Object-based image analysis (OBIA)-based gully erosion dynamics, sediment loading rate and sediment yield study in Lake Hawassa Sub-basin, Ethiopia

Abstract

Gully erosion is a serious environmental issue in Lake Hawassa Sub-basin. It has affected a large portion of the catchment, and a large amount of sediment has been transported into the lake. This condition needs to be addressed. However, before conservation techniques are implemented, the gully systems should be mapped, their extent and changes over time need to be measured, and their capacity to produce sediment from their catchments should be determined. The objective of this study were, therefore, to map and analyse the change in gully erosion and quantify the sediment loading rate and sediment yield from the active and connected gullies in Lake Hawassa Sub-basin using object-based image analysis (OBIA) with high-resolution SPOT 5 satellite image of the years 2011 and 2020. This method was integrated with field observation for mapping gully features of the study area, in comparison with automatic digitization carried out with the help of eCogenition Developer Version 9.1 and ArcGIS tools. Overall accuracy and Kappa coefficient were determined and were found to be 85.2% and 0.81, respectively, for the image of year 2020 and 81.1% and 0.76, respectively, for the image of year 2011. Based on the OBIA method, the extent of gullies (in area) were found to be 63.5 km2 in 2011 and 79.9 km2 showing a rapid increase between 2011 and 2020 (an increase of 16.4 km2 (24.4%) in the 10 years considered). The later result shows that 5.53% of the area of the Lake Hawassa Sub-basin is affected by gully erosion. The maximum gully density in the study area was found to be 589 km/km2 in 2011 and this increased to 884 km/km2 in 2020. The sediment loading rate from the Lake's catchment was found to be in the range of 12.62 to 38.59 ton per hectare per year. The sediment yield from the Lake's catchment was 8.83 to 27.02 t/ha/year. The total annual volume-based sediment yield at the Lake generated from the gully was 2.39 million cubic meter considering sediment delivery ratio of 70% for fully connected gullies. This result shows that 0.21% of the storage capacity of the Lake was being lost due to sedimentation from the gully system every year. From the result by dividing the total volume of the sediment by the surface area of the lake, one can see that a silt thickness of 2.51 cm was being deposited in the Lake every year.