
. (2002) show that
this effect results in significantly different slopes
of the rivers draining a range whose precipitation
pattern is strongly controlled by orographic
effects (Fig. 11.23). With the increasing availability
of high-resolution spatial records of rainfall
variation over the past decade (see Bookhagen
and Burbank, 2010), more realistic calculations of
upstream discharge should become more routine.
Anders
et al
. (2008) take this analysis further
and demonstrate the role of the phase of the pre-
cipitation in governing the erosional pattern in
the landscape. As snow falls at roughly 10-fold
slower rates than rain, the snow wafts farther
downwind than rain from its site of nucleation,
allowing snowfall to decouple from the local
slopes that generate the lift in the air mass; and
it falls more diffusely on higher elevations (see
Foster
et al
., 2010). They quantify this effect
using a “delay time” that captures the time it
takes for the precipitation to fall from its site of
nucleation. That this delay time ought to correlate
with the mean annual temperature is corroborated
in measurements (Fig. 11.24A). In models of
landscape evolution in which the phase of the
