Search

This study evaluated changes in land cover in the Chemoga watershed, headwater to the Blue Nile. Two sets of aerial photographs (1957 and 1982) and a multispectral Spot image (1998) were used as inputs to produce 3 GIS-based land cover maps of the area. The results show that during the last 41 years, forest cover increased at a rate of about 11 ha per annum in the 36,400-ha watershed. Woodlands and shrublands decreased between 1957 and 1982 but increased between 1982 and 1998, approximately to their previous levels. Farmland and settled areas gained from the other cover types (13% increase) in the first period but lost around 586 ha (2% decrease) in the second. Grassland and degraded land decreased, accounting for 4.8% of the total area of the watershed in 1982 and 3.5% in 1998, as against 9.6% in 1957. Riverine trees suffered the greatest destruction, shrinking by 79% over the 4 decades; much of this decline was due to cultivation. Marshlands increased in the first period and decreased in the se...

Population and environment debates regarding Africa, whether Malthusian or Boserupian in nature, focus on population levels as the driving force behind the relationship between environment and society. This article argues, instead, that how people adjust to their rise in numbers is more important than are population levels. It focuses on the role of local informal institutions, such as land tenure systems, but also on customs, norms, and networks, and their change over time in mediating the relationship between people and the environment. The article is based on fieldwork conducted between 1995 and 1998 in the Sahelian and Sudano-Sahelian zones of Africa, as well as on a review of colonial documents pertaining to the area written in the first half of the twentieth century. The article concludes that adaptations made to local, informal institutions within the past century have enabled an environmentally sustainable land use within the context of a rising population and growing scarcity of natural resources.

Population and environment debates regarding Africa, whether Malthusian or Boserupian in nature, focus on population levels as the driving force behind the relationship between environment and society. This article argues, instead, that how people adjust to their rise in numbers is more important than are population levels. It focuses on the role of local informal institutions, such as land tenure systems, but also on customs, norms, and networks, and their change over time in mediating the relationship between people and the environment. The article is based on fieldwork conducted between 1995 and 1998 in the Sahelian and Sudano-Sahelian zones of Africa, as well as on a review of colonial documents pertaining to the area written in the first half of the twentieth century. The article concludes that adaptations made to local, informal institutions within the past century have enabled an environmentally sustainable land use within the context of a rising population and growing scarcity of natural resources.

Eritrea’s coastal zone has been identified as an area of substantial development potential. About 14,000 ha of this 5 million ha area (i.e.

Hypotheses about the origin of vegetation pattern formation in semi-arid areas around the world almost all include a common feature of semi-arid areas: the presence of a positive feedback between plant density and water infiltration. We investigate whether this positive feedback and the spatial redistribution of runoff water are sufficient to explain vegetation pattern formation. For this purpose, we analyze a spatially explicit model consisting of partial differential equations using a method for demonstrating pattern formation (Turing analysis). Our analysis reveals that pattern formation can occur in semi-arid areas given only the positive feedback between plant density and local water infiltration coupled with the spatial redistribution of runoff water. Thus, slope and underlying heterogeneity are not essential conditions. Other factors in the model, such as herbivory, plant dispersal, rainfall, and drought tolerance of plants, appear to determine under what conditions pattern formation is likely but are not the primary factors that generate the patterns. The model is in agreement with field observations and indicates the conditions for which vegetation pattern formation can be expected in arid and semi-arid grazing systems.

Hypotheses about the origin of vegetation pattern formation in semi-arid areas around the world almost all include a common feature of semi-arid areas: the presence of a positive feedback between plant density and water infiltration. We investigate whether this positive feedback and the spatial redistribution of runoff water are sufficient to explain vegetation pattern formation. For this purpose, we analyze a spatially explicit model con- sisting of partial differential equations using a method for demonstrating pattern formation (Turing analysis). Our analysis reveals that pattern formation can occur in semi-arid areas given only the positive feedback between plant density and local water infiltration coupled with the spatial redistribution of runoff water. Thus, slope and underlying heterogeneity are not essential conditions. Other factors in the model, such as herbivory, plant dispersal, rainfall, and drought tolerance of plants, appear to determine under what conditions pattern formation is likely but are not the primary factors that generate the patterns. The model is in agreement with field observations and indicates the conditions for which vegetation pattern formation can be expected in arid and semi-arid grazing systems.

Atmospheric desert dust was collected over 36 months in ground-level collectors at four stations in the northern Negev desert, Israel. Three stations (Shivta, Sede Boqer and Avdat) are located in the desert itself whereas the fourth station (Sayeret Shaked) is situated at the desert fringe, in the transition zone between the true desert and the more humid area to the north. The dynamics of the dust in the three desert stations is similar and differs from that at the desert fringe station. In the desert, dust accumulation is highest in summer and lowest in winter, whereas the situation is reverse at the desert fringe. Also, the amount of accumulation is much higher in the desert itself. No important differences are observed with respect to the average grain size of the dust, but the temporal evolution in grain size variability at the desert fringe is different from that in the desert. Mineralogical analysis by means of X-ray diffraction shows that quartz and calcite, and to a lesser extent also dolomite, are the major constituents of the Negev dust. There is a good agreement between the quartz and calcite content of the dust samples and that of the outcropping rocks located slightly upwind of the four stations, suggesting that at least an important part of the dust collected is of rather local origin. However, the regular records of dust haze events in the northern Negev show that part of the dust that accumulates in this region is not of local origin, but originates from distant sources. This dust may later mix with the local dust.

Mulching with flattened crop residues is widely used to protect soils from wind erosion. Several wind tunnel and field experiments have shown decreased protection of some soil covers with increasing wind speed. In some studies, sediment transport was enhanced with flattened residue as compared with the bare soil condition. The purpose of this article was to determine the behavior of wind speed and sediment transport when the soil surface is covered with randomly applied, flattened crop residues. A literature review was conducted to evaluate recent insights in turbulent flow properties and related sediment transport. A conceptual model was then developed to explain decreasing soil protection of a certain residue quantity when the free stream wind speed increases. The main reason is the change in turbulent flow properties of the near-surface wind when no-erodible roughness elements are added to an otherwise smooth surface. The average wind speed is reduced by the roughness but the probability distribution of instantaneous wind speed becomes wider and positively skewed. If free stream wind speed increases, at a certain moment the changed turbulence will cause more wind gusts that exceed the threshold wind speed for soil particle movement than would occur over a bare surface, But it is likely that this only happens when soil cover is less than 10%. For higher soil covers, increasing wind speed will also cause a decrease in soil protection, but natural wind speeds are normally not sufficient to cause enhanced sediment transport, because average wind speed is sufficiently reduced.

The spatial pattern of medium-term (a few months) dry aeolian dust accumulation in rocky deserts is predicted using short-term deposition and erosion experiments in a wind tunnel. The predictions are tested in a field experiment set up in the northern Negev Desert of Israel. The results show that superimposing wind tunnel deposition and erosion maps usually leads to correct predictions of medium-term dust accumulation. The predictions are somewhat less confident near the inflection lines of windward hillslopes, where small-scale irregularities in the local topography make it difficult to locate the exact position of the areas of little accumulation. Elsewhere in the topography predictions are good, and the method works satisfactorily. Highest accumulation occurs on concave windward slopes and, to a lesser extent, on slopes parallel to the wind. Little accumulation occurs on the convex windward slopes and in dust separation bubbles. The smallest accumulation rates are observed immediately upwind of the top of pronounced hills and on leeslopes. The rate of dry dust accumulation measured during the field experiment varied from 17 to 93 g m-2 a-1, depending on the topographic position of the accumulation plots. For most plots, it was of the order of 30-60 g m-2 a-1.

In the Central Plateau of Burkina Faso, runoff on bare soil amounts to 40␘f annual rainfall and soil losses reach 4 to 8 Mg ha-? a-?, despite slopes of under 3ÐSeveral studies have shown that mulching the soil surface can reduce runoff by over 60ÐHowever, the scarcity of straw and the incompatibility of mulching with mechanical soil preparation have prevented the large-scale adoption of the technique. The study conducted in Saria village where annual rainfall is 800 mm,set out to evaluate the efficacy of sorghum-cowpea intercropping in reducing runoff and erosion. The study design comprised five plots set up so as to recover runoff and transported solids, on a Ferric lixisol with a slope of 0.7ÐThree years results showed that sorghum-cowpea intercropping reduces runoff by 20-30␌ompared to a sorghum monoculture and by 45-55␌ompared to a cowpea monoculture. Soil loss is also reduced with intercropping by at least a half compared to sorghumand cowpea monoculture. Moreover, it transpired that sorghum-cowpea intercropping is also beneficial in agricultural production terms, since the grain yield of the intercropped plots was double that obtained with sorghumor cowpea monocultures. The better crop production can be an asset for the widespread use of this technique in the country. An evaluation of the advantages of the legume in terms of nitrogen supplies to the cereal (N2 symbiotic fixation) and of added soil OM will be useful in improving the cost-effectiveness of such a technique.

In the Sheeb area in eastern Eritrea a Participatory Rural Appraisal (PRA) was carried out in two villages, one upstream and one downstream of the ephemeral rivers Laba and Mai-ule. The objectives of the study were to obtain a better understanding of farmer-managed spate irrigation systems and to enable the local people to perform their own farming system analysis. This paper describes the various PRA activities, such as mapping, diagramming and ranking of problems, that were undertaken with the participation of local people. The resource mapping revealed that lack of water and not scarcity of land is the main constraint affecting the development of the spate irrigation system in the Sheeb area. When there is a lack of rainfall in the highlands of Eritrea, there are only few floods per year in the Sheeb area. The development trend diagrams showed that the number of floods, the food production and the prevalence of pests vary considerably over time. The livelihood diagrams indicated that food, fodder, and water were only available during part of the year. In search of these resources, the farmers have developed a livelihood strategy of seasonal migration to the highlands of Eritrea. The PRA data collected in the two villages appeared to be very useful for the preparation of community action plans in the entire Sheeb area.

Metadata only record Soil and water conservation interventions in Africa have had a checkered history, calling into question the way in which soil and water conservation technologies have been studied in the past. This article draws on a case study from eastern Burkina Faso to explore an area usually ignored by soil and water conservation studies-the role of social institutions in guiding decisions regarding the use of technologies. It looks at soil and water conservation through the historical development of what the authors call the 'cultural economy', that is, a system of exchange in which the market economy has mixed with pre-existing forms of exchange. The approach adopted by the authors identifies concepts on which the cultural economy is based and uses these ideas to analyse institutions that affect the choice of soil and water conservation technologies. The article shows how this approach leads to a reconceptualization of the ways in which soil and water conservation technologies are to be considered.

In designing farm household surveys in the Sahel in West Africa much attention should be paid to various specific features of the farming systems. These features relate in particular to the social organisation of the farming communities. Within households, kinship relations have a strong bearing on decision-making with regard to the control and use of land, labour and capital resources. This need to be reflected in the surveys. Surveys have also to deal with specific property relations, with a distinction between collective and individual fields within households, and various common pool resources, including short and long term fallow land at village level. In survey design one also needs to be aware of the simultaneous application of land use techniques with a quite different intensity. The large households and the large number of fields complicate field and yield measurements. Finally surveys should have enough flexibility to incorporate new insights into these complex and elusive farming systems.

Peter Oksen's comments on our paper about farmer-herdsman relations in Burkina Faso raise some interesting issues, notably regarding problems of interpretation of oral and archival sources and regarding the broader relevance of insights gained from an in-depth case study. Before answering straightforwardly to his objections, it is useful to clarify the misunderstanding which appears to exist about the meaning we attribute to 'symbiosis' and 'symbiotic relations'. In our article we restricted the use of these terms to the way in which past relations between farmers and herdsmen, depicted as undifferentiated groups, are often represented. In discussions about the change of these relations, the emphasis is on progressive deterioration, again without attention paid to the possible existence of intra-group differentiation or of heterogeneity of relations across group boundaries. In this regard, it is remarkable that from colonial documents the interests of farmers and herdsmen emerge as equally irreconcilable as they are often considered today, and that, just as at present, competition over scarce natural resources constituted a major factor in inter-group relations. If we therefore reject 'symbiosis' as a correct description of formerly existing inter-group relations, we do not intend to imply that complementary links - such as those we describe for present-day relations between Mossi and Fulbe - did not exist in the past. A major aspect of our argument is that inter-group relations, whether past or present, cannot be subsumed under simplifying labels such as 'symbiosis'. Neither can changes in these relations be understood in terms of uni-directionally processes of deterioration. Hence, present-day 'complementary' links across the ethnic boundary - established by certain, but not all, Mossi and Fulbe actors - are but a manifestation of the continued presence of diversity of relations, not of 'symbiosis'. They point to mutual interests between certain actors belonging to different ethnic groups, not between the groups as such.

A modelling approach was chosen for analyzing the effects of competition between windbreaks and crops for soil water and radiation in the Sahel. The model has a high spatial and temporal resolution to account for the heterogeneity in a windbreak-cropping system. The model was parameterised for millet (Pennisetum glaucum (L.) R. Br.), the tree species Bauhinia rufescens Lam. and soil characteristics and weather data of Sadore, Niger. Simulations were run for two wet years (1992 and 1993) and for a dry cropping season (1989). Simulations showed a strong yield reduction up to 2 m from the windbreak due to shading and/or water competition. For Sahelian sites with no access to groundwater, competition for water between trees and crops is likely to occur in the beginning of the rainy season. Water competition was surprisingly highest in the wet year 1992, intermediate in the dry year 1987 and lowest in the wet year 1993. Simulation results indicate that light and water competition depends on rainfall distribution and the period between the seasonal onset of tree and crop growth.

This article critically assesses an interdisciplinary research effort to build a new tool for conducting farming systems analysis (FSA). It was envisioned that this new tool, named quantified farming systems analysis (QFSA), would overcome some of the problems that confront the classical FSA techniques. At the onset of this endeavour expectations were high and they have been presented at several international symposia (for example, at Bangkok, 1991, Colombo, 1992, and Wageningen, 1993). This article deals with one way of tackling the first phase of QFSA, i.e. the setting of research priorities for long-term development, which resulted in the farm level optimal resource allocation (FLORA) procedure. The manner in which this was accomplished and the extent to which the anticipated results were achieved are evaluated. It is concluded that, although the way in which FLORA was eventually developed differed from what had been envisaged, it offers a more objective and quantitative procedure for setting research priorities in view of long-term development goals.

The effectiveness of mulch and termite activity in the rehabilitation of the physical properties of crusted soil was studied in northern Burkina Faso (Province de Bam). A split plot design was used with three replications each being on one soil type. The soil types were Ferric Lixisol, Haplic Lixisol and Chromic Cambisol. The main factor was termite activity, and to this end dieldrin (0·50 kg a.i. ha−1) was used to create plots without termite activity next to plots with termite activity. The subplots consisted of Pennisetum pedicellatum mulch, wood Pterocarpus lucens mulch and composite (wood+straw) mulch, applied at rates of 3, 6 and 4 t ha−1, respectively. Two years after establishing the experiment, the combined effect of termite and mulch on the change in physical properties of the soil was measured. The parameters used for this assessment were porosity, saturated hydraulic conductivity and soil resistance to cone penetration. Soil water content was also measured. Termite activity was found to increase soil porosity, soil saturated hydraulic conductivity, improve soil water status and reduce soil bulk density and soil resistance to cone penetration. The only difference between bare plots and mulched plots without termites was in water content. This indicates that the mechanism whereby mulch improves the physical properties of crusted soil is mainly based on soil biology processes and to a limited extent on protecting soil against weather impact. © 1997 John Wiley & Sons, Ltd.

Olive production is an important and growing agricultural activity throughout the Mediterranean zone. At the same time, soil erosion is one of the environmental key problems in this zone. Actual erosion in olive production areas is high, in particular on sloping land. Several erosion risk factors are present here: rainfall erosivity, soil erodibility, steep slopes and poor ground cover. In this paper an inventory is made of the actual situation and trends of olive production and erosion hazards. Subsequently soil conservation options are briefly described.

Aeolian dynamics were measured during a wind erosion event on an agricultural field in Northern Germany. Because the topsoil was loamy sand, particle flow behaved as a sand–dust mixture and not as pure sand (>63 m) or pure dust (