4.1 Case 1: Spatially Uniform Decrease in Rainfall
The modelled catchment response to a spatially uniform decrease in
rainfall is characterized by adjustment to uniformly higherksn and an increase in steady state fluvial
relief. For this model run, rainfall is decreased from 2 to 1 m/yr,
causing discharge to decrease similarly by 50% (Movie S1).
Erosion rate decreases across the entire channel network following the
change in rainfall, driven by a decrease in erosional efficiency
directly proportional in magnitude (i.e., 50%; recall m = 1).
The resulting disequilibrium, with E < U , drives
surface uplift and upstream migration of a convex-up slope-break
knickpoint, and eventually a ~40% increase in fluvial
relief.
In ksn–Eavg space (Figure 4a),
the decrease in erosional efficiency driven by the change rainfall
causes the trunk river and tributary network to shift uniformly onto a
different erosional efficiency curve that describes the expected
relationship between ksn and erosion rate for a
given erosional efficiency at steady state. Specifically, both shift
from K=2 ·Kp to K=Kp(Kf = 0.5·Ki ). Following
this initial shift, the trunk river and individual tributaries
approximately follow this new curve (K=Kp ) during
adjustment toward higher erosion rates and ksn to
return to steady state, although in detail they deviate slightly. This
deviation is a result of averaging segments above and below the
migrating knickpoint into mean (upstream-averaged)ksn and erosion rate values. Insofar as the
relationship between channel steepness and erosion rate implies a given
erosional efficiency, these deviations imply an apparent erosional
efficiency different, albeit minor in this case, from the modelled
value.
In ksn-q –Eavg space
(Figure 4b), both the initial and final steady state conditions plot in
the same location on the K=Kp curve. Here, the
initial decrease in erosional efficiency causes the trunk and
tributaries to shift uniformly along this curve to lowerksn-q and erosion rate rather than shifting onto
a different curve. During adjustment both generally follow this curve to
return to steady state, minor transient deviations as seen forksn notwithstanding.