Mapping and quantifying the rate of soil and gully erosion in the dry areas of Jordan

Abstract

The arid and semi-arid regions of Jordan, also known as “Badia”, are said to be increasingly degraded and soil erosion creating large gullies has become a common phenomenon. There are concerns that the rate of gully erosion has increased during the last decades due to enhanced desertification. Bedouin people in the Jordan Badia depend on herding sheep, and enhanced desertification hinders grazing and reduces a key source of income. The Jordanian government deemed it a priority to develop the Badia and reduce the damage of existing gully erosion. The objective of this research is to determine how gullies are initiated and what their progression speed is in the Jordan Badia. A gully growth time lapse of the Wadi al Wala region inside the Jordan Badia has been made in Google Earth Pro and Google Earth Engine to determine gully growth over a period of 15 years. Rangeland Hydrology and Erosion Model (RHEM) is an event-based prediction model for runoff and water erosion that was used for surface runoff quantification. Cipoletti weirs were in addition used to directly measure runoff after events. A finite element model (Hydrus 2D) was used to simulate two-dimensional water movement inside gully channel walls. Initiation of gullies was connected to water velocity and volume, as these factors determined soil detachment and transport. The correlation between critical slope and drainage area determined gully head boundaries but were influenced by obstacles and phenomena on the hillslope. The growth speed was different inside the Wadi al Wala catchment. The lower catchment had an average growth of 6.1%, the middle catchment 4.05% and the high catchment 1.1% in 15 years. The growth did not increase gradually, as the largest rainfall events consisted often for >54% of yearly rainfall, which resulted in enhanced gully growth.