Preprints
https://doi.org/10.5194/esurf-2021-64
https://doi.org/10.5194/esurf-2021-64

  24 Aug 2021

24 Aug 2021

Review status: this preprint is currently under review for the journal ESurf.

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

Liesa Brosens1,2, Benjamin Campforts3, Gerard Govers2, Emilien Aldana-Jague4, Vao Fenotiana Razanamahandry2, Tantely Razafimbelo5, Tovonarivo Rafolisy5, and Liesbet Jacobs2,6 Liesa Brosens et al.
  • 1Research Foundation Flanders (FWO), Egmontstraat 5, 1000 Brussels, Belgium
  • 2Department of Earth and Environmental Sciences, KU Leuven, Leuven, Belgium
  • 3Institute for Arctic and Alpine Research, University of Colorado at Boulder, Boulder, CO
  • 4Earth and Life Institute, Georges Lemaître Centre for Earth and Climate Research, Université Catholique de Louvain, Louvain-la-Neuve, 1348, Belgium
  • 5Laboratoire des Radio Isotopes, Université d’Antananarivo, Antananarivo, Madagascar
  • 6Ecosystem & Landscape Dynamics, Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, Amsterdam, Netherlands

Abstract. Over the past 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, an interferometric SAR mission of the Deutsche Zentrum für Luft- und Raumfahrt providing near-global coverage and high resolution DEMs. Moreover, ongoing developments in unmanned aerial vehicle (UAV) technology has 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 time-consuming 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 12 m TanDEM-X DEM to i) estimate gully volumes, ii) establish an area-volume (A-V) relationship, and iii) determine mobilization rates, through comparison with a high resolution (0.2 m) UAV-SfM DEM and lower resolution (30 m) SRTM DEM. We did this for six study areas in the Lake Alaotra region (central Madagascar) where lavaka (gullies) are omnipresent and lavaka surface area changes over the period 1949–2010s are available for 699 lavaka. SRTM derived lavaka volume estimates were systematically too low, indicating that the SRTM DEM is too coarse to accurately estimate volumes of geomorphic features at the lavaka-scale (100–100 000 m2). 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 that is within the 95 % confidence interval of the UAV-SfM A-V relationship. Our calibrated area-volume relationship enabled us to obtain large-scale lavaka mobilization rates ranging between 18 ± 6 and 289 ± 125 ton ha−1 yr−1 with an average of 102 ± 41 ton ha−1 yr−1. These results indicate that current lavaka mobilization rates are two orders of magnitude higher than long-term erosion rates. With this study we demonstrate that the global TanDEM-X 12 m DEM can be used to accurately estimate volumes of gully-shaped features at the lavaka-scale (100–100 000 m2), 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.

Liesa Brosens et al.

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Liesa Brosens et al.

Data sets

Data for: "Is there an environmental crisis in Madagascar's highlands? Insights from lavaka demographics" Liesa Brosens https://doi.org/10.6084/m9.figshare.c.5236322.v1

Model code and software

Lavaka volumes: Data, PyGIS code and example Liesa Brosens https://doi.org/10.5281/zenodo.5155317

Liesa Brosens et al.

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Short summary
Obtaining accurate information on the volume of geomorphic features typically requires high-resolution topographic data, which is often not available. Here, we show that the globally available 12 m TanDEM-X DEM can be used to accurately estimate gully volumes after applying a correction. This allows to get a first estimate of the amount of sediment that has been mobilized by large gullies (lavaka) in central Madagascar over the past 70 years.