Hillslope sediment transport across climates and vegetative influences

Erosion occurs across the globe to varying extents according to the climatic, biological, and anthropological influences present. With the rise of modeling, computing power, and data shar ing, research is beginning to develop in several distinct, challenging directions in terms of being able to represent the likely causes of soil erosion across different contexts. While geologists, geo morphologists, hydrologists, agronomists, and soil geochemists all take an interest in erosion estimates for the sake of varying landscape evolu tion and environmental quality inquiries, debate has carried on over the decades regarding param eter estimation, conceptualization, and scale. Currently, the methods that dominate erosion prediction modeling rely on some form of the empirical relationship captured by the Universal Soil Loss Equation (USLE; or Revised/Modified Universal Soil Loss Equation), which estimates erosion using the experimental plot as the unit of analysis. By dividing erosion into six distinctive parameters based on precipitation, soil erodibil ity, slope length, slope gradient, landcover, and management practice, modeling efforts isolate the challenge as estimating these parameters through proxies given unavailable resources and time to replicate plot experiments that underpin the experimental relationship between the varia bles. Unfortunately, over 40 years of research has shown this endeavor to be problematic and likely functioning outside of the intended pur pose (Boardman, 2006). Wischmeier and Smith (1978) attest to the localized nature of the 30,000 years worth of plotbased data (Stocking, 1995). Rather than relying on these developed rela tionships between the factors studied in the USLE, other authors recommend revisiting the underlying influences that climate can have on the transport of sediment (Kinnell, 2004, 2005, 2010, 2014, 2015). Greater attention to the pro cesses that help conceptualize erosive processes are needed. For instance, beyond the USLE, at the other ends of the empirical spectrum are efforts focusing on the hydrological process representation (larger) or texturebased infil tration processes (smaller), which would then permit inclusion of sediment transport through separation of the flow above the soil and below the soil. Above the soil, if trying to calculate the overland flow that will later be used to esti mate erosion, Burt and McDonnell (2015) issue a wakeup call regarding further use of methods such as the Curve Number, which has 50 years of use and yet is known as something that has been shown to “not work.” Below the soil, the use of pedotransfer functions to determine infiltration or other hydraulic properties that would allow for hydropedological processes to be simu lated in hydrologic and erosion models also encounters challenges. Similar to the USLE, these smaller scale functions encounter issues in their expansion of use beyond the developed context in which they were first formulated, in some cases underestimating infiltration capabili ties by a factor of 10, especially in more diverse climates (Ramírez et al., 2017). McDonnell et al. (2007) recommend not being caught up in the heterogeneity and process complexity but rather moving forward toward classification, defining scaling behavior, and emergent properties. This sentiment can also be found in the works of Klemeš (1983) and Dooge (1986), who discuss searching for laws at the appropriate scale. Recently, this call for scaled process importance has been rearticu lated as serving as the fourth paradigm (Peters Lidard et al., 2017), needing rigorous hypothesis testing (Beven, 2018), and having the potential for integration with Earth System Models (Fan et al., 2019). Many who focus on physicsbased approaches rather than empirical ones are real izing that the smallscale matrixflow laws are continually disappointing efforts at larger basin scales (Beven, 2018). If erosion studies are to advance, especially from hydrological perspec tives, a clearer approach will need to arise that considers other approaches to scaling beyond the todate upscaling and downscaling. What is needed is more aggregation of context specific, climatedependent patterns and behavior from which research questions can be refined and developed further. Richardson et al. (2019) analyze a wide ranging data set from across sites in differ ent parts of North America, Europe, South America, and China to investigate how climate (specifically aridity), lithologies, and measure ment methodology interact with soil movement estimates using a diffusivitylike parameter to describe hillslope sediment flux. Their analysis finds that large controls on the sediment trans port parameter investigated by Culling (1963), D, can be represented through a measure of how wet the climate is below 250 mm and above 500 mm of annual precipitation. Within each of these ranges, however, the movement of soil has several potential influences, notably vegetation and lithology. Richardson et al. (2019) compile a data set spanning middle latitudes in combination with their new estimates of the sediment transport coefficient (D) as calculated through the combi nation of highresolution topographic data with published erosion rates (E ), soil densities (ρs), and bedrock densities (ρr) to contextualize the following equation into varying climate classifications: