Comparative analysis of the Copernicus, TanDEM-X, and UAV-SfM digital elevation models to estimate lavaka (gully) volumes and mobilization rates in the Lake Alaotra region (Madagascar)

Liesa Brosens, Benjamin Campforts, Gerard Govers, Emilien Aldana-Jague, Vao Fenotiana Razanamahandry, Tantely Razafimbelo, Tovonarivo Rafolisy, Liesbet Jacobs

Research output: Contribution to JournalArticleAcademicpeer-review

Abstract

© 2022 Liesa Brosens et al.Over the past few decades, developments in remote sensing have resulted in an ever-growing availability of topographic information on a global scale. A recent development is TanDEM-X (TerraSAR-X add-on for digital elevation measurements), an interferometric synthetic aperture radar (SAR) mission of the Deutsche Zentrum für Luft- und Raumfahrt, providing near-global coverage and 12m resolution digital elevation models (DEMs). Moreover, ongoing developments in uncrewed aerial vehicle (UAV) technology have enabled acquisitions of topographic information at a sub-meter resolution. Although UAV products are generally preferred for volume assessments of geomorphic features, their acquisition remains a time-consuming task and is spatially constrained. However, some applications in geomorphology, such as the estimation of regional or national erosion quantities of specific landforms, require data over large areas. TanDEM-X data can be applied at such scales, but this raises the question of how much accuracy is lost because of the lower spatial resolution. Here, we evaluated the performance of the 12m TanDEM-X DEM to (i) estimate gully volume, (ii) establish an area-volume relationship, and (iii) determine mobilization rates through comparison with a higher-resolution (0.2m) UAV structure-from-motion (SfM) DEM and a lower resolution (30m) Copernicus DEM. We did this for six study areas in the Lake Alaotra region (central Madagascar), where lavaka (gullies) are omnipresent and surface area changes over the period 1949-2010s are available for 699 lavaka. Copernicus-derived lavaka volume estimates were systematically too low, indicating that the Copernicus DEM is too coarse to accurately estimate volumes of geomorphic features at the lavaka scale (100-105m2). Lavaka volumes obtained from TanDEM-X were similar to UAV-SfM volumes for the largest features, whereas the volumes of smaller features were generally underestimated. To deal with this bias we introduce a breakpoint analysis to eliminate volume reconstructions that suffer from processing errors as evidenced by significant fractions of negative volumes. This elimination allowed the establishment of an area-volume relationship for the TanDEM-X data with fitted coefficients within the 95% confidence interval of the UAV-SfM relationship. Our calibrated area-volume relationship enabled us to obtain large-scale lavaka mobilization rates ranging between 18±3 and 311±82tha-1yr-1 for the six different study areas, with an average of 108±26tha-1yr-1 for the full dataset. These results indicate that current lavaka mobilization rates are 2 orders of magnitude higher than long-term erosion rates. With this study we demonstrate that the global TanDEM-X 12m DEM can be used to accurately estimate volumes of gully-shaped features at the lavaka scale (100-105m2), where the proposed breakpoint method can be applied without requiring the availability of a higher-resolution DEM. Furthermore, we use this information to make a first assessment of regional lavaka erosion rates in the central highlands of Madagascar.
Original languageEnglish
Pages (from-to)209-227
JournalEarth Surface Dynamics
Volume10
Issue number2
DOIs
Publication statusPublished - 14 Mar 2022
Externally publishedYes

Funding

Financial support. This research has been supported by YouReCa, the Fonds Wetenschappelijk Onderzoek (grant nos. 11B6921N, 12Z6518N, V436719N, and V436319N). Acknowledgements. This research would not have been possible without the help of our local partners at Laboratoire des RadioIsotopes and the local authorities of the Ambatondrazaka and Ampara-faravola district and the authorization from the Aviation Civil de Madagascar to fly the UAV over their territories. Kristof Van Oost’s expertise was crucial for obtaining high-quality UAV DEMs. Ny Ri-avo Voarintsoa, Rónadh Cox, and Steven Bouillon are wholeheartedly thanked for their insights and inspiring discussions on this topic. This research is part of the MaLESA project funded by a KU Leuven Special Research Fund grant (An integrated approach for assessing environmental changes in soil-covered landscapes: the case of Madagascar). We are very grateful for the constructive feedback provided by Benjamin Purinton, Amaury Frankl, and Rónadh Cox, which has drastically improved the methodological rigour and quality of this paper.

FundersFunder number
Fonds Wetenschappelijk OnderzoekV436719N, 11B6921N, V436319N, 12Z6518N
KU Leuven

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