Evaluating land degradation by gully erosion through soil erosion indices and rainfall thresholds

Abstract

Among the land degradation processes, gully erosion is the one that poses more environmental and societal challenges in arid regions. Predicting spatio-temporal gully development in a region under changing conditions is important to adopt proper mitigation measures. Here we investigate the Ghapan-Olya watershed in Golestan province in Iran, which is impacted by many erosional landforms, including gully phenomena. We applied the pixel-based distributed LANDPLANER model, to predict the location of rainfall induced gullies. Different input maps were prepared using UAV data, the region's soil, and seasonal land use information. Our modelling results from the application of topographic thresholds and the erosion index calculation were compared with eld observations in locations with and without gullies and the application of quantitative metrics such as sensitivity, speci city, fallout, precision, and recall. Our study revealed that the spatial density of the gully's location is more repetitious in the areas with altitude about 200-300m, steeper slope (between 15 to 30 degrees), and lower average accumulation value (< 100) in the south-east facing slope. Higher values of erosion index and topographic threshold were obtained for the min curve number, where intense rainfall is more frequent and where land use and cover conditions are more favourable for gully occurrence. The greater values of soil erosion indices were provided for the CN3 scenario (autumn) when the daily rainfall is 80 mm (6.27), continued by the summer scenario (CN2) with 80 mm daily rainfall (4.88), and spring (CN1) again with 80 mm daily rainfall (2.99).In addition, topographic threshold maps illustrate the greatest amount of soil erosion for the CN scenario (without considering daily rainfall) in autumn. Our results provide accurate prediction parameters to simulate the natural hazard under changing conditions.