3.1 Results and discussion
3.1.1 Causes of gully formation and development
In the study area, construction of gravel and asphalt roads, footpaths, concentrated surface runoff from farmlands, overgrazing and improper design of soil and water conservation practices were found to be the causes of gully erosion (formation and development). However, the main asphalt and gravel roads passing through the study watershed were found to be the major causes for the formation and development of gullies (Figure 3), which is in line with many research findings, for instance: Montgomery et al. (1994); Wemple et al . (1996); et al . (1996) and Moyerson et al. (2000). In the highlands of Ethiopia, Birhane and Mekonen (2009); Rijkee et al . (2015) and Mekonnen et al. (2015) also reported that construction of roads and footpaths have a major role on the formation and development of gullies. Hence, an appropriate runoff discharge mechanism following roadsides that means stone-pave drainage lines along the roads should be constructed to arrest the problem.
Concentrated surface runoff from farmlands and inappropriately designed soil conservation practices (SWCPs), were found to be the second important causes for gully formation and development, which agreed well with different research results conducted in the northwest highlands of Ethiopia, for example; at Debre Mewi watershed (Tibebu et al.,2010; Mekonnen and Melesse, 2011; Zegeye et al. 2014) and Minizr catchment (Mekonnen et al., 2017). Concentrated surface runoff from farmlands was also an important cause of gully formation and development in the southern part of Ethiopia (Gebreslassie et al., 2014). After their formation gullies serve as soil/sediment transfer pathways and increase runoff connectivity between upstream and downstream parts of the watershed (Mekonnen et al., 2017). Therefore, implementing properly designed SWCPs within fields that can reduce the energy of concentrated surface runoff and can increase water infiltration in the run-on area of gully head watershed is highly important. Constructing check dams that might reduce runoff concentration and disconnect the runoff transfer role of gullies will also be another important solution as recommended by Mekonnen et al (2017) in the northwest highlands of Ethiopia. Moreover, institutional integration is vital that means the road construction authority and ministry of agriculture have to work together. Principally, the road construction authority should construct stone-faced runoff discharging waterways along roads during road construction to discharge the excess runoff collected following roadsides, and the ministry of agriculture will construct check dams within the developed gullies and implement SWCPs within fields, which will enhance water infiltration and reduce surface runoff concentration.
3.1.2. Land use and gully characteristics
Table 1 shows the gully characteristics of different land use/cover types. From the twenty-two gullies formed within the study watershed, three (13.6%) were formed on grazing land, fourteen (63.6%) were formed on cultivated lands and five (22.7%) were formed on both cultivated and grazing lands. Longer and deeper gullies were found on the cultivated lands. The most probable reason will be the nature of the soil that soils in farmlands were deep and can be easily eroded with runoff water at greater depth. From the total length of the investigated gullies, 70,523.89 m (74%) was found on the cultivated lands while 30,224.6 m (26%) was on the grazing lands. The average width of gullies in cultivated and grazing lands was 7.4 m and 9.4 m, respectively. Gully density was found to be 45.3 m ha-1 on grazing lands and 15.2 m ha-1 on cultivated lands. Higher gully density was found on grazing lands than on cultivated land, which was because of the lower area coverage of grazing lands than cultivated lands. Similarly, Belay & Bewket (2013) reported that a large percentage of gullies were located on cultivated and grazing lands.
Almost all of the gullies in grazing land were discontinuous (not actively expanding) because of treatments like area closure and grass plantation, whereas ~70% of the gullies found in cultivated lands were continuous which means gully depth, width and length were increasing in every direction. One important cause for the continuous development of gullies in cultivated lands was that gullies were serving as runoff discharging drainage channels or runoff transfer pathways coming from cultivated land. Farmers’ are leading the runoff from their cultivated land into gullies considering gullies as waterways. Therefore, properly designed waterways within the cultivated lands are essential that will carry and transfer the runoff from cultivated lands to appropriate drainage channels like permanent rivers.
3.1.3. Gully surface area and gully to area ratio
The total length of the 22 gullies was 9,093 m. The longest gully length was 1,299 m and the shortest was 76 m with the mean length of 413 m, which was very long compared with previous findings in Ethiopia. For example, the longest and shortest gullies reported by Osore & Moges (2014) in Alalicha watershed, southern Ethiopia was 427.4 m and 108 m, respectively.
The total surface area occupied by gullies was 100,748.5 m2 (~ 10 ha). This means gully erosion in the area is competing for productive agricultural lands by reducing the farmers’ land size quantitatively. Different studies found that gully erosion is reducing the size of agricultural lands significantly in different parts of Ethiopia. For example, in the northwest highlands of Ethiopia Tibebu et al ( 2010) found ~17.4 ha land loss at Debre Mewi watershed. In the southern part of Ethiopia, Belay and Bewket (2012) reported ~4.7 ha land loss at Bora watershed, Osore & Moges (2014) reported ~2.6 ha land loss in Alalicha watershed. Gully to watershed area ratio was found to be 0.02, which means for every 1000 units of land ~20 units of land was damaged by gully erosion and becoming out of production in the watershed. In the Kilie catchment central highlands of Ethiopia, Woldegiorgis et al. (2007) found 0.136.
3.1.4. Gully density and category
Gully density, total gully length divided by watershed area, was found to be 17.2 m ha-1, which implies that the watershed was severely degraded because gully density between 10 and 25 m ha-1 was categorized as severely degraded (Valentin et al., 2005). Gully density reported by (Gebreslassie et al., 2014) in Huluka watershed, central rift valley of Ethiopia, was 16.1 m ha-1, which agreed well with the finding of this study. In the northern highlands of Ethiopia, different studies found different results, for example; Osore & Moges (2014) found 8.9 m ha-1 in Alalicha watershed, and Belay and Bewket (2013) found 6.7 m ha-1 in Bonda watershed, and Tarekegn et al. (2010) found 67 m ha-1 in Kilie catchment, central highlands of Ethiopia.
Pathak et al. (2006) and Sargeant et al . (1984) classified gullies as small (< 1 m), medium (1-5 m) and large (> 5m) based on depth, and small (<5 m), medium (5-10 m) and large (>10 m) based on length. In this study, in terms of depth 91% of the investigated gullies were medium and 9% were large, and in terms of length, all gullies were under the large category.
3.1.5. Rate of gully development
Based on the measurement of eight gullies in the year 2017 rainy season (June to September 2017), average gully depth was 3.26 m and 3.59 m before and after the rainy season respectively, and gully width was 12.18 m and 12.68 m before and after the rainy season, respectively (Table 2). Within a rainy season, gully depth increased by 0.33 m and gully width increased by 0.5 m.
Figure 4 shows gully depth and width before and after the rainy season and Figure 5 shows the relationship between gully depth and width expansions. Gully depth increment and gully width expansion showed a positive correlation using a linear model. That means as gully depth increases, and gully width also increases (Figure 4) with a correlation coefficient of 0.88 (Figure 5). Gully length, surface area, and volume showed, respectively, 64 m, 2727 m2 and 23,553 m3 increment in a rainy season (Table 5).
3.1.6. Soil loss and land competition
In four decades (~40 years) the volume of soil loss from 22 gullies was 235,532 m3 or 340,956.7 t with surface area coverage of ~100,748.5 m2 or ~10 ha. Within a rainy season from the investigated eight gullies, ~23,553 m3 or 34,387.4 t soil was lost with an average bulk density of 1.46 g cm-3 that means bulk density was found to be 1.34 g cm-3 (upper), 1.46 g cm-3 (middle) and 1.57 g cm-3 (lower) parts of the gully depth. The result shows that ~10 ha of productive land was lost. Loss of land and reduction of crop production were found as the major impacts of gully erosion ( Desta et al., 2012; Yitbarek et al., 2012).
The rate of gully erosion was found to be ~62 t ha-1 yr-1. Different studies found different results, for example, in the semi-arid rift valley of Ethiopia, (Mukai, 2017) found 16·2 t ha -1yr-1; in Debre Mewi watershed, northwest highlands of Ethiopia Tibebu et al. ( 2010) found 530 t ha-1yr-1, and Nyssen et al. (2006) reported 6.2 t ha-1 yr-1. According to Zegeye et al. ( 2014), in Debre Mewi watershed, northwest highlands of Ethiopia, the rate of gully erosion was 127 t ha -1yr-1. In Alalicha watershed, southern Ethiopia Osore and Moges ( 2014) reported ~2.12 t ha-1 yr-1. All the above study results show that the rate of gully erosion was different, which will be due to the difference in land use/cover, rainfall, soil type, etc.
3.1.7. Impact of gully erosion on crop production
The main crop grown in the study watershed was the native crop Teff (Eragrostis teff, E. abyssinica ) with a productivity of 2400 kg ha-1. Taking into account this productivity and the area lost (10 ha) due to gully erosion, on average ~24,000 kg of Teff grain yield was being lost annually. The total grazing land lost was 30,225 m2 and ~14 t animals feed (grass) was lost annually with an average animal forage productivity of 4.5 t ha-1. In the Dangila district, northwestern highlands of Ethiopia, Belay, and Bewket (2012) reported that 46,265 m2 lands were damaged due to gullies, from this, 1,401 m2 was cropland showing 502 kg of crop yield reduction annually. In Alalicha watershed, southern Ethiopia ~25,761 m2 land was a loss due to gullies (Osore and Moges 2014). Such research results indicated that gully erosion is significantly reducing the limited resource that is land and farmers’ income in Ethiopia.
3.1.8. Treatment practices
Gully erosion is becoming a priority agenda in Ethiopia. It is damaging all resources available on land and hence affecting human existence. Based on this study, the investigators recommended possible preventive and remedial measures as follows: (i) Institutional integration-ministry of road construction and agriculture should work together. During constructing roads the ministry of road construction is not bothering about soil erosion/gully erosion occurring because of road construction and it is pushing the responsibility to the ministry of agriculture. Hence their integration is vital; (ii) technical standards of soil and water conservation practices (SWCP) - experts/ professionals should properly put the design/ layout of SWCP since inappropriate designs are leading to surface runoff concentration and hence formation and development of gullies; (iii) immediate action – immediate actions like removing the young gully at its rill stage through plowing is a best remedial measure; (iv) constructing check dams - for large and deep gullies, constructing check dams combining with vegetative practices will help to rehabilitate the gully or at least it helps to stop its development (Mekonnen et al. 2015).
4.1. Conclusion
In this study, even though construction of gravel and asphalt roads, footpaths, concentrated surface runoff, overgrazing and inappropriate design of soil and water conservation practices were the causes of gully formation and development, road construction (both gravel and asphalt) and concentrated surface runoff from farmlands were found to be the main causes. Most of the gullies located in farmlands were active (expanding in all directions/dimensions) compared with the gullies located in grazing lands. Gully formation and development was different for different land use/cover type, which was serious on farmlands than grazing lands.
Gully erosion also played an important role in soil and land losses. About 340, 956.7 t of soil was lost and ~10 ha of land damaged (became out of production). The annual rate of soil loss due to gully erosion was found to be 62 t ha-1 with an average gully density of 16.4 m ha-1. Gully erosion also greatly reduced crop production (Teff grain yield) and livestock forage due to land competition. Farmers are losing ~24,000 kg Teff grain yield and ~14 t animals’ forage annually.
Gully erosion is competing for the agricultural land, reducing farmers’ income affecting crop production and animals forge in the study area. Therefore, properly designed biological and physical soil and water conservation practices, maintenance of roads and properly diverting runoff generated along the roads to nearby natural waterways were found to be possible solutions to rehabilitate the developed gullies and protect new gully formation.
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Table1: Shows the characteristics of the investigated 22 gullies on different land use/cover types at Genbo Wonz watershed, north-west highlands of Ethiopia from 1977 to 2017