Bedrock erosion by root fracture and tree throw: A coupled biogeomorphic model to explore the humped soil production function and the persistence of hillslope soils

In 1877, G. K. Gilbert reasoned that bedrock erosion is maximized under an intermediate soil thickness and declines as soils become thinner or thicker. Subsequent analyses of this “humped” functional relationship proposed that thin soils are unstable and that perturbations in soil thickness would lead to runaway thinning or thickening of the soil. To explore this issue, we developed a numerical model that simulates the physical weathering of bedrock by root fracture and tree throw. The coupled biogeomorphic model combines data on conifer population dynamics, rootwad volumes, tree throw frequency, and soil creep from the Pacific Northwest (USA). Although not hardwired into the model, a humped relationship emerges between bedrock erosion and soil thickness. The magnitudes of the predicted bedrock erosion rates and their functional dependency on soil thickness are consistent with independent field measurements from a coniferous landscape in the region. Imposed perturbations of soil erosion during model runs demonstrate that where bedrock weathering is episodic and localized, hillslope soils do not exhibit runaway thinning or thickening. The pit-and-mound topography created by tree throw produces an uneven distribution of soil thicknesses across a hillslope; thus, although episodes of increased erosion can lead to temporary soil thinning and even the exposure of bedrock patches, local areas of thick soils remain. These soil patches provide habitat for trees and serve as nucleation points for renewed bedrock erosion and soil production. Model results also suggest that where tree throw is a dominant weathering process, the initial mantling of bedrock is not only a vertical process but also a lateral process: soil mounds created by tree throw flatten over time, spreading soil over bedrock surfaces

A stochastic sediment supply model for a semi-arid landscape

We investigated the surficial processes that deliver sediment from hillslopes into channels in a mediterranean landscape and determined how these processes are controlled by climate and landscape characteristics (e.g. topography, vegetation). Watersheds in semi-arid regions of the United States are subject to a variety of disturbances, including grazing, fires, and vegetation cover changes and, while the consequences of sediment loading are well documented, presently our ability to predict the spatial and temporal pattern of delivery is poor. The processes that we have investigated are: shallow landslides, overland flow, dry ravel, and bioturbation. Through field experiments and monitoring, we have developed physically-based transport equations for each individual process. These transport equations are used as the governing equations for a computer model, driven by random sequences of rainstorms and fires, that predicts the spatial and temporal patterns of sediment delivery. As downstream problems become more closely linked to watershed conditions and perturbations, this type of fundamental research is relevant to issues such as the health of riverine ecosystems and the siltation of reservoirs. We anticipate that our fieldwork and modeling results will help land managers estimate the influx of sediment from surface processes according to different land-use practices

Frequency-magnitude distribution of debris flows compiled from global data, and comparison with post-fire debris flows in the western U.S.

Forecasting debris flow hazard is challenging due to the episodic occurrence of debris flows in response to stochastic precipitation and, in some areas, wildfires. In order to facilitate hazard assessment, we have gathered available records of debris flow volumes into the first comprehensive global catalog of debris flows (n = 988). We also present results of field collection of recent debris flows (n = 77) in the northern Rocky Mountains, where debris flow frequency increases following wildfire. As a first step in parameterizing hazard models, we use frequency–magnitude distributions and empirical cumulative distribution functions (ECDFs) to compare volumes of post-fire debris flows to non-fire-related debris flows. The ECDF of post-fire debris flow volumes is significantly different (at 95% confidence) from that of non-fire-related debris flows, suggesting that the post-fire distribution is composed of a higher proportion of small events than that of non-fire-related debris flows. The slope of the frequency–magnitude distribution of post-fire debris flows is steeper than that of non-fire-related debris flows, corroborating evidence that small post-fire debris flows occur with a higher relative frequency than non-fire-related debris flows. Taken together, the statistical analyses suggest that post-fire debris flows come from a different population than non-fire-related debris flows, and their hazard must be modeled separately. We propose two possible non-exclusive explanations for the fact that the post-fire environment produces a higher proportion of small debris flows: 1) following fires, smaller storms or effective drainage areas can trigger debris flows due to increased runoff and/or decreases in root strength, resulting in smaller volumes and increased probability of failure, and 2) fire increases the probability and frequency of debris flows, causing their distribution to shift toward smaller events due to limitations in sediment supply

Prediction of sediment-bound nutrient delivery from semi-arid California watersheds

Soil carbon (C), nitrogen (N), and phosphorus (P) are lost from hillslopes in particulate forms through soil erosion. The fate of the eroded C (e.g., sequestration or oxidation) may affect the global C budget, and delivery of N and P to waterbodies can lead to eutrophication. Whereas the magnitude of particulate nutrient losses may be similar to or greater than dissolved losses, it is rarely estimated. We couple a sediment delivery model with measurements of C, N, and P in soil to account explicitly for hillslope sediment transport processes that yield sediment-bound nutrients to fluvial networks. The model is applied to a site in California dominated by coastal sage scrub and gopher-rich grasslands. Although the magnitude of sediment delivery predicted by the model has been tested with reservoir sedimentation records, no data exist to test the predicted rates of nutrient delivery. Nevertheless, the model results are provocative; it predicts that losses of particulate C from sage covered hillslopes (23 kg/ha/yr) are nearly double that from grassland hillslopes (13 kg/ha/yr), despite a lower annual sediment yield from the sage hillslopes. The model predicts similar average annual N and P losses for sage and grasslands but dramatic differences in the frequency and magnitude of delivery events. Nutrient delivery from grasslands is chronic whereas delivery from the coastal sage is highly episodic, with large pulses driven by fire frequency. These results suggest that changes in the vegetation community can alter the delivery regime of sediment-bound C, N, and P

A stochastic sediment delivery model for a steep Mediterranean landscape

It is a truism in geomorphology that climatic events operate on a landscape to drive sediment transport processes, yet few investigations have formally linked climate and terrain characteristics with geomorphological processes. In this study, we incorporate sediment transport equations derived from fieldwork into a computer model that predicts the delivery of sediment from hillslopes in a steep Mediterranean landscape near Santa Barbara, California. The sediment transport equations are driven by rainstorms and fires that are stochastically generated from probability distributions. The model is used to compare the rates and processes of sediment delivery under two vegetation types: coastal sage scrub and grasslands. Conversion of vegetation from sage to exotic grasses is a common land management strategy in the region and may also be engendered by regional climate change due to global warming. Results from the model suggest that (1) approximately 40% more sediment is delivered from grasslands (98 t km−2 yr−1) than the sage scrub (71 t km−2 yr−1) and (2) chronic soil creep processes dominate under grasslands whereas catastrophic processes dominate under coastal sage scrub. Results from the model also suggest that changes in the spatial distribution of vegetation arising from climate change will have a greater effect on sediment delivery than changes in the magnitude and frequency of meteorological events

Modern erosion rates in the High Himalayas of Nepal

Current theories regarding the connections and feedbacks between surface and tectonic processes are predicated on the assumption that higher rainfall causes more rapid erosion. To test this assumption in a tectonically active landscape, a network of 10 river monitoring stations was established in the High Himalayas of central Nepal across a steep rainfall gradient. Suspended sediment flux was calculated from sampled suspended sediment concentrations and discharge rating curves. Accounting for solute and bedload contributions, the suspended sediment fluxes were used to calculate watershed-scale erosion rates that were then compared to monsoon precipitation and specific discharge. We find that, in individual watersheds, annual erosion rates increase with runoff. In addition, our data suggest average erosion rate increases with discharge and precipitation across the entire field site such that the wetter southern watersheds are eroding faster than the drier northern watersheds. The spatially non-uniform contemporary erosion rates documented here are at odds with other studies that have found spatially uniform long-term rates (105–106 yr) across the pronounced rainfall gradient observed in the region. The discrepancy between the modern rates measured here and the long-term rates may be reconciled by considering the high erosional efficiency of glaciers. The northern catchments were much more extensively glacierized during the Pleistocene, and therefore, they likely experienced erosion rates that were significantly higher than the modern rates. We propose that, in the northern watersheds, the high rates of erosion during periods of glaciation compensate for the low rates during interglacials to produce a time-averaged rate comparable to the landslide-dominated southern catchments