Geomorphic process signatures reshaping sub‐humid Mediterranean badlands: 2. Application to 5‐year dataset

Badland landscapes exhibit high erosion rates and represent the main source of fine sediments in some catchments. Advances in high‐resolution topographic methods allow analysis of topographic changes at high temporal and spatial scales. We apply the Mapping Geomorphic Processes in the Environment (MaGPiE) algorithm to infer the main geomorphic process signatures operating in two sub‐humid badlands with contrasting morphometric attributes located in the Southern Pyrenees. By interrogating a 5‐year dataset of seasonal and annual topographic changes, we examine the variability of geomorphic processes at multiple temporal scales. The magnitude of geomorphic processes is linked to landform attributes and meteorological variables. Morphometric differences between both adjacent badlands allow us to analyse the role of landform attributes in the main geomorphic process reshaping landscapes subjected to the same external forcing (i.e. rainfall and temperature). The dominant geomorphic process signatures observed in both badlands are different, despite their close proximity and the same rainfall and temperature regimes. Process signatures determining surface lowering in the gently sloping south‐facing badland, characterized by lower connectivity and more vegetation cover, are driven by surface runoff‐based processes, both diffuse (causing sheet washing) and concentrated (determining cutting and filling, rilling and gullying). The steeper, more connected north‐facing slopes of the other badland are reshaped by means of gravitational processes, with mass wasting dominating topographic changes. In terms of processes determining surface raising, both mass wasting and cutting and filling are most frequently observed in both badlands. There is a clear near‐balanced feedback between both surface‐raising and ‐lowering processes that becomes unbalanced at larger temporal scales due to the thresholds overcome, as the volume associated with surface lowering becomes higher than that associated with raising‐based processes. Rainfall variables control surface flow processes, while those variables associated with low temperature have a significant relation with mass movement‐based processes and other localized processes such as regolith cohesion loss. Finally, our results point out that morphometry (slope and connectivity) together with vegetation cover are key factors determining geomorphic processes and associated topographic changes

‘Local gradient’ and between-site variability of erosion rate on badlands in the Karoo, South Africa

Site-average values of local gradient, defined as the steepest slope angle measured at a point, are a powerful predictor of long-term rates of soil loss as measured by erosion pins on the non-channel floor portions of ten badland study sites in the Karoo area of South Africa. Local gradient may be easily measured using a smartphone clinometer. The successful use of local gradient here is in strong contrast to the previous failure of other site-specific attributes, including other measures of gradient and relief, to explain between-site variation in erosion rate on these study sites. Each measurement of local gradient may be thought of as a sample of the site’s microtopography. Microrelief is a strong determinant of the emergent patterns of inter-channel overland flow, and hence of the patterns of inter-channel erosion by flow. Local gradient changes most rapidly during the initial stages of channel incision. When channels are established, local gradient changes more slowly leading to almost-parallel retreat of channel sidewalls. A sensitivity analysis suggests that measurements of local gradient are not all equal with regard to prediction of long-term erosion rate. A greater share of predictive power is contributed by measurements made on very steep or vertical channel side wall areas, and a lesser share is contributed by measurements made on interfluves

On the continua in two-dimensional nonadiabatic magnetohydrodynamic spectra

The equations for the continuous subspectra of the linear magnetohydrodynamic (MHD) normal modes spectrum of two-dimensional (2D) plasmas are derived in general curvilinear coordinates, taking nonadiabatic effects in the energy equation into account. Previously published derivations of continuous spectra in the 2D ideal MHD case and in 1D nonadiabatic MHD are thus complemented with calculations for 2D nonadiabatic MHD spectra. The nonadiabatic MHD spectrum contains a thermal continuum in addition to the well-known Alfven and slow continua. These thermal continuum modes are exponentially decaying or growing in time. The latter unstable modes are widely accepted as an explanation for the onset of marfes in tokamaks and the most favored hypothesis for prominence formation in the solar corona. A numerical code has been developed to calculate the continuum modes. An investigation of a typical \u27pre-MARFE\u27 (multifaceted asymmetric vadiation from the edge) equilibrium is presented. (C) 2000 American Institute of Physics. [S1070-664X(00)03604-1]