Quantifying long-term changes in gully networks and volumes in dryland environments: The case of Northern Ethiopia

Understanding historical and present gully development is essential when addressing the causes and consequences of land degradation, especially in vulnerable dryland environments. For Northern Ethiopia, several studies exist on the severity of gully erosion, yet few have quantified gully development. In this study, gully network and volume development were quantified over the period 1963–2010 for an area of 123 km2, representing the regional variability in environmental characteristics. Gully networks were mapped from small-scale aerial photographs and high-resolution satellite images. For the latter, visualizing Google Earth images in 3D proved to be very suitable to investigate gully erosion. From the changes in networks and volumes over the period 1963–2010, the occurrence of one cut-and-fill cycle is apparent. From a largely low-dynamic gully system in the 1960s, network expansion and increased erosion rates in the 1980s and 1990s caused the drainage density and volume to peak in 1994. The average gully density (Dtotal) was then 2.52 km km− 2 and the area-specific gully volume (Va) 60 × 103 m3 km− 2. This coincides with soil losses by gully erosion (SLg) of 17.6 t ha− 1 y− 1 over the period 1963–1994. By 2010, improved land management and the region-wide implementation of soil and water conservation measures caused 25% of the gully network to stabilize, resulting in a net infilling of the gully channels over the period 1994–2010. The study validates previous findings that land degradation by gullying was indeed severe in Northern Ethiopia in the second half of the 20th century, but also shows that when proper land management is applied, a gully can be transformed into a linear oasis, which increases the resistance of gullies to further erosion.