5. Discussion
The central themes we have explored so far are how the spatial rainfall
pattern influences the channel profile morphology, and how temporal
changes in rainfall pattern affect erosion rates and profile morphology
during periods of transient adjustment. We have detailed the expected
response to along-stream variations in erosional efficiency caused by
spatial rainfall gradients according to the SPM and have shown how the
transient response to a change in rainfall pattern is fundamentally
different from a spatially uniform change in rainfall. A change in
rainfall pattern will always result in spatially variable changes of
erosion rates that also change with time during the transient response.
In some circumstances a given location may, over time, experience both
elevated and reduced erosion rates (and channel steepness values)
relative to equilibrium in response to a single change in rainfall. It
is important to keep in mind, however, that the nature of transient
response depends strongly on the initial conditions at the time of the
change in rainfall pattern. Therefore, there is a complex relationship
between the transient response at any given location or time and both
the change in mean rainfall and the final rainfall pattern. In the
following discussion, we focus on highlighting some implications for the
different expectations that follow from changes in rainfall pattern and
discussing examples where conventional expectations based on spatially
uniform changes in rainfall can potentially lead researchers astray.
Where possible, we attempt to identify additional information or
strategies that may be leveraged by future studies.
5.1 Revising Expectations for Erosional and Morphological
Responses to Changing Climate 5.1.1 Relative Nature of Erosional Response
To this point, our choice of a steady state initial condition with
spatially uniform rainfall has been convenient, as have been the terms
top-heavy and bottom-heavy to describe typical orographic rainfall
patterns. While idealized, this provides an intuitive starting point for
understanding how more complicated – but almost certainly more
realistic – climate change scenarios might play out. Recall, according
to the SPM, transient climate-driven changes in erosion rate are
dictated by a relative change in discharge. Where discharge is
increased, erosion rates increase in response and river gradient
declines toward a new equilibrium steepness; thus, a river subjected to
an increase in discharge can be considered locally, if transiently,
oversteepened relative to equilibrium, and vice versa. As we have shown,
because discharge generally accumulates non-linearly downstream within a
river basin, a change in rainfall pattern can create circumstances where
the relative change in discharge inverts along the river length – at
position xsc – producing a complex transient
response (Figures 2 & 3). This implies that the river is simultaneously
oversteepened and understeepened on either side of positionxsc . These transient states dictate whether
erosion rates initially increase or decrease following the change in
rainfall, respectively, not whether the new rainfall pattern is itself
top-heavy or bottom-heavy, and the positions of these transient states
shift throughout adjustment.
The nature of landscape response to relative changes in discharge
implies, for instance, that relaxation of a bottom-heavy rainfall
gradient can cause a complex transient response resembling a change from
uniform to top-heavy rainfall patterns. That is, a weaker bottom-heavy
gradient is relatively top-heavy compared to an extreme bottom-heavy
gradient; similarly, a gentler top-heavy gradient is relatively
bottom-heavy compared to an extreme top-heavy gradient, and vice versa
(Figure 7). Thus, for example, in the case of a change in climate that
causes an extreme bottom-heavy rainfall gradient to become less
bottom-heavy and results in a complex transient response (e.g.,
Figure 7a), rainfall and erosion rate are expected to increase in the
headwaters of the catchment and decrease near the outlet as seen for
Case 4 (uniform to top-heavy). This response is not consistent with
expectations for any uniform increase or decrease in rainfall, even if
such a shift accurately reflects the change in mean rainfall. Therefore,
neither the final rainfall pattern alone (i.e., modern observed pattern)
nor accurate inference about the relative change in mean rainfall
(wetter or drier) necessarily allow a robust prediction of changes in
erosion rate within a catchment following a change in climate where
rainfall patterns have changed significantly.
Interestingly, changes in climate do not need to involve extreme changes
in rainfall patterns (e.g., reversal from top-heavy to bottom-heavy), or
to occur over short timescales to drive complex transient responses.
Indeed, even subtle changes in rainfall pattern potentially driven by
minor, commonly occurring variations temperature and atmospheric
conditions (e.g., Mutz et al., 2018; Roe et al., 2003; Siler & Roe,
2014), may induce complex responses and significantly, if temporarily,
alter the spatial pattern of erosion in a catchment (Figures 3 & 7).
Indeed, such climate changes may have occurred in the Peruvian Andes and
eastern-central Himalaya in the transition from Pliocene to Pleistocene
climates, the latter represented by Last Glacial Maximum conditions
(LGM; Figure 7c & 7d). Even if rivers in each of these ranges were in a
transient state during Pliocene time, any adjustment toward equilibrium
with the Pliocene rainfall pattern that occurred would then be in
disequilibrium with the Pleistocene (LGM) rainfall pattern, and would
have driven a complex response.
As transient adjustments proceed relatively more rapidly where rainfall
is more concentrated (i.e., erosional efficiency is higher), changes in
rainfall pattern have the potential to produce spatially distinct
effects different from what would be expected from considering uniform
changes in mean climate. Transient adjustments may therefore be
relatively enhanced or underdeveloped in different locations within the
same catchment, or adjustment to quasi-equilibrium may be essentially
complete in some locations while others reflect only an incipient
response to the climate change. We noted an example of this behavior in
Case 4, where low-elevation dry tributary catchments preserve transient
conditions the longest, contrasting with the notion that headwater
catchments should be the last to equilibrate. Similarly segregated
conditions occur in Case 3, where adjustment to quasi-equilibrium is
essentially complete in wet low-elevation catchments long before the
migrating trunk knickpoint even reaches drier high-elevation catchments.
Because such complex, climate change-driven landscape adjustments are
not reasonably captured by a conceptual framework based on spatially
uniform changes in rainfall (e.g., compare Cases 3 & 4 to Cases 1 &
2), apparent inconsistencies between expectations and observations have
the potential to give a false impression about the primary forcing(s)
controlling erosion rates.
Additionally, if large-scale changes in rainfall patterns like we model
develop incrementally over long timescales (e.g., millions of years),
they could still result in complex transient responses.
Greenhouse-icehouse transitions and orogenic growth are among many
geologically significant events that may cause temporally distinct,
sustained, and dramatic changes to climate and/or circulation patterns
where complex responses could arise (Mutz et al., 2018; Poulsen et al.,
2010; Roe et al., 2003; Zachos et al., 2001), If, for example, the
bottom-heavy gradient in Case 3 instead develops over several million
years, regardless any added complexity to the general trajectory of this
change in rainfall pattern, the result is that it supports a 30%
increase fluvial relief despite also increasing total rainfall by
~80%, and channel steepness patterns fundamentally
change as the catchment adjusts. Gradual changes in rainfall patterns
cause morphological adjustments to become more diffuse, and induce
relatively smaller transient changes in erosion rate than abrupt
changes. However, the spatial pattern of erosion is still significantly
affected (i.e., in excess of a factor or two from steady state) so long
as the timescale over which the rainfall pattern evolves does not far
exceed that of the catchment adjustment timescale, which may be several
million years for large river basins (Roe et al., 2003; Whipple, 2001).
As such, the general characteristics of the classes of transient
behavior following changes in rainfall pattern toward relativelybottom-heavy or top-heavy conditions remain intact, even for long-term
transient responses.