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The importance of dambos in African agriculture
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The importance of dambos in African agriculture Beryl Turner
Dambos are seasona~iy wa~er~~ged areas found in the headwaferzones of drainage systems or alongside streams. Their importance lies in the fact that they are widespread, and can provide grazing in the dry season as well as being moist enough to grow dry season crops without irrigation. In addition, it may be possible to use them in small-scale irrigation schemes. The author feels that their value in savanna areas is severely underrated. Dr Beryl Turner can be contacted at The Green, Litton, Skipton, North Yorkshire, BD23 5QJ, UK.
‘J.M. Rattray, R.M.M. Cormack and RR. Staples, ‘The vlei areas of S. Rhodesia and their uses’, Rhodesian Agricultural Journal, Vol 50, pp 465-463; B. Turner, The Fadama Lands of Central Northern Nigeria: Their C~assi~~tiofl, Spatiat l/ariafion, Present and Potential Use, PhD thesis, Universi~ of London, UK, 1977; and R. Whitlow, ‘Dambos in Zimbabwe: a review’, Zeitschrift fiir G~mo~hof~ie, NF Supplementband 52, March 1985, pp 115-146. 2Zeitschrift fiir Geomorphologie, NF Saup plementband 52, 1985. 3A. Low, Agricultural Development in Southern Africa, James Currey, London, UK, 1986; and D.J. Pratt and M.D. Gwynne, Range/and Management and Ecology in East Africa, Hodder and Stoughton, London, UK, 1977. 4M. Bell and N. Roberts, ‘Development theory and practice in human and physical geography’, Area, Vol 18, No 1, 1986, pp 3-8.
LAND USE POLICY October 1986
Dambos are seasonally waterlogged, shallow, grassy depressions found in the headwater zones of drainage systems or alongside streams. The term “dambo’ is of Bantu origin and used in central and eastern Africa but is often used, as in this article, to include similar features found elsewhere in Africa, such as the vleis of Zimbabwe and South Africa, the fadamas of Nigeria and the bolis of Sierra Leone. Dambos are best developed on the ancient plateau surfaces which cover much of inland Africa. They are most common in the savanna zones, although they do extend into both drier and wetter areas in some places. They are very conspicuous features when seen from the air. Their grassy vegetation contrasts with the surrounding woodland or cultivated areas, particularly in the dry season when they appear green in a predominantly brown landscape. Their importance lies in the fact that they are widespread, providing grazing for cattle and wildlife during the dry season as well as being moist enough to grow dry season crops without irrigation. Dambos may vary in width from a few metres to over one kilometre and in plan form are usually lobate in headwater areas, becoming linear downstream. They often form a dendritic network following the drainage pattern, but disappear in actively eroding areas where gradients become too steep and where the water table has been lowered. They occur on
different rock types - most frequently on granites and gneisses, but also on schists, limestones, shales and sandstones. They are found mainly where the rainfall is between 500 and 1500 mm. The area they occupy varies from 2% to over 30% of the total land area with an average of about 10% over extensive areas. ’ Figure 1 shows a typical network of dambos in northern Nigeria. Dambos have not received the attention they deserve. Although a recent issue of Zeitschrift fiir Geomorphologit? has been devoted to their geomorphology, and many local (often unpublished) reports exist in government departments or in local journals in several African countries, little has been written about their land use in international journals. Textbooks often contain no mention of dambos.’ Recently, Bell and Roberts have used dambos as an example of the need for integration between physical and human geography in assessing development potential.” Although African agriculture and animal husbandry have been the focus of worldwide attention in recent years because of the devastating droughts, almost all the emphasis has been on the staple rainfed grain crops, the declining prices fetched by the major cash crops, the problems of overgrazing and possible solutions offered by large-scale mechanized irrigation schemes. Many supposed experts on several
343
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‘B. Turner, ‘Changing land use patterns in the fadamas of northern Nigeria’, in E. Scott, ed, Life Before the Drought, Allen and Unwin, London, UK, 1984. ‘Whitlow, op tit, Ref 1.
Figure 1. Dambos northern Nigeria.
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Africa do not seem to realize that in spite of the savanna areas having little or no rain for five to seven months, many crops can be grown in dambos in this dry season, both with and without irrigation. In addition, dry season dambo grazing is often a crucial factor limiting animal carrying capacities. Individual dambos are mainly small and not suitable for large-scale mechanized agriculture, but collectively their area is considerable. They are ideal for growing vegetable crops in the dry season to supplement the staple grain crops and this often provides a source of income where local village or urban markets are accessible. Dambo cultivation makes use of the surplus labour available in the dry season,’ whereas wet season cultivation is often limited by labour shortages. Dambos have distinct physical characteristics. Dambos without stream channels are usually gently sloping with a wetter central zone and drier margins while streamside dambos may similarly slope up gradually from a stream channel or be in the form of river terraces. Many dambos retain moisture throughout the year (peren-
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nial dambos) although some, particularly in the lower rainfall areas, dry up before the end of the dry season (seasonal dambos). The level of the groundwater table determines which crops can be grown during the different seasons. Because the growing season in the dambos is much longer than on the surrounding upland, a greater biomass is produced and under natural conditions the soils become richer in nutrients and organic matter. Sheetwash and throughflow move water, nutrients and fine soil particles into the dambo from adjacent areas thus increasing their nutrient status and clay content. Soils are generally hydromorphic and very variable, often changing rapidly over short distances both horizontally and vertically. In high rainfall areas such as northern Zambia and Malawi, and also over dry Kalahari sand areas in Zimbabwe peaty soils are found, often overlying sand.(’ Parent rock, as well as rainfall, affects acidity and in Zambia a distinction is drawn between ‘sweet’ dambos on calcareous parent rock and ‘sour’ dambos which are more acidic and
near Dutsen Wai,
Source: From Sheet 125 of a series of six 1:lOO 000 land use maps compiled and drawn by Beryl Turner from 1963 aerial photographs and 1973-75 ground survey, published by the Institute for Agricultural Research, Samaru, Zaria, Nigeria, 1975.
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LAND USE POLICY
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Many dambos are ideally suited to small-scale cultivation relying on groundwater resources. Most vegetable crops thrive best in the dry season when pests and diseases such as mildew are much less prevalent. Small plots can be cultivated by hoe and the height of ridges is determined by the moisture level. Vegetables can be chosen, according to their rooting depth, to correspond to the differing groundwater levels in different parts of the dambo. Because most vegetables have a short growing season, double or even treble cropping is often introducing successively possible, deeper rooting crops as the dry season progresses. Sugar cane, which has a 12 month growing season, can be grown in perennial dambos and swamp rice can be grown in dambos which are flooded in the wet season. In the lower rainfall areas, where rainfall is also more variable from year to year, it is often possible to plant some of the staple grain crops (maize, sorghum or millet) in the dambos if the rains are late in starting, as an insurance against failure of the upland crop. However, in wet years they will not do well because of waterlogging. Irrigated
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found over granite and gneiss. The former support a vegetation which is much more nutritious for grazing animals - dominantly grasses and herbs.’ The sour dambos contain more sedges. Occasional trees are found along streams or on termite mounds within the dambo, but waterlogging prevents the growth of most trees. During the dry season there is a serious shortage of grazing for both cattle and wildlife. Crop residues provide some fodder for the early dry season but there is no native tradition in Africa of growing fodder crops or making hay or silage. The dambo grazing is therefore particularly important during the late dry season and early wet season. Groundwater
7S. Heerv, ‘The veoetation ecoloav of dambo gr&slands (around Choma, yambia) in relation to dry season grazing’, Research memorandum 27, Department of Agriculture, Lusaka, Zambia, 1979, 66PP. ‘Turner, op cif, Ref 1. ‘K. Vickery, ‘Aspects of Plateau Tonga economic history’, in R. Palmer, ed, Zambian Land and Labour Studies, Ill, National Archives of Zambia, 1976, pp 55-67. “%I. Whitlow, ‘Vlei cultivation in Zimbabwe,’ Zimbabwe Agricultural Journal, Vol80, 1983, pp 123-l 36.
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Small-scale irrigation in streamside dambos may be possible by a gravity fed system of channels, but it is usually
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necessary to lift the water using either shadoufs, as in northern Nigeria, or diesel pumps. Shadoufs consist of a pole, balanced on a pivot, with a clay weight at one end and a rope suspending a bucket or calabash at the other. The materials needed for construction are readily available and they can be operated by one man, though preferably with a partner directing water into channels. About 0.1 ha can be irrigated from one shadouf, with a maximum lift of about three metres. A much larger area, about four hectares, can be irrigated using a diesel pump and the water can be lifted to a greater height. However, apart from the initial cost, shortages of fuel, risk of mechanical breakdown and lack of mechanics able to mend pumps, make this type of irrigation much more risky for an individual farmer. In dambos without stream channels wells are often dug and water lifted by hand and carried to nearby plots. The extent to which dambos are cultivated at present varies considerably from place to place. In the area south of Kano in northern Nigeria, a densely populated area where only 3% of the land is fadama (dambo), over 89% of it is cultivated. However, in the wetter and less densely populated area 100 km further south, 15% of the land is dambo but only 21% of it is cultivated. These differences are related to distance from market as well as to availability of dambo and population density.8 There was a large increase in the area of dambo cultivated near to Zaria between 1963 and 1973. In Zambia, although historical records show that the margins of dambos were the most favoured areas for cultivation in Southern Province at the beginning of this century (partly because clearance of the woodland was not necessary in such areas),” only small areas of dambo are now cultivated, and even close to the towns uncultivated dambos are common. Whitlow has described the changes in cultivation in Zimbabwe.“’ Legal restrictions since the 1920s reduced the areas of dambo cultivated although formerly these were more widespread.
345
Since independence there has been an increase in cultivation on both large and small farms. The bolis in Sierra Leone are often used for cultivation of rice and vegetables” and Gosnell reports an increase in cultivation in dambos in Kenya.12 The natural vegetation of the dambos includes many species of value as medicinal plants as well as reeds and palms used for making mats and haskets. Clays and sands are dug for buiiding purposes and for pottery and dambos are also essential in providing water supplies to many villages and watering places for cattle and other animals. Erosion
“R Mlckel, ‘Zur Entstehung und geiikologischen Stellung der Bolis in Sierra Leone/ Westafrika’, B&ichte der Naturforschenden Gesellschaft zu Freiburo im Breisoau, Vol 69, 1979, pp 47-71; a& A.C. MiGngton, F. Helmisch and G.J. Rhebergen, ‘Inland valley swamps and boiis in Sierra Leone: hydrol~ical and ~dological considerations for agricultural development’, Zeitschrirl fiir Geomor~hotog;e, NF Supplementband 52, March 1985; pp2Oi-222. ‘*J.M. Gosnell. ‘Vleiland develooment in West Kenya’, &st African Agricultural and forestry Journal, October 1963, pp QQ105. 13H.A.S. Johnston, The Fulani Empire of Sokoto, Oxford University Press, London, UK, 1967. j4J.R.E. Hindson, ‘The protection of dambos by means of contour seepage furrows’, Mimeo report, Ministry of Agriculture, Lusaka, Zambia, 1964, 107~~. 15A. W~ndram, R. Faulkner, M. Beli, N. Roberts, P. Hotchkiss and R. Lambert, ‘The use of dambos in small scale rural development’, 11 th conference of the Water and Waste Engineering for Developing Countries group, Dar es Salaam, 1985, pp F4.1 -F4.4. ‘%attray et al, op tit, Ref 1. “Heery, op tit, Ref 7; and Gosnell, op tit, Ref 12. “Rattray et al, op cit. Ref 1.
346
and degradation
The continued existence of a dambo depends on the maintenance of the delicate ecological balance between water, soils and vegetation. Dambos may be destroyed directly by erosion or they may dry out, losing their water-storing capacity. Such degradation affects not only the dambo itself, but also areas downstream which depend on the slow release of-water from the dambo throughout the dry season. Dambos with peaty soils are particularly susceptible to damage as drying out leads to oxidation of the organic matter content. The main dangers are in 1) overuse of the water supply (either for domestic supplies, for watering stock or for irrigation) leading to lowering of the water table, and 2) erosion of the soil, particularly by gully formation. If ditches are constructed to drain dambos which are seen as too wet for cultivation or grazing, there is a severe danger of increasing the surface runoff, thus causing a rapid fall in the water table in the early dry season. Gully erosion often occurs at the end of drainage ditches or irrigation channels, or where footpaths or cattle tracks cross dambos. Exposure of bare soil which is then easily eroded by the first rains can be caused by overgrazing or large-scale, especially mechanized, cultivation. These both also cause compaction of the soil which increases surface runoff and destroys the soil structure. Burning also results in exposure of the soil
and destroys organic matter thus reducing the water holding capacity. The later the burning, the more the damage caused. Land use of the surrounding upland areas also has a considerable effect on the moisture content of the dambos. In Zimbabwe, wheat was grown on dambos on commercial farms in the 1930s and 1940s and this led to a drying-out of the dambos and gully erosion, and resulted in legislation to restrict cultivation of dambos. However, examples of successful dambo cultivation for many decades can be found in northern Nigeria where irrigation in dambos has also been practised for at least 130 years.‘l Fertility is maintained by manuring with cattle manure and household wastes. In Zambia, Hindson suggested dambos could be protected by means of contour seepage furrows, but avoidance of degradation in general depends mainly on small-scale cultivation with little or no drainage, overgrazing, trampling, exposure of soils or burning, and careful control of water levels and maintenance of fertility. ” In many parts of Africa dambos are underutilized and their potential remains unrecognized. There is a great need for small-scale development on the level of the individual farmer and his family using their indigenous knowledge reinforced by improvements such as provision of better seeds and fertilizer, handpumps, etc. Windram et at have recently reported the development of hand pumps able to irrigate up to one hectare with a lift of up to six metres.” Improved education is necessary both to introduce new ideas and to enable understanding of the problems and dangers of soil erosion and degradation. Other improvements which have been suggested include mowing the dambo grasses and making hay or silage instead of burnjng,‘~ seeding with improved grasses and legumes” and use of rotations and organic fertilizers to maintain fertility.‘s Improvement of dambo cultivation and grazing provides an ideal means of such smallscale development. There is some conflict of use be-
LAND USE POLICY
October
1986
tween cultivation and grazing especially in areas where these are practised by different groups. The migratory Fulani cattle owners in northern Nigeria have found the increase in fadama cultivation, pa~icuiarly close to the towns and large villages where they sell their milk, prevented their access to traditional grazing and this has led to conflict with farmers. Depending on local conditions, multiple use of the dambos can be practised if a
LAND USE POLICY October 1986
balance is maintained between the need for grazing land and cultivation. Features similar to dambos are found in other savanna areas of the world and have been described in South America and Australia. The effective use made of dambos in the most densely populated parts of rural Africa, particularly northern Nigeria, may provide an example of relevance not only in the rest of Africa but in many other parts of the world.
347
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The importance of dambos in African agriculture Beryl Turner
Dambos are seasona~iy wa~er~~ged areas found in the headwaferzones of drainage systems or alongside streams. Their importance lies in the fact that they are widespread, and can provide grazing in the dry season as well as being moist enough to grow dry season crops without irrigation. In addition, it may be possible to use them in small-scale irrigation schemes. The author feels that their value in savanna areas is severely underrated. Dr Beryl Turner can be contacted at The Green, Litton, Skipton, North Yorkshire, BD23 5QJ, UK.
‘J.M. Rattray, R.M.M. Cormack and RR. Staples, ‘The vlei areas of S. Rhodesia and their uses’, Rhodesian Agricultural Journal, Vol 50, pp 465-463; B. Turner, The Fadama Lands of Central Northern Nigeria: Their C~assi~~tiofl, Spatiat l/ariafion, Present and Potential Use, PhD thesis, Universi~ of London, UK, 1977; and R. Whitlow, ‘Dambos in Zimbabwe: a review’, Zeitschrift fiir G~mo~hof~ie, NF Supplementband 52, March 1985, pp 115-146. 2Zeitschrift fiir Geomorphologie, NF Saup plementband 52, 1985. 3A. Low, Agricultural Development in Southern Africa, James Currey, London, UK, 1986; and D.J. Pratt and M.D. Gwynne, Range/and Management and Ecology in East Africa, Hodder and Stoughton, London, UK, 1977. 4M. Bell and N. Roberts, ‘Development theory and practice in human and physical geography’, Area, Vol 18, No 1, 1986, pp 3-8.
LAND USE POLICY October 1986
Dambos are seasonally waterlogged, shallow, grassy depressions found in the headwater zones of drainage systems or alongside streams. The term “dambo’ is of Bantu origin and used in central and eastern Africa but is often used, as in this article, to include similar features found elsewhere in Africa, such as the vleis of Zimbabwe and South Africa, the fadamas of Nigeria and the bolis of Sierra Leone. Dambos are best developed on the ancient plateau surfaces which cover much of inland Africa. They are most common in the savanna zones, although they do extend into both drier and wetter areas in some places. They are very conspicuous features when seen from the air. Their grassy vegetation contrasts with the surrounding woodland or cultivated areas, particularly in the dry season when they appear green in a predominantly brown landscape. Their importance lies in the fact that they are widespread, providing grazing for cattle and wildlife during the dry season as well as being moist enough to grow dry season crops without irrigation. Dambos may vary in width from a few metres to over one kilometre and in plan form are usually lobate in headwater areas, becoming linear downstream. They often form a dendritic network following the drainage pattern, but disappear in actively eroding areas where gradients become too steep and where the water table has been lowered. They occur on
different rock types - most frequently on granites and gneisses, but also on schists, limestones, shales and sandstones. They are found mainly where the rainfall is between 500 and 1500 mm. The area they occupy varies from 2% to over 30% of the total land area with an average of about 10% over extensive areas. ’ Figure 1 shows a typical network of dambos in northern Nigeria. Dambos have not received the attention they deserve. Although a recent issue of Zeitschrift fiir Geomorphologit? has been devoted to their geomorphology, and many local (often unpublished) reports exist in government departments or in local journals in several African countries, little has been written about their land use in international journals. Textbooks often contain no mention of dambos.’ Recently, Bell and Roberts have used dambos as an example of the need for integration between physical and human geography in assessing development potential.” Although African agriculture and animal husbandry have been the focus of worldwide attention in recent years because of the devastating droughts, almost all the emphasis has been on the staple rainfed grain crops, the declining prices fetched by the major cash crops, the problems of overgrazing and possible solutions offered by large-scale mechanized irrigation schemes. Many supposed experts on several
343
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‘B. Turner, ‘Changing land use patterns in the fadamas of northern Nigeria’, in E. Scott, ed, Life Before the Drought, Allen and Unwin, London, UK, 1984. ‘Whitlow, op tit, Ref 1.
Figure 1. Dambos northern Nigeria.
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Africa do not seem to realize that in spite of the savanna areas having little or no rain for five to seven months, many crops can be grown in dambos in this dry season, both with and without irrigation. In addition, dry season dambo grazing is often a crucial factor limiting animal carrying capacities. Individual dambos are mainly small and not suitable for large-scale mechanized agriculture, but collectively their area is considerable. They are ideal for growing vegetable crops in the dry season to supplement the staple grain crops and this often provides a source of income where local village or urban markets are accessible. Dambo cultivation makes use of the surplus labour available in the dry season,’ whereas wet season cultivation is often limited by labour shortages. Dambos have distinct physical characteristics. Dambos without stream channels are usually gently sloping with a wetter central zone and drier margins while streamside dambos may similarly slope up gradually from a stream channel or be in the form of river terraces. Many dambos retain moisture throughout the year (peren-
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nial dambos) although some, particularly in the lower rainfall areas, dry up before the end of the dry season (seasonal dambos). The level of the groundwater table determines which crops can be grown during the different seasons. Because the growing season in the dambos is much longer than on the surrounding upland, a greater biomass is produced and under natural conditions the soils become richer in nutrients and organic matter. Sheetwash and throughflow move water, nutrients and fine soil particles into the dambo from adjacent areas thus increasing their nutrient status and clay content. Soils are generally hydromorphic and very variable, often changing rapidly over short distances both horizontally and vertically. In high rainfall areas such as northern Zambia and Malawi, and also over dry Kalahari sand areas in Zimbabwe peaty soils are found, often overlying sand.(’ Parent rock, as well as rainfall, affects acidity and in Zambia a distinction is drawn between ‘sweet’ dambos on calcareous parent rock and ‘sour’ dambos which are more acidic and
near Dutsen Wai,
Source: From Sheet 125 of a series of six 1:lOO 000 land use maps compiled and drawn by Beryl Turner from 1963 aerial photographs and 1973-75 ground survey, published by the Institute for Agricultural Research, Samaru, Zaria, Nigeria, 1975.
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LAND USE POLICY
October
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Many dambos are ideally suited to small-scale cultivation relying on groundwater resources. Most vegetable crops thrive best in the dry season when pests and diseases such as mildew are much less prevalent. Small plots can be cultivated by hoe and the height of ridges is determined by the moisture level. Vegetables can be chosen, according to their rooting depth, to correspond to the differing groundwater levels in different parts of the dambo. Because most vegetables have a short growing season, double or even treble cropping is often introducing successively possible, deeper rooting crops as the dry season progresses. Sugar cane, which has a 12 month growing season, can be grown in perennial dambos and swamp rice can be grown in dambos which are flooded in the wet season. In the lower rainfall areas, where rainfall is also more variable from year to year, it is often possible to plant some of the staple grain crops (maize, sorghum or millet) in the dambos if the rains are late in starting, as an insurance against failure of the upland crop. However, in wet years they will not do well because of waterlogging. Irrigated
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found over granite and gneiss. The former support a vegetation which is much more nutritious for grazing animals - dominantly grasses and herbs.’ The sour dambos contain more sedges. Occasional trees are found along streams or on termite mounds within the dambo, but waterlogging prevents the growth of most trees. During the dry season there is a serious shortage of grazing for both cattle and wildlife. Crop residues provide some fodder for the early dry season but there is no native tradition in Africa of growing fodder crops or making hay or silage. The dambo grazing is therefore particularly important during the late dry season and early wet season. Groundwater
7S. Heerv, ‘The veoetation ecoloav of dambo gr&slands (around Choma, yambia) in relation to dry season grazing’, Research memorandum 27, Department of Agriculture, Lusaka, Zambia, 1979, 66PP. ‘Turner, op cif, Ref 1. ‘K. Vickery, ‘Aspects of Plateau Tonga economic history’, in R. Palmer, ed, Zambian Land and Labour Studies, Ill, National Archives of Zambia, 1976, pp 55-67. “%I. Whitlow, ‘Vlei cultivation in Zimbabwe,’ Zimbabwe Agricultural Journal, Vol80, 1983, pp 123-l 36.
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cultivation
Small-scale irrigation in streamside dambos may be possible by a gravity fed system of channels, but it is usually
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necessary to lift the water using either shadoufs, as in northern Nigeria, or diesel pumps. Shadoufs consist of a pole, balanced on a pivot, with a clay weight at one end and a rope suspending a bucket or calabash at the other. The materials needed for construction are readily available and they can be operated by one man, though preferably with a partner directing water into channels. About 0.1 ha can be irrigated from one shadouf, with a maximum lift of about three metres. A much larger area, about four hectares, can be irrigated using a diesel pump and the water can be lifted to a greater height. However, apart from the initial cost, shortages of fuel, risk of mechanical breakdown and lack of mechanics able to mend pumps, make this type of irrigation much more risky for an individual farmer. In dambos without stream channels wells are often dug and water lifted by hand and carried to nearby plots. The extent to which dambos are cultivated at present varies considerably from place to place. In the area south of Kano in northern Nigeria, a densely populated area where only 3% of the land is fadama (dambo), over 89% of it is cultivated. However, in the wetter and less densely populated area 100 km further south, 15% of the land is dambo but only 21% of it is cultivated. These differences are related to distance from market as well as to availability of dambo and population density.8 There was a large increase in the area of dambo cultivated near to Zaria between 1963 and 1973. In Zambia, although historical records show that the margins of dambos were the most favoured areas for cultivation in Southern Province at the beginning of this century (partly because clearance of the woodland was not necessary in such areas),” only small areas of dambo are now cultivated, and even close to the towns uncultivated dambos are common. Whitlow has described the changes in cultivation in Zimbabwe.“’ Legal restrictions since the 1920s reduced the areas of dambo cultivated although formerly these were more widespread.
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Since independence there has been an increase in cultivation on both large and small farms. The bolis in Sierra Leone are often used for cultivation of rice and vegetables” and Gosnell reports an increase in cultivation in dambos in Kenya.12 The natural vegetation of the dambos includes many species of value as medicinal plants as well as reeds and palms used for making mats and haskets. Clays and sands are dug for buiiding purposes and for pottery and dambos are also essential in providing water supplies to many villages and watering places for cattle and other animals. Erosion
“R Mlckel, ‘Zur Entstehung und geiikologischen Stellung der Bolis in Sierra Leone/ Westafrika’, B&ichte der Naturforschenden Gesellschaft zu Freiburo im Breisoau, Vol 69, 1979, pp 47-71; a& A.C. MiGngton, F. Helmisch and G.J. Rhebergen, ‘Inland valley swamps and boiis in Sierra Leone: hydrol~ical and ~dological considerations for agricultural development’, Zeitschrirl fiir Geomor~hotog;e, NF Supplementband 52, March 1985; pp2Oi-222. ‘*J.M. Gosnell. ‘Vleiland develooment in West Kenya’, &st African Agricultural and forestry Journal, October 1963, pp QQ105. 13H.A.S. Johnston, The Fulani Empire of Sokoto, Oxford University Press, London, UK, 1967. j4J.R.E. Hindson, ‘The protection of dambos by means of contour seepage furrows’, Mimeo report, Ministry of Agriculture, Lusaka, Zambia, 1964, 107~~. 15A. W~ndram, R. Faulkner, M. Beli, N. Roberts, P. Hotchkiss and R. Lambert, ‘The use of dambos in small scale rural development’, 11 th conference of the Water and Waste Engineering for Developing Countries group, Dar es Salaam, 1985, pp F4.1 -F4.4. ‘%attray et al, op tit, Ref 1. “Heery, op tit, Ref 7; and Gosnell, op tit, Ref 12. “Rattray et al, op cit. Ref 1.
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and degradation
The continued existence of a dambo depends on the maintenance of the delicate ecological balance between water, soils and vegetation. Dambos may be destroyed directly by erosion or they may dry out, losing their water-storing capacity. Such degradation affects not only the dambo itself, but also areas downstream which depend on the slow release of-water from the dambo throughout the dry season. Dambos with peaty soils are particularly susceptible to damage as drying out leads to oxidation of the organic matter content. The main dangers are in 1) overuse of the water supply (either for domestic supplies, for watering stock or for irrigation) leading to lowering of the water table, and 2) erosion of the soil, particularly by gully formation. If ditches are constructed to drain dambos which are seen as too wet for cultivation or grazing, there is a severe danger of increasing the surface runoff, thus causing a rapid fall in the water table in the early dry season. Gully erosion often occurs at the end of drainage ditches or irrigation channels, or where footpaths or cattle tracks cross dambos. Exposure of bare soil which is then easily eroded by the first rains can be caused by overgrazing or large-scale, especially mechanized, cultivation. These both also cause compaction of the soil which increases surface runoff and destroys the soil structure. Burning also results in exposure of the soil
and destroys organic matter thus reducing the water holding capacity. The later the burning, the more the damage caused. Land use of the surrounding upland areas also has a considerable effect on the moisture content of the dambos. In Zimbabwe, wheat was grown on dambos on commercial farms in the 1930s and 1940s and this led to a drying-out of the dambos and gully erosion, and resulted in legislation to restrict cultivation of dambos. However, examples of successful dambo cultivation for many decades can be found in northern Nigeria where irrigation in dambos has also been practised for at least 130 years.‘l Fertility is maintained by manuring with cattle manure and household wastes. In Zambia, Hindson suggested dambos could be protected by means of contour seepage furrows, but avoidance of degradation in general depends mainly on small-scale cultivation with little or no drainage, overgrazing, trampling, exposure of soils or burning, and careful control of water levels and maintenance of fertility. ” In many parts of Africa dambos are underutilized and their potential remains unrecognized. There is a great need for small-scale development on the level of the individual farmer and his family using their indigenous knowledge reinforced by improvements such as provision of better seeds and fertilizer, handpumps, etc. Windram et at have recently reported the development of hand pumps able to irrigate up to one hectare with a lift of up to six metres.” Improved education is necessary both to introduce new ideas and to enable understanding of the problems and dangers of soil erosion and degradation. Other improvements which have been suggested include mowing the dambo grasses and making hay or silage instead of burnjng,‘~ seeding with improved grasses and legumes” and use of rotations and organic fertilizers to maintain fertility.‘s Improvement of dambo cultivation and grazing provides an ideal means of such smallscale development. There is some conflict of use be-
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1986
tween cultivation and grazing especially in areas where these are practised by different groups. The migratory Fulani cattle owners in northern Nigeria have found the increase in fadama cultivation, pa~icuiarly close to the towns and large villages where they sell their milk, prevented their access to traditional grazing and this has led to conflict with farmers. Depending on local conditions, multiple use of the dambos can be practised if a
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balance is maintained between the need for grazing land and cultivation. Features similar to dambos are found in other savanna areas of the world and have been described in South America and Australia. The effective use made of dambos in the most densely populated parts of rural Africa, particularly northern Nigeria, may provide an example of relevance not only in the rest of Africa but in many other parts of the world.
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