Can woody vegetation in valley bottoms protect from gully erosion? Insights using remote sensing data (1938–2016) from subhumid NW Ethiopia

Abstract

Remote sensing data can be used to improve our understanding of environmental change over longer periods. Here, we used a series of orthorectified aerial photographs of 1938–1940 and 1957 and recent satellite images of 2014–2016 to explore trends and patterns of woody vegetation cover in the Lake Tana Basin of northwestern Ethiopia, and to understand how land cover changes influence gully dynamics. Woody vegetation was mapped using a supervised image classification and gully networks were mapped on-screen from randomly chosen sites distributed across the region. Woody vegetation cover has drastically changed over the past 78 years. From an overall cover of about 20% in 1938–1940 and 1957, woody vegetation cover dropped to 9.8% in 2014–2016. Areas that were subjected to agricultural extensification saw woody vegetation cover drop by nearly 70%. Paddock trees, woodlots, and forests have largely been transformed into cropland. Remarkably, most valley bottoms were vegetated in 1938–1957. Concomitant to the woody vegetation loss, gully networks expanded over the study period. From 1938–1957 to 2014–2016, gully density increased from 0.5–1 to 1.9 km km−2. The interrelation between both was most remarkable for valley bottoms, where the loss of woody vegetation has been paralleled by the development of gully networks and multiplication of gully heads. Based on visual observations, we hypothesize that as part of indigenous knowledge on erosion control, past societies may have allowed vegetation regrowth in valley bottoms to protect against erosion.