Levelling

Abstract: Land-levelling measures are widely used in the Souss valley, South Morocco, for the implementation of land use change. However, their impact on soil erosion in this region is unclear. This paper presents the approach of combining punctual process analysis through experimental rainfall simulation and gully mapping as well as volume quantification analysing on a local scale using unmanned aerial vehicle (UAV) remote sensing data. Thus, the influence of the impacts of land levelling in the catchment area on the linear soil erosion by gullies can be elucidated. Soil surface characteristics, modified by land levelling, lead to higher runoff generation and sediment production. Mean runoff coefficients from 54% to 58% are found in levelled study sites, and 38% to 47% are found in undisturbed areas. Mean sediment loads reach 48.6 g m− 2 to 81 g m− 2 under the influence of levelling, but only 9.3 g m− 2 to 23.7 g m− 2 without it. Quantification of soil erosion by UAV data showed that a gully in a levelled study site eroded about 720 m3 of soil within only one rain period. The surface of the catchment area was lowered 0.054 m on average due to land levelling, whereas in study sites without land levelling, the monitoring did not show significant differences of shape and extent of the gullies at two different points in time. The strong influence of land levelling can be documented with the connection of these two methods. A clear amplification of soil erosion is induced by land-levelling measures.

Abstract: A global geopotential model, like EGM2008, is not capable of representing the high-frequency components of Earth’s gravity field. This is known as the omission error. In mountainous terrain, omission errors in EGM2008, even when expanded to degree 2,190, may reach amplitudes of 10 cm and more for height anomalies. The present paper proposes the utilisation of high-resolution residual terrain model (RTM) data for computing estimates of the omission error in rugged terrain. RTM elevations may be constructed as the difference between the SRTM (Shuttle Radar Topography Mission) elevation model and the DTM2006.0 spherical harmonic topographic expansion. Numerical tests, carried out in the German Alps with a precise gravimetric quasigeoid model (GCG05) and GPS/levelling data as references, demonstrate that RTM-based omission error estimates improve EGM2008 height anomaly differences by 10 cm in many cases. The comparisons of EGM2008-only height anomalies and the GCG05 model showed 3.7 cm standard deviation after a bias-fit. Applying RTM omission error estimates to EGM2008 reduces the standard deviation to 1.9 cm which equates to a significant improvement rate of 47%. Using GPS/levelling data strongly corroborates these findings with an improvement rate of 49%. The proposed RTM approach may be of practical value to improve quasigeoid determination in mountainous areas without sufficient regional gravity data coverage, e.g., in parts of Asia, South America or Africa. As a further application, RTM omission error estimates will allow refined validation of global gravity field models like EGM2008 from GPS/levelling data.

Abstract: Land subsidence poses a serious risk to the low-lying coastal city of Bangkok, Thailand; major flooding occurred there in 1983 and again in 2011. Extreme water pumping in the past led to subsidence rates of up to 120 mm year−1. Although water extraction is now controlled, maximum rates measured by levelling today are still up to 20 mm year−1. In this study, we apply interferometric synthetic aperture radar InSAR time-series analysis to study subsidence in Bangkok between October 2005 and March 2010. We validate the InSAR results, by comparing levelling rates and find good agreement between the two techniques. We detect approximately 300,000 coherent pixels overall, with an average density of 120 observations per km2. This is two orders of magnitude greater than the density of levelling benchmarks and reveals subsiding areas that are missed by the levelling network.