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Earth Surface Processes and Landforms Assessment of gully erosion 31, rates Earth Surf. Process. Landforms 167–185 (2006) Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/esp.1317

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Assessment of gully erosion rates through interviews and measurements: a case study from northern Ethiopia Jan Nyssen,1,2* Jean Poesen,1 Maude Veyret-Picot,1 Jan Moeyersons,3 Mitiku Haile,2 Jozef Deckers,4 Joke Dewit,1 Jozef Naudts,5 Kassa Teka2,6 and Gerard Govers1 1 2 3 4 5 6

Physical and Regional Geography, K.U. Leuven, Redingenstraat 16, B-3000 Leuven, Belgium Department of Land Resource Management and Environmental Protection, Mekelle University, PO Box 231, Mekelle, Ethiopia Royal Museum for Central Africa, B-3080 Tervuren, Belgium Institute for Land and Water Management, K.U. Leuven, Vital Decosterstraat 102, B-3000 Leuven, Belgium ADCS Food Security Project, PO Box 163, Adigrat, Ethiopia St. Mary’s School, PO Box 12, Wukro, Ethiopia

*Correspondence to: J. Nyssen, Institute for Land and Water Management, K.U. Leuven, B3000 Leuven, Belgium. E-mail: [email protected]

Received 5 May 2004; Revised 6 August 2005; Accepted 11 August 2005

Abstract Gullying has been widespread in the Ethiopian Highlands during the 20th century. It threatens the soil resource, lowers crop yields in intergully areas through enhanced drainage and desiccation, and aggravates flooding and reservoir siltation. Knowing the age and rates of gully development during the last few decades will help explain the reasons for current land degradation. In the absence of historical written or photographic documentation, the AGERTIM method (Assessment of Gully Erosion Rates Through Interviews and Measurements) has been developed. It comprises measurements of contemporary gully volumes, monitoring of gully evolution over several years and semi-structured interview techniques. Gully erosion rates in the Dogu’a Tembien District, Tigray, Ethiopia, were estimated in three representative case-study areas. In Dingilet, gullying started around 1965 after gradual environmental changes (removal of vegetation from cropland in the catchment and eucalyptus plantation in the valley bottom); rill-like incisions grew into a gully, which increased rapidly in the drier period between 1977 and 1990. The estimated evolution of the total gully volume in the other areas show patterns similar to those of the Dingilet gully. Average gully erosion rate over the last 50 years is 6·2 t ha−1 a−1. Since 1995, no new gullies have developed in the study area. Area-specific shortterm gully erosion rates are now on average 1·1 t ha−1 a−1. The successful application of the AGERTIM method requires an understanding of the geomorphology of the study area and an integration of the researchers with the rural society. It reveals that rapid gully development in the study area is some 50 years old and is mainly caused by human-induced environmental degradation. Under the present-day conditions of ‘normal’ rain and catchment-wide soil and water conservation, gully erosion rates are decreasing. Copyright © 2006 John Wiley & Sons, Ltd. Keywords: gully; Ethiopia; eucalyptus; Vertisol; AGERTIM; land use; watershed management

Introduction Gullies are a common feature throughout the Ethiopian Highlands. Induced environmental degradation comprises not only the loss of soil volume and of arable land, the triggering of landslides (Nyssen et al., 2002a) or off-site sedimentation problems (Nigussie et al., 2005), but also the creation of efficient links between uplands and valley bottoms, through which runoff and sediment are evacuated. Furthermore, gullies enhance desiccation of the land and decreased crop yield (Van Damme, 2004) and contribute to the lowering of the ground water table in the surrounding areas (Moeyersons, 2001): field observations reveal that with the deepening of gullies, the position of springs in gully beds is lowered. Gullies have been defined as recently developed drainage lines of ephemeral streams with steep banks and a nearly vertical gully head (Poesen et al., 2003) and are distinguished from rills by the former technology-related notion of the Copyright © 2006 John Wiley & Sons, Ltd.

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Figure 1. The study areas for gully erosion rates in the northern Ethiopian Highlands. Contour interval is 200 m.

impossibility of being obliterated by ‘conventional’ tillage operations (FAO, 1965), or by a critical cross-section of one square foot (930 cm2) (Poesen et al., 1996). The phenomenon of gully development is not restricted to the Highlands of Ethiopia but seems to be a phenomenon on subcontinental scale in Africa (Moeyersons, 2000, 2001). Gully systems in Ethiopia can often be considered as discontinuous ephemeral streams (as defined by Bull, 1997), comprising a hillslope gully, an alluvial–colluvial cone at the foot of the hill and renewed incision with gully head formation further downslope in the valley. Pediments dissected by gullies are common in many areas (Riché and Ségalen, 1971; Ogbaghebriel and Brancaccio, 1993; Ogbaghebriel et al., 1997). In valley bottoms, initial gully heads often coincide with sinking polygonal structures in Vertisols (Nyssen et al., 2000a). In Tigray, which is the northernmost and driest region of the Ethiopian Highlands (Figure 1), the change in hydrological behaviour of catchments has been attributed to an overall lowering of the infiltration capacity of the soil, due to depletion of the vegetation (Hunting, 1974; Virgo and Munro, 1978; Machado et al., 1998). Brancaccio et al. (1997) explain present-day processes of channel incision by the increased erosive power of runoff enhanced by a smaller sediment load, associated with the advanced soil erosion on the hillslopes where bedrock increasingly crops out. Active gullying induced by road building on pediments in Ethiopia is described in Ogbaghebriel and Brancaccio (1993). In a case study along the Mekelle–Adwa road, built in 1994, Nyssen et al. (2002b), analysed how road building, through the enlargement of drainage areas and runoff concentration, induces an artificial passing of the critical catchment size at which gully heads are formed for a given slope gradient. Assessments of gully erosion volumes and rates in Ethiopia are scarce. Field measurements in other parts of the world have shown that the development of a gully network is fast at the beginning and then gradually slows down (Graf, 1977; Rutherfurd et al., 1997; Nachtergaele et al., 2002). Furthermore, Nachtergaele et al. (2002) observed that after the first years of active gully development, vegetation developed on the gully bottom, which led to a reduction of runoff velocity and to sediment deposition, hence a decrease in gully volume from a certain moment onwards. Such an evolution is not possible in most gullies in Ethiopia, which are used as rangeland or cattle tracks. However, in Ethiopia, sediment deposition and gully stabilization take place after the building of gully control structures such as check dams (Nyssen et al., 2004a). Historical documents describing the development of gullies in the Ethiopian Highlands are completely lacking. Aerial photographs are generally at a scale of 1:50 000, which does not allow the recognition of gullies, let alone the measurement of their dimensions. Nor can their evolution be assessed. Simple, short interviews with occasionally Copyright © 2006 John Wiley & Sons, Ltd.


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Figure 2. Gully system draining the Argak’a catchment; note the subhorizontal structural relief. This figure is available in colour online at www.interscience.wiley.com/journal/espl

encountered people generally yield unreliable data, for a multitude of reasons. Two options remain open to gain more insight into gully development rates: (1) direct monitoring of gully volume evolution in the field; and (2) adaptation of existing interview techniques to our specific need of gaining information on the age, the development and the controlling factors of gullies. Therefore, the objectives of this study are (1) to develop a methodology for gully erosion rate assessment over the last decades in the absence of written or photographic information, (2) to study the long- and short-term rates of gully development in the Ethiopian Highlands, and (3) to correlate this evolution with changes in environmental characteristics.

Study Methods Study area The study area is located in the uplands of the Geba catchment (Tigray Highlands), near Hagere Selam (13°40′ N, 39°10′ E), the main town of the Dogu’a Tembien district (Figure 1), at elevations between 2200 and 2750 m a.s.l. Local geological formations comprise Mesozoic Antalo limestone and Amba Aradam sandstone and Tertiary basalt flows with interbedded silicified lake deposits. They form subhorizontal layers and give rise to stepped slope profiles (Figure 2). There are also Quaternary formations, consisting of alluvium, colluvium and travertine. Around 65 per cent of the study area is arable land. The agricultural system in the northern Ethiopian Highlands has been characterized as a ‘grain-plough complex’ (Westphal, 1975). The main crops are barley (Hordeum vulgare L.), wheat (Triticum sp.) and tef (Eragrostis tef ), an endemic cereal crop. Various species of pulses constitute another important part of the crop rotation. Soil tillage is carried out with ox-drawn ard ploughs. Steep slopes (>0·3 m m−1) are mainly under rangeland, parts of which have been set aside recently to allow vegetation recovery (exclosures). In some flat areas, crop cultivation was abandoned to increase the grazing area. In many cases, especially if there is risk of gullying, the thalweg is occupied by rangeland. Mean annual rainfall is 750 mm, concentrated in three months (mid-June to mid-September). Rain is particularly erosive, due to its large drop sizes (Nyssen et al., 2005). Rainfall and runoff on soils, which have lost most of their natural vegetation by century-long human activity, cause serious soil erosion. The predominance of moderate to steep slopes induces natural vulnerability to sheet and rill erosion, despite the high clay content and high rock fragment cover, causing generally low soil erodibility (Nyssen, 2001).

Monitoring of gully erosion Four gully systems, located in different environments, and representative of the Tigray Highlands in terms of dimensions and location in the landscape, were studied in detail (Table I): (1) the gully in the village of Dingilet, (2) the Copyright © 2006 John Wiley & Sons, Ltd.


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Table I. Studied gully systems in the Dogu’a Tembien highlands

Location Dingilet Argak’a Kuliheni Ziban Khunale Harena/ Khunale† May Leiba Agerba

Number of gully segments

Drainage area (ha)

Type of measurement*

Period of measurement/monitoring

14 20 7 15

108 156 14 9

G G G

1999–2001 1999–2001 1999–2001 2000–2001

59 157 60

647 1788 264

G g g G

2001 2002 2002

Study Veyret-Picot, 2001 Nyssen, 2001 Nyssen, 2001 Nyssen, 2001 Nyssen, 2001 De Wit, 2003 Naudts, 2002; Kassa, 2003

* G, AGERTIM (detailed monitoring, several cross-sections, several years, detailed interviews); g, ‘rapid’ AGERTIM † Area including the four gullies above

Figure 3. Example of a measured gully cross-section (‘Dingilet 10’ in 2000). The gully has a width (w) of 6·05 m, a depth (d) of 3·59 m and the area (A) of the cross-section is 15·63 m2. Dots indicate the slope breaks measured by theodolite.

Argak’a gully system (Figure 2), (3) the Kuliheni gully which was induced by building the Mekelle–Adwa main road, and (4) the Ziban Khunale gully, in a Vertisol area, which started incising after a large part of the catchment was changed from arable to overgrazed rangeland. In addition, gully volumes (V, in m3) were measured in three wider areas, covering a total of 2853 ha: V = Σ Li Ai

(1)

where Li = length of considered gully segment (m) and Ai = representative cross-sectional area of the gully segment (m2). Field monitoring involved the detailed measurement, at yearly intervals, of gully cross-sections marked by paint on rocks and trees on the banks. Each cross-section was representative of a gully segment (≤50 m long). Earlier measurements in 1998 by measuring tape were not taken into account, since they suffered not only from lack of accuracy, but also from the difficulty of calculating volumes as the shape of most gully cross-sections cannot simply be generalized to a triangle or a rectangle (Figure 3). Measurements by theodolite proved to be much more accurate. For this purpose, in March 1999, 2000 and 2001, at each gully section, a heavy electric cable was lowered into the gully between corresponding marks on both banks, to represent a straight transect line. The position of each break of slope along the cable was recorded. Surveyed cross-sections from the three subsequent years were superposed, and the marks fitted. The area of corresponding cross-sections (A) as well as their greatest depth and horizontal width was then calculated using MapInfo software (Figure 3).

Interview techniques The long-term development of the gully in Dingilet was studied through interviews. This gully is located near the village and crossed by some intensively used footpaths. First interviews with some of the farmers invariably resulted Copyright © 2006 John Wiley & Sons, Ltd.



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Figure 4. Group discussion on gully development in Dingilet. The presence of tension cracks (middle), here used as a scale model for the gully (left), might explain why a narrowing of the gully was measured in some places. This figure is available in colour online at www.interscience.wiley.com/journal/espl

in the statement ‘when I was a child, there was no gully here; it was a green marshy area’. Such statements also came from younger people, who reported the collective perception of the history of this area, rather than their own experience. Showers (1996) developed the HEIA (Historical Environmental Impact Assessment) method, based on interviews and historical reference documents. Our method not only complements such historical evidence with measurements of the short-term evolution, but aims also at improving the interview techniques. Since individuals, for a variety of reasons, can give imprecise answers, it seemed most suited to adapt existing interview and especially Participatory Rural Appraisal (PRA) techniques (Bryceson et al., 1981; Young and Hinton, 1996). In borrowing some techniques from PRA, we worked in close contact with the farmers, and methods and attitudes closely followed the recommendations of PRA. The adjective ‘participatory’ cannot, however, be given to the method we developed, since it is essentially a methodology set up by the researcher for the aims of the research without two-way reciprocity. The method included the following steps: (i) construction of a base map of the area, (ii) walks with key informants, (iii) semi-structured group interviews and (iv) the reconstruction of a time line. Depending on the spatial area to be covered, this method was applied with a varying level of detail. For larger areas, a rapid assessment was made, whereby the area was visited with one or two key informants and the age of the various gully segments established through a series of targeted questions. Some gullies were studied in more detail. First, a base map of the area, based on an existing DTM, was constructed. This map included the main geographical features and was enhanced during two walks with key informants. During these half-day walks along the gully and in its wide surroundings, the informants were invited to locate and name a maximum number of points along the studied gully (e.g. crossing places, confluences, large trees) and to talk about the gully and all interesting phenomena related to it, such as interrupted footpaths, terraces in the gully bank, stones brought to cross a previously muddy area. A tentative age was already given for these phenomena by the key informant. The gullied area was then visited with three age groups (farmers aged around 30, 40 and 50 years), composed of four or five people (Figure 4). The situation in the area when they were young, and observed events that interfered with people’s daily lives (e.g. building of a bridge over the gully) were discussed in a semi-structured way, with respect to characteristics related to the event (‘how deep was the gully?’). For relative and absolute timing, the farmers’ estimated age at that moment, as well as an event calendar (Table II) were used. It was easier to discuss the time line with the age groups in this way, than going through a boring matrix ranking exercise (Young and Hinton, 1996), which becomes impossible when there are more than five events to rank. Possible contradictions were discussed to a certain extent and the results of the three age groups cross-checked, and controlled with additional informants. Events related to the gully, from which approximate dimensions could be deduced, were combined with dimensions measured by theodolite. Sequences of cross-sections and longitudinal profiles of the gully were reconstructed. Copyright © 2006 John Wiley & Sons, Ltd.


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J. Nyssen et al. Table II. Event calendar used in the interviews Year

Event

1990 1989 1988

Downfall of the Derg government Last important mobilization for the guerilla war Land reform

1983

Extreme drought

1978

Country-wide repression (Key Shebir)

1975 1974

Alphabetisation campaign by Zamatcha Downfall of Haile Selassie I

Table III. Examples of questions asked during semi-directed interviews with individual farmers (‘rapid AGERTIM method’) • • • •

Introductory questions (age, location of his home and his land) Did the gully exist in this place when you were a child/a shepherd/when you started ploughing? Did the gully exist during the 1978/1991 land reform? (it would then have been used as a plot boundary) Any other informations he can give about the gully segment and its evolution

The farmers’ perceptions of the causes of gullying were also noted, which gave hints for further investigations, especially on changing land use in the catchment. Care was taken not to abuse the people’s willingness to talk and therefore interview sessions were limited to a maximum duration of 3 h. Young and Hinton (1996) insist on the necessity of carefully planning such research: our plan was written down and discussed in advance among researchers. This method, referred to as AGERTIM (Assessment of Gully Erosion Rates Through Interviews and Measurements), allows assessment of long-term gully extension. In combination with precise measurements of short-term changes, the evolution of approximate gully dimensions since incision can be reconstructed and gully erosion rates calculated: RL = V Bd/ T C

(2)

RS = (V − V0)Bd/ T C

(3)

where RL = area-specific long-term gully erosion rate (t ha−1 a−1), RS = area-specific short-term gully erosion rate (t ha−1 a−1), V = total gully volume (m3), V0 = initial gully volume, at the beginning of the considered time span (m3), T = time span considered (years), Bd = soil bulk density, 1·2 t m−3 (Nyssen et al., 2000b) and C = catchment area (ha).

Results Harena-Khunale area In this area, four gullies were monitored in detail, and overall, the AGERTIM method was applied in a ‘rapid’ way (Table III). Throughout the observation period (1999–2001), width and plan area of the four monitored gullies continued to increase. The evolution of the total gully volumes (Table IV) shows a decrease in total volume during some years in some gullies. In many sections gully depth decreases, i.e. there is aggradation, which could often be related to check dam building. As a consequence, the width:depth ratio of the monitored gullies increased from an average of 3·0 to 3·4 between 1999 and 2001 (Figure 5). The larger width:depth ratio of the Argak’a gully is probably due to its age (‘very old’, according to our informants), as compared to Kuliheni (6 years) and Dingilet (35 years). Where present, gully heads continued to retreat (Ziban Khunale, Kuliheni; Figure 6), increasing the length of the corresponding gullies. The Ziban Khunale gully as well as the lower segment of the Kuliheni gully are situated in Vertisol areas. The measured volume does not include subsurface tunnels and pipes, which have been discussed elsewhere (Nyssen et al., 2000a). Though their volume is probably much less than that of the gullies (maximum length of pipes is around 30 m), the existence of pipes might have led to an underestimation of gully development in some cases. Copyright © 2006 John Wiley & Sons, Ltd.



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Table IV. Evolution of plan area and volume of the monitored gullies Gully plan area (m2) Year

Kuliheni

Argaka

Dingilet

1994 1999 2000 2001

0 1072 1139 1209

10 192 10 398 10 701

6954 7066 7401

Gully volume (m3) Ziban Khunale 0 425 463

Year

Kuliheni

Argaka

Dingilet

1994 1999 2000 2001

0 2079 2057 2218

22 805 22 922 23 877

23 053 23 377 22 940

Ziban Khunale 0 499 558

Figure 5. Gully width versus depth for the monitored gullies between 1999 (full trendlines) and 2001 (dotted trendlines); r 2 ranges between 0·39 and 0·77.

Figure 6. Retreating gully head in Kuliheni. Since the collapse of the stone bund at the gully head, the middle branch is draining most of the runoff and has fully captured the side branches. Copyright © 2006 John Wiley & Sons, Ltd.


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J. Nyssen et al. Table V. Long- and short-term rates of gully erosion in the Dogu’a Tembien highlands

Location

Total volume at last monitoring (m3)

Year of first gully incision

RL (t ha−1 a−1)

RS (t ha−1 a−1)

17 052 23 877 2218 558 126 864 714 857 34 106

1965 around 1951 1994 1995 around 1951 around 1937 around 1935

5·0 3·7 27·0 13·0 4·7 7·4 2·3

−4·6‡ 4·1 6·0 8·0 nd nd nd

6·2

1·1

(2853)

(287)

Dingilet* Argak’a Kuliheni Ziban Khunale Harena/Khunale† May Leiba Agerba Area-weighted average rate (area considered, in ha)

RL, Area-specific long-term rate (since incision); RS, area-specific short-term rate (1999–2001); nd, no data * Ten sections out of 14 were considered † Wide area, including the four gullies above ‡ Net sediment deposition

Table VI. Age of the studied gully segments in the Harena-Khunale area, as estimated by local informants Age

n

‘Old’ (30–60 years) 1941–1950 1951–1960 1961–1970 1971–1980 1981–1990 1991–2000 Total

38 4 1 3 3 9 1 59

The total length of all gullies in the Harena-Khunale area is c. 12 691 m, with a total volume of 126 864 m3 (Table V). For 59 gully segments, the approximate age was obtained from local informants (Table VI). Most gullies were considered ‘old’ – at least older than our informants (30–60 years old) – but gully generation continued throughout the second half of the 20th century. This information allowed the age of gully development in this catchment to be estimated at about 50 years, which leads to an estimated average soil loss rate by gully erosion (RL) of 4·7 t ha−1 a−1 for the Harena-Khunale area.

May Leiba catchment In this 18 km2 catchment, the AGERTIM method was applied in a ‘rapid’ way (Table III). Total length of all gullies in the May Leiba catchment is 31·4 km (De Wit, 2003), with a total volume of 714 857 m3 (Table V). For 157 gully segments, the approximate age was obtained from local informants (De Wit, 2003) and ranged between 4 and 65 years. Summing the rates of individual gullies allowed estimation of average RL at 7·4 t ha−1 a−1 for the May Leiba catchment. For our study area, this value is on the high side, which is probably related to the larger catchment size, producing more runoff.

Agerba The total length of the five gullies in Agerba is 11 707 m (Kassa, 2003), with a total volume of 34 106 m3 (Table V). For 60 gully segments, the approximate age was obtained through group discussions (Kassa, 2003). This information Copyright © 2006 John Wiley & Sons, Ltd.



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Figure 7. Evolution of gully volume in Agerba village.

Figure 8. Longitudinal profiles of soil surface and of gully bottom levels since 1965 in Dingilet, based on interviews and measurements. Vertical exaggeration is 10×. A local base level, composed of large boulders in the gully bed, explains the less deep gully at section 10.

allowed the start of gully development around Agerba to be estimated at 67 years, and led to an estimated average RL of 2·3 t ha−1 a−1. Figure 7 represents the historical evolution of the summed volumes of the five gullies in the village of Agerba.

Long-term evolution of the gully in Dingilet The valley bottom in Dingilet is used as common grazing land and is densely planted with eucalyptus trees. Several footpaths cross the 5–8 m deep gully, in which some temporary springs are located, witness that the gully drains the water table. Results of the group discussions (Figure 4) are reported by Veyret-Picot (2001); only a short overview of the most remarkable events related to the development of this gully will be given here. According to all of the farmers interviewed, before 1965 the valley bottom in Dingilet was a marshy area, with water present throughout the year. Near section 4 (Figure 8), many large stones are embedded in the topsoil near the gully. They were brought in before 1956, before gullying started, to allow the crossing of a muddy area during the rainy season. A small pond existed 20 m upslope, which was used as a cattle drinking place; its disappearance Copyright © 2006 John Wiley & Sons, Ltd.


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Figure 9. Cross-section 12 of the gully in Dingilet, and its evolution (distances in m). Solid line represents measured topography; dashed lines represent tentative cross-sections, as derived from interview sessions. Note the remnants of stability levels and the aloe fence on the right bank. This figure is available in colour online at www.interscience.wiley.com/journal/espl

indicates the incision by a gully between 1977 and 1987. Gullying would have started here from 1972 onwards. At section 6, 100 m upslope, stones were put in the gully bottom to build a crossing place in a shallow gully in 1972; this crossing place was washed away around 1977. Fifty metres upslope, the thalweg was crossed by a major footpath to the village of Ruwaksa; if a shallow gully could be jumped in 1971, it was necessary to descend into the gully and to climb out at the other side in 1978. When the gully became too deep at this place (cross-section 7), around 1984, a bridge was built, some 100 m upslope, at a place where the gully dimensions were smaller. The gully was already deep enough at that place for a man to walk under the bridge. One or two years later, the gully size increased also at this place and the bridge collapsed. Over the next 200 m, there are signs of short periods of stability: small terraces in the gully bank show the position of the gully bed around 1978 and 1988. The age of aloe shrubs, planted along the gully bed at that time to prevent cattle from entering nearby land, helped to date these ‘terraces’ (Figure 9). Several events were related to a second small pond in the marshy valley bottom at cross-section 14: a girl drowned there before 1973, an old man drowned in shallow water in 1980, and the pond emptied after it was tapped by the regressive gully head around 1982. All interviews indicate that gully incision started in the Vertisol area near sections 5 and 6, between 1960 and 1970. Dates obtained from the interviews have been integrated in a chronosequence of longitudinal profiles of the studied gully over 500 m, i.e. between sections 4 and 14 (Figure 8). Gully segments between cross-sections 1 and 4 did not evolve very much. At the upper end, the gully size gradually decreases, to reach the foot of an active debris fan, some 300 m upslope of cross-section 14. From Figure 8, it appears that the development of this gully is not linear over time. The most important gully incision took place around the period 1977–1981, as evidenced by the results of the interviews at sections 6, 7, 9 and 11. Field monitoring (1999–2001) showed that the rate of incision also varied along the gully: at some sections aggradation was observed, while others showed deepening during that period. Copyright © 2006 John Wiley & Sons, Ltd.



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Figure 10. Evolution of Kuliheni and Ziban Khunale gully volumes versus time since incision (t).

Discussion Causes of gully development Sudden environmental changes, leading to larger runoff response to rainfall, could be recognized in Kuliheni and Ziban Khunale in the early 1990s, shortly before gully incision started. The Kuliheni gully was incised very rapidly after the building of the Mekelle–Adwa road (Figure 10), due to improved runoff concentration and spectacular increase of the drainage area (from 0·1 to 8·6 ha), both of which result in increased stream power during storms (Nyssen et al., 2002b). In Ziban Khunale (Figure 10), the upper part of the drainage area, with a slope gradient of 0·1–0·15 m m−1, was converted from arable to intensively used rangeland, reportedly because of a lack of grazing land in the nearby village of Khunale. Besides this land use change, gully incision was also enhanced by the presence of Vertisols, which are more sensitive to gullying than other soils, due to their swell–shrink properties (Nyssen et al., 2000a). Aerial photo interpretation of the watershed draining towards the Argak’a gully (Naudts, 2001) also shows important changes in land use: the area of land with poor or no vegetation increased from 16 per cent in 1963 to 20 per cent in 1994. In addition, many shrubs and trees located within arable land disappeared. The area covered by dense vegetation decreased, from 10·1 to 4·0 per cent between 1963 and 1994. Due to exclosure policy, this area increased again to 5·6 per cent in 2000. In Dingilet and Agerba, the major causes of gullying stressed by the interviewed farmers were changes in land use. On most arable land in the catchments, Rumex nervosus and other shrubs, as well as uncultivated strips of land, which were omnipresent in between the fields until 1960 and formed obstacles to runoff, gradually disappeared. Only on some 30 per cent of the arable land in Dingilet are there still shrubs between cultivated lands (Figure 11). On the steep basalt slope, which separates the lower and the upper part of this 108 ha catchment, marginal fields in between rangeland were abandoned in the 1980s and turned into overgrazed rangeland; the entire slope became an exclosure, planted with eucalyptus and other trees in 1997. In the upper part of the catchment, belonging to the hamlet of Haddish Addi, bare rock crops out in the fields. A farmer in his forties told us: ‘Since I received this plot, 10 years ago, its length decreased by 2 m due to the increase of rock outcrop. All fields in this area have the same problem. Look my neighbour: his land is now cut into two by a rock outcrop; that wasn’t the case when he received it. However, I didn’t see, nor hear from the old people, that arable land once covered the whole area. These rock outcrops have always been here, but their extent was less’. On several of these rock outcrops plough marks are present, indicating the presence of soil and arable land in former times. In this area, there are several fields with soil some 10 cm thick over bare rock. Due to small water storage capacity, there is much rilling, down to the bedrock. Such land is expected to become another rock outcrop within a few years. In the upper part of the catchment, bare rock outcrop was estimated to occupy 25 per cent of the area; in addition, in 10 per cent of the area there are shallow soils (Leptosols) over bedrock. If we accept that rock outcropping started some generations (say 100 years) ago, this means a yearly increase by 0·25 per cent of the bare rock area. Accordingly, at least till 1997, the year of large-scale conversions to exclosures, there has been a gradual increase in runoff response. Copyright © 2006 John Wiley & Sons, Ltd.


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Figure 11. Sketch map of the Dingilet gully catchment. 1, Catchment of the studied gully, with major land units in 2001; 2, valley bottom with rangeland and eucalyptus trees; 3, dispersed village with arable land, gardens, eucalyptus trees and fences; 4, arable land; 5, arable land with many shrubs; 6, steep slope, exclosure since 1997 with eucalyptus plantations; 7, contour lines (m a.s.l., vertical interval: 100 m); 8, rural access road; 9, studied gully; 10, reference numbers of some of the measured cross-sections; 11, other gullies; 12, waterfall; 13, Dingilet church.

Finally, there were also important changes in land use at the site where the Dingilet gully is located. Between 1950 and 1960, planting of eucalyptus trees in the marshy, clay-rich bottomland started. An influential priest was powerful enough to plant his trees on this communal land. He planted others in a nearby field, which he irrigated with water from the marshland. The oldest trees of the village, or their trunks, invariably stand along the gully bank, in the middle of the thalweg. Ever since, many more trees were planted in the valley bottom, which is not marshy anymore. Their roots are visible in the gully banks, as witnesses of recent incision.

Phases in gully development In Agerba, the AGERTIM method was applied as part of a participatory study on the evolution of agricultural systems. Outcomes of this detailed study (Naudts, 2002) are correlated with the evolution of gully volumes. Several types of land use and land cover conversion, which took place during the last half century, could be recognized. Most of these changes lead to larger runoff response to storms. An analysis of the derivative of the curve representing gully evolution in Agerba (Figure 7) allows four phases in the evolution of gully erosion to be identified. 1. Before 1977, gully development was slow. Village elders still remember that the places which are now gullied were marshy areas. After planting eucalyptus, these lands dried up and gully development started. Even before 1960 some plots in these areas had to be abandoned due to gully erosion. 2. From 1977 till 1989, there was a continuous acceleration of gully erosion. Most probably, the degradation of vegetation cover on steep slopes, accompanied by an increase of surface runoff, caused this acceleration. This is also the period of the great drought which contributed to vegetation depletion. In this period, the village was often flooded and several houses had to be moved due to flooding and gully development. 3. After 1989, gully erosion rates increased even more strongly. This period corresponds paradoxically to the gradual introduction of soil- and water-conservation (SWC) programmes. However, gully evolution in this period is probably linked to the degradation of the biophysical environment in previous years. After drought and civil war, Copyright © 2006 John Wiley & Sons, Ltd.



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Figure 12. Rapid gully development in overgrazed private woodlots near Dingilet valley bottom (September 2001). This figure is available in colour online at www.interscience.wiley.com/journal/espl

agricultural activities improved around 1989, but the land had not yet recovered, leading to increased erosion phenomena. In this period, some plots had to be abandoned in Agerba, due to gully erosion, though several SWC measures had been taken. 4. The decrease of gully erosion rates after 1998, resulting in a relatively low long-term gully erosion rate, is linked to the stabilization of gully systems, which is clearly the consequence of SWC activities: check dams have been built in Agerba’s gullies starting from 1996. The establishment of exclosures on steep slopes (Naudts, 2001) is also important in this positive evolution. In the other areas, a similar evolution could be recognized. During a first stage, due to water and tillage erosion, the area of rock outcrops in the upper part of the Dingilet catchment increased. Larger runoff volumes are also the consequence of the removal of shrubs growing in between cultivated land. A decrease of the area covered by dense vegetation, as observed by aerial photo interpretation in the Argak’a area, has the same consequences. Hence, increased runoff and gullying are a consequence of increased pressure on the land. In addition, during stage 1 there has been widespread planting of eucalyptus trees in the marshy areas of the Dingilet and Agerba valley bottoms on smectite-rich soils since the 1950s. There are strong indications that there is not merely a correspondence in time between eucalyptus plantation on one hand, and gully incision on the other, but also a causal relationship. Eucalyptus trees planted in a moist area result in very high evapotranspiration rates (Cosandey, 1999), drying up rivers and marshes (Scott and Lesch, 1997; Le Maitre et al., 1999). In Australia, Greenwood et al. (1985) measured an evapotranspiration rate of 2300–2700 mm a−1 for a eucalyptus plantation, as compared to 390 mm a−1 in nearby grazing land. Growing eucalyptus trees in the valley bottoms certainly led to a drying of the area, including the development of deep cracks in the areas with Vertisols, located between sections 5 and 8 in Dingilet (Figure 8), which is an ideal situation for pipe erosion, tunnelling and gully development (Figure 12). It is for such reasons that an active policy of removal of eucalyptus and other exotic phreatophytes from wetlands is carried out in South Africa (WFW, 2005). It is evident that, in an already degraded environment, a dry spell, such as that of the 1980s (stage 2), has a very negative impact, not only on agricultural production but also on the environment (i.e. overgrazing, cracking of Vertisols, groundwater depletion). Runoff coefficients in such periods are larger. This rapid gully development between 1977 and 1990 (Figures 8 and 13), with some time lag (stage 3), corresponds to a long-lasting period with many dry years observed in the Ethiopian Highlands (Conway, 2000; Nyssen et al., 2004b). Due to less biomass availability, pressure on the land was deemed to increase during that period, leading to increased runoff response in the case of extreme events or during more humid years (Casenave and Valentin, 1992; Valentin, 1996), which resulted in increased Copyright © 2006 John Wiley & Sons, Ltd.


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Figure 13. Evolution of Dingilet gully volume between cross-sections 4 and 14 since incision (t)

gully erosion rates. In addition, growing eucalyptus trees completely dried the valley bottoms, leading to the development of soil cracks, pipes and tunnels, ultimately resulting in gully development. On the other hand, since 1990, and even more since 1995 (stage 4), human reaction to land degradation, population growth and impoverishment has led to agricultural intensification, including increasing efforts to conserve soil and water in the area. Naudts (2001) shows that in Argak’a the area covered by dense vegetation increased from 4·0 per cent to 5·6 per cent between 1994 and 2000; in Dingilet, 17 per cent of the catchment was transformed from rangeland to exclosure. From environmental data collected by Feoli et al. (1994) at altitudes of 1500 –2300 m a.s.l. in Central Tigray, we calculate that after one year of exclosure, grass cover reaches 79 (±24) per cent (n = 9), compared to 16 (±12) per cent on grazed slopes (n = 8). As a consequence of land conversion to exclosures, infiltration rates increase in the upper catchments (Descheemaeker et al., 2005), which can explain the observed stabilization of gully volumes over the last few years. It should be noted that these exclosures have been planted with eucalyptus trees, with relatively good survival rates in some areas. The growing of these trees might well result in less grass and shrubs, hence increased runoff, the effect of which could not yet be included in this study. In general, we observed that stages in gully development do not correspond exactly to the change of triggering factors, such as climate and changes in the agricultural system (Naudts, 2002). The evolution of gully erosion lags behind land use and land cover changes.

Evolution of gully erosion rates Independently computed, area-specific long-term rates of soil loss by gully erosion (RL) in the various catchments have a similar order of magnitude, i.e. between 2·3 and 7·4 t ha−1 a−1 (Table V). Large short-term rates (RS) in the younger Ziban Khunale and Kuliheni gullies (13–27 t ha−1 a−1) are due to the fact that these gullies are not stabilized. Hence, we estimate that the average RL of 6·2 t ha−1 a−1 is a representative figure to be used in sediment budgeting. It should also be noted that soil lost by gully erosion is not necessarily transported very far. The volume of sediment deposited in the debris fan, which interrupts the discontinuous Kuliheni gully, is slightly over half of the total gully volume (1115 m3 compared to 2218 m3). A comparison between the present-day change in volume of the four gullies monitored in detail, and their total volume and estimated age, shows an important slow-down in gully development (Table V). At present, the average RS is 1·1 t ha−1 a−1, which is an order of magnitude less than current sheet and rill erosion rates measured in the study area: 9·7 t ha−1 a−1 (weighted average over different land uses; Nyssen, 2001) or 18 t ha−1 a−1 on cropland treated with stone bunds (Desta et al., 2005). Such a non-linear increase in gully volume was reported by Graf (1977) and Rutherfurd et al. (1997), and is usually attributed to decreasing catchment area and decreasing stream power (Nachtergaele et al., 2002). There is strong evidence supporting the application of negative exponential functions to the change in volume of the Kuliheni and Ziban Khunale gullies (Figure 10). Gully incision was very rapid here, due to sudden environmental changes, i.e. road building in Kuliheni and conversion, in Ziban Khunale, of large tracts of arable to rangeland by decision at village level. Copyright © 2006 John Wiley & Sons, Ltd.



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Figure 14. Evolution of gully segment volumes in the 756-ha Harena-Khunale study area. Dingilet, Ziban Khunale and Kuliheni gully evolutions as presented in Figures 10 and 13; other gully evolutions expressed as negative exponential functions taking into account estimated year of first incision and volume in 2000. The curve for ‘old’ gullies indicates the evolution of total volume of gullies existing before 1950, according to our informants; total gully volume (bold line) in the study area is the sum of all gully segment volumes. Note the change in scale on the vertical axis.

In Dingilet and Agerba, gullying started after gradual environmental changes (removal of vegetation between cultivated fields and eucalyptus plantation in the valley bottom), and rill-like incisions grew into gullies, which increased rapidly until 1990–1998, when a decay in volumetric changes started. The sigmoidal evolution of gully volume (Figures 7 and 13) contrasts with the model developed by Graf (1977) and Rutherfurd et al. (1997), probably due to the fact that there were gradual land-use and land-cover conversions, inducing larger storm runoff volumes; this evolution was reinforced by a dry decade. This, together with expansion of the number of eucalyptus trees in the valley bottoms, induced an exponential increase in gully volume. The decay of gully volume expansion in recent years is attributed to less interannual rain variability and increased implementation of SWC techniques in the gullies and in the catchment. The estimated evolution of the gully volume in the 756-ha Harena-Khunale area is shown in Figure 14. The evolution of individual gully segments is represented as ‘classic’ (Graf, 1977) negative exponential functions, similar to those in Figure 10. Total gully volume increased rapidly from 1980 onwards and shows a flattening around 1995. This evolution of total gully volume shows a pattern similar to that of the Dingilet gully. Note that the curve for the evolution of total gully volume was obtained without ‘applying’ a sharp volume increase to individual gully segments in the second half of the 1980s, though that was observed in Dingilet and Agerba. The gully erosion rates in a 1788ha area draining to the May Leiba reservoir show a similar evolution (De Wit, 2003). The RL value of 6·2 t ha−1 a−1 (Table V) should be seen as a long-term average. In critical periods, such as the drought of the 1980s, with its below-average vegetation cover, gully volumes increase rapidly (Casenave and Valentin, 1992), both as a consequence of the development of existing gullies, as shown in Dingilet, and of the incision of new gullies. On the other hand, since 1995, no new gullies have been formed in the study area, which explains the flattening of the main curve in Figure 14. Under the present-day conditions of ‘normal’ rain and catchment-wide SWC, the gully system seems to function as a sediment transfer system, rather than being a sediment source. Copyright © 2006 John Wiley & Sons, Ltd.


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Figure 15. Main controlling factors of gully evolution in the northern Ethiopian Highlands are of natural (dry spells) and anthropogenic origin (land use and cover changes, road building, but also SWC).

Conclusions The AGERTIM method (Assessment of Gully Evolution Rates Through Interviews and Measurements) developed for this research allowed us to reconstruct gully evolution in the Tigray Highlands. This method complements measurements of short-term gully evolution with historical evidence. Interview techniques for this purpose were improved by adapting Participatory Rural Appraisal (PRA) methods. The AGERTIM method could be successfully applied, since it was based on an understanding of the properties and geomorphological characteristics of gullies. Personal knowledge of the study area and integration with the rural society enhanced the methodology: Veyret-Picot (2001), Kassa (2003) and Naudts (2002) as well as De Wit (2003) each stayed one month in the villages of Dingilet, Agerba and Adi Koilo (May Leiba) to assess gully erosion rates. Gullying at all monitored sites can be related to environmental changes that induce larger runoff responses to rainfall (Figure 15). Sudden environmental changes could be recognized in Kuliheni and Ziban Khunale in the early 1990s, a short time before gully incision started. The Kuliheni gully was clearly induced by road building. In Ziban Khunale, the upper part of the drainage area was converted from arable to overused rangeland, which has a greater runoff response, induced by decreased surface roughness, compaction of fine-textured soils, increased bulk density of the topsoil, soil structure decay and decreased hydraulic conductivity. More widespread is the impact of ongoing degradation processes (Nyssen et al., 2004b). Due to erosion, the area of rock outcrops increases. Larger runoff volumes are also due to the removal of shrubs growing in between cultivated fields and to the decrease of the area covered by dense vegetation, both consequences of increased pressure on the land and lack of resources that would allow the improvement of farm productivity without mining the environment. In addition, gully development in Dingilet and Agerba started shortly after eucalyptus trees were planted in the valley bottom, which led to a drying and cracking of the vertic soil and to gully formation. Our measurements show that in all cases, there is an exponential decay in gully development, as shown by Graf (1977), Rutherfurd et al. (1997) and Nachtergaele et al. (2002). However, based on our data, one can question the Copyright © 2006 John Wiley & Sons, Ltd.



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Figure 16. Flood regulator (left; arrow indicates metal pipe allowing gradual outflow of runoff stored behind dam) and brushwood check dams (in areas lacking stones) are new gully control techniques in the study area that have been successfully tested at Dingilet gully since 2002. This figure is available in colour online at www.interscience.wiley.com/journal/espl

statement that incipient gullying is always rapid. Except in the case of rapid environmental changes, the development of the gully systems shows a sigmoidal pattern, with a strong increase during the period with the worst environmental conditions. Average long-term soil-loss rate by gully erosion is 6·2 t ha−1 a−1, whereas the actual short-term rate is 1·1 t ha−1 a−1. A decrease of gully development was observed during 1990–2001, and especially in the most recent years, both by monitoring and through interviews (Figures 13 and 14). Together with improved climatic conditions, the implementation of SWC techniques in the study area explains this positive evolution. The role of check dams in the gully channel has been discussed elsewhere (Nyssen et al., 2004a). Within the catchments, more and more stone bunds are built on hillslopes, favouring infiltration and decreasing the runoff response (Desta et al., 2005; Vancampenhout et al., 2005). The now widespread setting-up of exclosures on steep slopes results in increased vegetation cover and a reduction of peak runoff rates (Descheemaeker et al., 2005). Recommendations resulting from this paper are to continue the catchment protection policy, including exclosures and stone bund building, in order to reduce gully erosion risk. With respect to eucalyptus planting, evidence was found that this tree, if planted in wetlands and Vertisol areas, leads to increased gully erosion. For the steep slopes, the impact of eucalyptus planting on gully development could not yet be measured. Fears exist, however, that, in comparison to exclosures with natural regrowth, runoff response will be more important, due to direct impacts of eucalyptus plantations on the soil in the form of hydrophoby (Scott, 2000) and reduced soil cover by understorey vegetation (Fiedler and Gebeyehu, 1988; Descheemaeker et al., 2005). Needless to say, the present policy of catchment protection and stabilization of gullies by check dams and exclosures should be continued. Experiments with small flood regulators and brushwood check dams proved very successful in stabilizing gullies (Figure 16).

Acknowledgements This study was carried out in the framework of the Zala-Daget project (VLIR, Belgium) and of research programme G006598.N funded by the Fund for Scientific Research – Flanders, Belgium. Thanks go to Berhanu Gebremedhin Abay, Romha Assefa and Girmay Hailemariam for assistance to all the fieldwork. Numerous farmers agreed to share their knowledge with us. The local Agricultural Office, REST (Relief Society of Tigray) branch, May Zegzeg Integrated Watershed Management Project office and the authorities of the concerned villages and district facilitated the research. Useful comments by two anonymous reviewers are gratefully acknowledged.

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