Soil microtopography is a property of critical importance in many earth surface processes but is often difficult to quantify. Advances in computer vision technologies have made image‐based three‐dimensional (3D) reconstruction or Structure‐from‐Motion (SfM) available to many scientists as a low cost alternative to laser‐based systems such as terrestrial laser scanning (TLS). While the performance of SfM at acquiring soil surface microtopography has been extensively compared to that of TLS on bare surfaces, little is known about the impact of vegetation on reconstruction performance. This article evaluates the performance of SfM and TLS technologies at reconstructing soil microtopography on 6 m × 2 m erosion plots with vegetation cover ranging from 0% to 77%. Results show that soil surface occlusion by vegetation was more pronounced with TLS compared to SfM, a consequence of the single viewpoint laser scanning strategy adopted in this study. On the bare soil surface, elevation values estimated with SfM were within 5 mm of those from TLS although long distance deformations were observed with the former technology. As vegetation cover increased, agreement between SfM and TLS slightly degraded but was significantly affected beyond 53% of ground cover. Detailed semivariogram analysis on meter‐square‐scale surface patches showed that TLS and SfM surfaces were very similar even on highly vegetated plots but with fine scale details and the dynamic elevation range smoothed out with SfM. Errors in the TLS data were mainly caused by the distance measurement function of the instrument especially at the fringe of occlusion regions where the laser beam intersected foreground and background features simultaneously. From this study, we conclude that a realistic approach to digitizing soil surface microtopography in field conditions can be implemented by combining strengths of the image‐based method (simplicity and effectiveness at reconstructing soil surface under sparse vegetation) with the high accuracy of TLS‐like technologies. Copyright © 2015 John Wiley & Sons, Ltd.

No abstract

Encroachment of sagebrush (Artemisia spp.) shrublands by pinyon (Pinus spp.) and juniper (Juniperus spp.) conifers (woodland encroachment) induces a shift from biotic-controlled resource retention to abiotic-driven loss of soil resources. This shift is driven by a coarsening of the vegetation structure with increasing dominance of site resources by trees. Competition between the encroaching trees and understory vegetation for limited soil and water resources facilitates extensive bare intercanopy area between trees and concomitant increases in run-off and erosion that, over time,propagate persistence of the shrubland-to-woodland conversion. We evaluated /journal/eco 1 of 28 plant community physiognomy and thereby can reverse the soil erosion feedback on sagebrush shrublands in the later stages of woodland encroachment.

No abstract

No abstract

Field observations and subsequent support from controlled lab experiments have shown the linkage between vertical hydraulic gradients and initiation of rills or incised channels. Nevertheless, current erosion prediction models do not include the hydraulic gradient effect in the erodibility parameters. The objectives of this study were to: (i) quantify the impact of a subsurface hydraulic gradient on rill erodibility and critical shear stress; and (ii) determine the intrinsic critical shear stress of the soil, which is the critical shear stress of the soil without the influence of the vertical hydraulic gradient. Two series of concentrated flow experiments were conducted on a silt loam soil subjected to hydraulic gradients varying from −2 to 2 m m−1 The first series measured soil loss at a given shear stress, while the second was a direct measurement of the critical shear stress by calculating the shear stress at the point of incipient motion of particles. The intrinsic critical shear stress was determined using a fluidized bed approach. We found that the average erodibility under an upward or positive hydraulic gradient was 5.64 times larger than that under a downward hydraulic gradient. The critical shear stress decreased from 1.0 to 0.2 Pa as the hydraulic gradient was increased from −2 to 2 m m−1 Using the fluidized bed approach, we obtained an inherent critical shear stress value of 1.61 Pa. We concluded that the presence of positive pore pressure near the soil surface was a major factor in reducing soil cohesion. We also concluded that with some improvements, the fluidized bed concept can be applied to determine the critical shear stress independent of the hydraulic gradient effect.

Recent developments in low-cost structure-from-motion (SfM) technologies offer new opportunities for geoscientists to acquire high-resolution soil microtopography data at a fraction of the cost of conventional techniques. However, these new methodologies often lack easily accessible error metrics and hence are difficult to evaluate. In this research, a framework was developed to evaluate a SfM approach for soil microtopography measurement through assessment of uncertainty sources and quantification of their potential impact on overall 3D reconstruction. Standard deviations of camera interior orientation parameters estimated from SfM self-calibration within five different soil surface models were several orders of magnitude larger than precisions expected from pattern-based camera calibration. Sensitivity analysis identified the principal point position as the dominant source of calibration-induced uncertainty. Overall, surface elevation values estimated from both technologies were similar in magnitude with a root mean square (RMS) of elevation difference of 0Á2 mm. Nevertheless, the presence of deformation in either SfM or traditional photogrammetric point clouds highlights the importance of quality assurance safeguards (such as a judicious choice of control points) in SfM workflows for soil microtopography applications.

No abstract