Search for: All records

Climate change and unsustainable land management practices have resulted in extensive soil degradation, including alteration of soil structure (i.e., aggregate and pore size distributions), loss of soil organic carbon, and reduction of water and nutrient holding capacities. Although soil structure, hydrologic processes, and biogeochemical fluxes are tightly linked, their interaction is often unaccounted for in current ecohydrological, hydrological and terrestrial biosphere models. For more holistic predictions of soil hydrological and biogeochemical cycles, models need to incorporate soil structure and macroporosity dynamics, whether in a natural or agricultural ecosystem. Here, we present a theoretical framework that couples soil hydrologic processes and soil microbial activity to soil organic carbon dynamics through the dynamics of soil structure. In particular, we link the Millennial model for soil carbon dynamics, which explicitly models the formation and breakdown of soil aggregates, to a recent parameterization of the soil water retention and hydraulic conductivity curves and to solute and O₂diffusivities to soil microsites based on soil macroporosity. To illustrate the significance of incorporating the dynamics of soil structure, we apply the framework to a case study in which soil and vegetation recover over time from agricultural practices. The new framework enables more holistic predictions of the effects of climate change and land management practices on coupled soil hydrological and biogeochemical cycles.

Incipient valley formation in mountainous landscapes is often associated with their presence at a regular spacing under diverse hydroclimatic forcings. Here we provide a formal linear stability theory for a landscape evolution model representing the action of tectonic uplift, diffusive soil creep, and detachment‐limited fluvial erosion. For configurations dominated by only one horizontal length scale, a single dimensionless quantity characterizes the overall system dynamics based on model parameters and boundary conditions. The stability analysis is conducted for smooth and symmetric hillslopes along a long mountain ridge to study the impact of the erosion law form on regular first‐order valley formation. The results provide the critical condition when smooth landscapes become unstable and give rise to a characteristic length scale for incipient valleys, which is related to the scaling exponents that couple fluvial erosion to the specific drainage area and the local slope. The valley spacing at first instability is uniquely related to the ratio of the scaling exponents and widens with an increase in this ratio. We find compelling evidence of sediment transport by diffusive creep and fluvial erosion coupled with the specific drainage area equation as a sufficient mechanism for first‐order valley formation. We finally show that the predictions of the linear stability analysis conform with the results of numerical simulations for different degrees of nonlinearity in the erosion law and agree well with topographic data from a natural landscape.

The objective of this comment is to correct two sets of statements in Litwin et al. (2022,https://doi.org/10.1029/2021JF006239), which consider our research work (Bonetti et al., 2018,https://doi.org/10.1098/rspa.2017.0693; Bonetti et al., 2020,https://doi.org/10.1073/pnas.1911817117). We clarify here that (a) the specific contributing area is defined in the limit of an infinitesimal contour length instead of the product of a reference contour width (Bonetti et al., 2018,https://doi.org/10.1098/rspa.2017.0693), and (b) not all solutions obtained from the minimalist landscape evolution model of Bonetti et al. (2020,https://doi.org/10.1073/pnas.1911817117) are rescaled copies of each other. We take this opportunity to demonstrate that the boundary conditions impact the obtained solutions, which has not been considered in the dimensional analysis of Litwin et al. (2022,https://doi.org/10.1029/2021JF006239). We clarify this point by using dimensional analysis and numerical simulations for a square domain, where only one horizontal length scale (the side lengthl) enters the physical law.

Intensive agricultural land use can have detrimental effects on landscape properties, greatly accelerating soil erosion, with consequent fertility loss and reduced agricultural potential. To quantify the effects of such erosional processes on hillslope morphology and gain insight into the underlying dynamics, we use a twofold approach. First, a statistical analysis of topographical features is conducted, with a focus on slope and gradient distributions. The accelerated soil erosion is shown to be fingerprinted in the distribution tails, which provide a clear statistical signature of this human‐induced land modification. Theoretical solutions are then derived for the hillslope morphology and the associated creep and runoff erosion fluxes, allowing us to distinguish between the main erosional mechanisms operating in disturbed and undisturbed areas. We focus our application on the landscape at the Calhoun Critical Zone Observatory in the US Southern Piedmont, where severe soil erosion followed intensive cotton cultivation, resulting in highly eroded and gullied hillslopes. The observed differences in hillslope morphologies in disturbed and undisturbed areas are shown to be related to the disruption of the natural balance between soil creep and runoff erosion. The relaxation time required for the disturbed hillslopes to reach a quasi‐equilibrium condition is also investigated. © 2019 John Wiley & Sons, Ltd.