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.