One of the objectives of the ALCOM project is to gather information about the small water bodies in southern Africa. The data given below are based on an article by Elin Ersdal of the ALCOM staff, written for ALCOM News. ALCOM is compiling an inventory of dams in southern African countries in order to assess their potential for fish production. This regional inventory is based on national inventories of different countries and so far has national data from Botswana, Lesotho, Malawi, Zimbabwe and one province in Zambia. It is designed to give information on the name of each dam, its location (province, district and grid reference or coordinates), capacity, surface area, type of ownership, dam use, rainfall, soil type and altitude. The coverage varies widely, and information is not complete for any dam. Since the dams are usually constructed for storing water, only capacity is recorded in the original databases and surface area was estimated from capacity, using a slightly different approach in each country. Information about the fish species present, access to the dam, its condition and permanence is also available.
Table 10. Percentage distributions by number and by area of various sizes of dam in Botswana, Lesotho, Malawi, Zambia (Eastern Province) and Zimbabwe
|5 – 10||56%||8%||14%||26%||49%||19%||56%||26%||49%||9%|
|10 – 15||10%||5%||1%||7%||13%||12%||21%||24%||18%||9%|
|15 – 20||4%||3%||1%||7%||2%||3%||8%||18%||11%||10%|
|20 – 50||3%||6%||1%||27%||1%||3%||4%||20%||5%||11%|
|50 – 100||1%||6%||<1%||15%||0%||0%||0%||0%||1%||7%|
|Total number||331||369||755||213||10 747|
|Total area (ha)||4 685||424||2 138||1 180||117 662|
(Data from Ersdal, unpublished)
A total of 12 430 dams are listed in the inventory of dams in the region, with most of them located in Zimbabwe (Table 10). Surface areas (measured or estimated from their capacity) are available for 6 580 of them, giving a total area of 112 000 ha. Assuming that those dams that are listed without a capacity or area value are of median size, then the total area will increase to 126 000 ha. That is almost a quarter of the size of Lake Kariba, or half of Lake Cahora Bassa. Most of the dams (80%) are 5 ha or less in area and they account for only 10% of the total area. In contrast, dams larger than 100 ha account for 55% of the total area.
This estimation is of the area at full supply level but, for most of the year, the area will be much less. Some dams dry out and it is some time before fishing can be resumed in them. Small dams are generally more productive than larger dams and if annual fish production is conservatively estimated at 100 kg/ha, total production will be 12 600 t/yr.
The inventory for Botswana lists 328 smaller dams in the eastern and southeastern parts of the country and three larger ones constructed by the Water Utility Corporation. Most of them (314) are reasonably well documented, whilst very little information is available for the remaining 14. The inventory excludes the very small reservoirs constructed by the local communities under drought relief programmes. Many dams were built for livestock watering in areas where people stay for short periods of the year with their cattle. On average, these dams are 10.8 km away from the nearest village (based on data from 72 dams). This will obviously limit the utilization of their fish resources.
The inventory is based on secondary sources, and in most cases the only available data are storage capacities which, except for the three large Water Utility Corporation dams, vary between 2 000 and 2 800 000 m3. Only 42 dams have a storage capacity that exceeds 70 000 m3. A rough estimate of their areas was made by assuming that the mean depth was 0.5 m in smaller reservoirs, 1.0 m in medium-sized ones (10 000 to 100 000 m3), 1.5 m in larger ones (100 000 to 250 000 m3) and 2.0 m in the largest (250 000 to 500 000 m3). Including an estimate for the dams of unknown size, the total area of water comes to 4 685 ha, the average size is 16.0 ha and the median is 1.8 ha. Reservoirs built by the Water Development Section after 1988 are reported to be perennial, and should give a safe yield in 19 out of 20 years. This applies to 10% of the dams built after 1988, which contribute 35% of the area.
Data on the species composition of the fish is available from only 16 dams. Clarias gariepinus was found in nine of them, Oreochromis spp. in six, and small Barbus species in two. Small labeos, large Barbus species, common carp and eels were all recorded in only one dam. The uses to which the dam is put is recorded for only 38 dams. The most common use is to provide water for livestock and domestic purposes (20 out of 38). Sixteen of the 38 dams are used for irrigation.
A total of 590 dams have been identified in Lesotho. Siltation is a serious problem and 160 dams were found to be completely silted up, and many more are seriously silted. Approximately 40 others are washed away, which leaves 369 functional dams. Their total area was 424 ha with a mean of 0.8 ha and a median of 0.07 ha.
The common carp (Cyprinus carpio) and catfish (Clarias gariepinus) are the most frequently recorded species, being found in 51% and 23% respectively of the reservoirs visited. Other species present included bluegill sunfish (Lepomis macrochirus), Barbus aeneus, Labeo capensis, Labeo umbratus and Micropterus salmoides. Some smaller dams are without any fish.
Most dams are used to provide water for livestock (80 % of the dams) and household purposes (64%), followed in importance by irrigation (25%), fish stocking (25%), municipal water supply (10%), angling (10%), and artisanal brick making (5%).
The dams in Malawi are old, 6 of them having been built before 1950, 125 in the 1950s, 111 in the 1960s, 17 in the 1970s, and only two since 1980. Little information is available on their size; 44% are reported to be in good condition, 25% in fair condition and 30% in poor condition, whilst 2.1% have been breached. Their total area was estimated from the area/capacity relationship of dams in the neighbouring Eastern Province of Zambia, where the dams are similar in age and construction. The total, mean and median areas were estimated to be 2 138, 7.3 and 2.1 ha respectively. Information about their use is available for 287 dams, and 147 of them have multiple uses. Domestic use (53%), irrigation (59%) and water for livestock (37%) are the most important, followed by industry (13%) and fishing (11%).
Data on the species of fish is available from 42 dams, and some of them have up to 5 species. The most important are Oreochromis spp. (found in 58% of the dams), Tilapia spp. (29%) and Clarias gariepinus (83%). Other species include small Barbus species (60%), small Labeo (2%) and bass (12%).
An inventory for the Eastern Province of Zambia, based mainly on information from the Department of Water Affairs, is available. It lists 227 reservoirs, most of them constructed more than 30 years ago. The purpose of building them at that time was to provide water in new areas for the resettlement of people.
Their areas were estimated from the relationship between area and capacity in which
A = 0.2135 C0.74010
where A = area (ha) and C = capacity (× 103 m3). The calculation was based on data from dams less than 40 ha in extent and therefore excluded relatively large ones like Lutembwe 2 (105 ha). The mean and median areas were 5.8 and 3.0 ha respectively. This calculation assumes that all reservoirs are still in their design condition, which is unlikely, since 52 of them (area = 224 ha) are known to be breached. It is likely that others have breached or silted up without being reported. Some dams may also be smaller than they were designed to be (the opposite is less likely), so although the total surface area is around 1 180 ha, in reality it is probably less than 900 ha.
Some data on fish are available from 15 dams, and up to 9 species have been recorded from one of them (Lutembwe 1). Barbus paludinosis and Labeo cylindricus are the most widely distributed species, in 14 dams, followed by Oreochromis macrochir in 13 and Tilapia rendalli in 9 dams.
Although the inventory for Zimbabwe is by far the largest, with 10 747 entries, relatively little information is available on each dam. The dam database is compiled by Department of Water Development, Ministry of Lands, Agriculture and Water Development, and the AGRITEX Fisheries Unit has analysed the data using a geographical information system (GIS) (Chimowa and Nugent, undated). Efforts to calculate area were unsatisfactory, partly because of the poor quality of the available data and partly because data for small dams were lacking. The relationship from the Eastern Province of Zambia was therefore used, and gave an estimate of 117 662 ha for the total surface area, with a mean and median of 16 and 3.0 ha respectively.
Sixty percent of these dams were privately-owned and located in commercial farming areas, but they tended to be smaller than those in communal lands, making up only 47% of the total area. Of the remaining area, 35% is in communal areas, and 5% in resettlement areas. Information about their use is available for 8 227 dams; 75% is used for agriculture, 1.6% is used for urban, government and institutional water users, 0.5% is used by industry or mining and 20.9% for other purposes.
The inventory has no data on the species of fish found in these dams, but information on the main targeted species is available from the AGRITEX Fisheries Unit, who have surveyed one hundred dams. Clarias gariepinus was the most widely distributed species, being found in 84% of the dams, followed by Oreochromis mossambicus (79%) and O. macrochir (23%). Other species included Tilapia rendalli (53%), Labeo cylindricus (61%), Micropterus salmoides (22%), Barbus marequensis (16%) and Mormyrus longirostris and Labeo altivelus (both in 2% of the dams).
ALCOM is aware that other SADC countries are also compiling dam inventories, but these have so far not been incorporated into the regional inventory. Hopefully this will happen in the future. All the inventory files are available on a computer in ALCOM's offices in Harare, and can be made available to interested users. The inventory format can also be made available.