4.3.1 Case 3: Trunk Response
In this case, because the change in rainfall is spatially variable along
the trunk river, the initial change in erosion rate is also variable.
The trunk river experiences an approximately 80% increase in erosion
rate at the outlet (E > U ), and a decrease of 67%
in the headwaters (E < U ), corresponding to the change
in upstream average rainfall along its length. Atxsc (located ~27 km upstream from
the outlet), Qf = Qi ,Sf = Si , and so
immediately following the change in rainfall E = U .
Enhanced incision at the outlet produces a concave-up knickpoint;
however, as this knickpoint migrates upstream it progressively sharpens
and eventually evolves into an oversteepened convex-up knickpoint,
contrasting with expectations for the increase in rainfall (e.g. ,
Case 2). Oversteepening is a consequence of the upstream decrease in
erosional efficiency driven by the rainfall gradient that is exacerbated
by, but does not depend on, differing modes of adjustment upstream and
downstream of xsc related to the complex
response. This is analogous to knickpoint behavior described by Forte et
al. (2016) and Darling et al. (2020) where modelled lithologic contacts
demarcate similar relative variations in erosional efficiency (i.e. hard
rocks over soft rocks). Lastly, we note that everywhere upstream ofxsc over-adjusts during the transient response,
which is a characteristic of complex responses in general, leading to
variable modes of adjustment in time and space. The overadjustment we
observe is essentially the whiplash response described by Gasparini et
al. (2006, 2007), but notably results here without sediment flux. This
continuous evolution of the trunk knickpoint has important consequences
for signals passed to tributaries.