5.4 Caveats and Limitations
Several important caveats should be kept in mind when evaluating the
results and subsequent discussion presented here. First, and foremost,
our modelling efforts explicitly assume that landscapes are inherently
sensitive to climate (through rainfall) in a manner described by the
SPM. While the intuitive support for such sensitivity is strong, and
evidence from natural landscapes is mounting that broadly support
predictions of the SPM (e.g., Adams, et al., 2020; Harel et al., 2016;
Lague, 2014), there remains a large amount of uncertainty about the
strength of the sensitivity to climate.
Following the core assumption that the SPM is broadly applicable, we
note that our model setup is very simple. We assume that all
precipitation is rainfall, all rainfall is converted directly to runoff,
and we impose constant rainfall gradients that act precisely along the
trunk stream and basin axis and that span the entire length of our
modelled river basin. While some river basins set within mountain-belt
scale orographic precipitation patterns may indeed experience rainfall
patterns consistent with this simple geometry, we note that non-linear,
and even non-monotonic, rainfall gradients are common for large river
basins or those characterized by high local relief (Roe, 2005). As noted
previously, however, the framework we have developed translates well to
these more complex scenarios. In addition, we model tributary catchments
that are uniform in size that experience uniform rainfall. Preliminary
model investigations suggest that although allowing tributary sizes and
rainfall to vary causes some dispersion in the relationships illustrated
here, the overall behavior we describe remains the dominant signal.
Nevertheless, both aspects warrant further investigation.
Lastly, we emphasize that we use a simple version of the SPM where,
among other simplifications, we treat erosion exclusively as detachment
limited and do not explicitly model erosional thresholds. While assuming
continuum detachment-limited conditions is common, particularly for
describing erosion in mountain settings, it may not always be
appropriate even in these settings to describe transient behavior
(Lague, 2014; Whipple & Tucker, 2002). Related to this, our treatment
of Kp , while incrementally more complex than
spatially uniform erosional efficiency, remains highly simplified. We do
not explore the myriad of other factors that control erosional
efficiency (K ), and the likelihood that rainfall, or more broadly
climate, will influence rock erodibility
(~Kp ) and size distribution of
sediment delivered to rivers (Ferrier et al., 2013; Murphy et al., 2016;
Neely & DiBiase, 2020; Riebe et al., 2015; Sklar et al., 2017).