Gully erosion monitoring in semi-arid landscapes

The objective of this paper is to present an overview and first results of a research project on gully monitoring which employs large-scale aerial photography taken from remote-controlled platforms. The resulting high-resolution images (pixel sizes < 10 cm) are employed for photogrammetric and GIS analysis in order to quantify gully development with linear, areal and volumetric measurements at various sites in semi-arid landscapes of Spain and Africa. Linear headcut retreat rates for 12 gullies were analysed in order to investigate their relation to patterns of runoff and infiltration behaviour in the gully headcut surroundings. Retreat rates for South Moroccan sites (0-0.31 m a(-1)) were found to be lower than those obtained for Spanish gullies (0.07-0.51 m a(-1)); however, by far the highest maximum retreat was observed in the West-African Sahel (3.16-9.85 m a(-1)). Infiltration measurements, runoff coefficients and erosion rates show differing ranges but always high variability for the study regions. Ranges of minimum and maximum values for the soil erosion parameters result in the same ranking of study regions as ranges for maximum linear headcut retreat, indicating a clear association between runoff behaviour and gully headcut retreat with respect to their spatio-temporal variability. 2D change quantification with detailed maps derived from the large-scale aerial photography provides additional information on the differences in headcut retreat behaviour which cannot be described by linear measures, illustrating the benefits of high-resolution aerial photography for monitoring and understanding gully erosion processes.