1.1 Introduction
Gully erosion is one of the major global problems confronting agricultural production. Although the problem is as old as the agricultural practices, its extent and adverse impact on crop production, land competition, groundwater depletion, etc. are getting worse year after year. In Africa ~29 million ha of land was affected by gully erosion (Hurni et al., 2010) and productivity of agricultural lands had significantly declined (Osore & Moges, 2014; Mekonnen et al. , 2015; Rijkee, et al ., 2015; Mukai, 2017).
Gully erosion is the most prevalent form of soil erosion in Ethiopia, which dissects farmlands, impedes tillage operations, damaged agricultural and residential areas, and restricts the free movement of animals and humans (Daba et al., 2003; Moges & Holden, 2008; Bewket & Teferi, 2009; Mekonnen et al., 2015). In the Ethiopian highlands, gullies covered ~7.6 million ha ( Poesenet al., 2006; Frankl et al ., 2012) which severely affected soil productivity, reduced land size, damaged roads, and buildings (Daba et al., 2003; Avni, 2005; Nyssen et al., 2006; Rijkee et al., 2015), specifically estimated gully surface area coverage as 1.7 million m2 over 30 years in the eastern highlands of Ethiopia. The rate of gully erosion was reported as 6.2 t ha-1 y-1 for northern Ethiopia (Nyssen et al., 2006) and the annual volume/mass of soil eroded only from four gullies was 1941.3 m3 (2717.8 t) in the north-west highlands of Ethiopia( Mekonnen et al., 2017).
In the north-west highlands of Ethiopia, gully erosion has also been identified as a major source of sediment for man-made reservoirs and natural lakes, which affects the water holding capacity of such water reservoirs and hence agricultural productivity by reducing irrigation water availability (Mekonnen et al., 2017) and it significantly reduces both grazing lands ( Tebebu et al., 2010; Mekonnen et al ., 2015), effective soil depth and groundwater availability thus hindering the free movement of moisture within the soil system (Moges & Holden, 2007) and the sizes of farmlands ( Poesen et al., 2003; Mekonnen & Melesse, 2011; Yitbarek et al ., 2012).
As part of the north-west highlands of Ethiopia, in this study area, Genbo Wonz watershed gully erosion is a priority problem and there was no study conducted to investigate the cause, rate of expansion and its impacts on crop production, except the district agricultural office reports explaining the severity (YDARDO, 2015). Since the causes of gully erosion are site-specific, many researchers recommended site-specific investigation on the causes of gully formation and development, its impacts in reducing agricultural lands and crop production( Tebebu et al., 2010; Zegeye, et al ., 2014; M. Mekonnen et al., 2015; Rijkee et al., 2015). Moreover, YDARDO (2015) reported the necessity of clear causal investigation to implement that could help to implement appropriate cause-based interventions. Therefore, the objectives of this study at Genbo Wonz watershed in the northwest highlands of Ethiopia were to; (i) identify site-specific real causes of gully formation and development for appropriate cause-based intervention planning and treatments; (ii) estimate long-term soil loss because of gully erosion, (iii) quantify land loss resulting from gully erosion and assess its adverse impact on crop production.