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).