DISTINGUISHING SEDIMENT DERIVED FROM SOIL EROSION VERSUS CHANNEL EROSION IN SEDIMENT BUDGETS AND WATERSHED HYDRO-EROSION MODELS

Excessive sedimentation is among the most common causes of impairment of stream ecosystems throughout the world. Determining the relative proportion of fine sediment derived from terrestrial soil erosion processes versus channel erosion processes has proven exceedingly difficult at the scale of large watersheds (>1000 km²). Yet, this distinction is paramount for understanding watershed sediment dynamics and providing actionable information in support of watershed management decisions and restoration actions. Much of the difficulty in constraining the relative amounts of sediment derived from these two sources can be attributed to (1) limitations in measuring or estimating very small rates of erosion over enormous areas, (2) immense variability of those rates in time and space, and (3) complex processes governing sediment storage and transport across the landscape and channel network. In this study we explore fundamental limitations of watershed scale hydro-erosion models to predict a priori the relative proportion of sediment derived from soil erosion versus channel erosion, despite widespread use of such models for that purpose. Specifically, we show that very disparate, yet physically reasonable, model parameterizations can result in similar predictions of sediment flux, suggesting an equifinality problem that cannot be overcome by modeling alone. Additionally, the benefits of incorporating probabilistic approaches to account for variability and uncertainty of both data and model is presented. Similarly, we explore the implications of the variability and uncertainties that are inherent to sediment budgets based exclusively on measurements of morphological change over time. Uncertainties in such estimates often render morphological budgets indeterminate as to the relative proportion of sediment flux derived from channel versus terrestrial sources. We use a variety of real and hypothetical landscapes to explore uncertainties in both approaches and we demonstrate that geochemical tracers, measured and modeled in combination with sediment budgeting and watershed scale modeling, can provide an important, and in some cases necessary, independent constraint on sediment dynamics.