A FISHERIES GIS FOR ZIMBABWE:
AN INITIAL ANALYSIS OF THE NUMBERS, DISTRIBUTION AND SIZE OF ZIMBABWE'S SMALL DAMS
|FAO/UNDP ZIM/88/021||“SUPPORT FOR RURAL AQUACULTURE EXTENSION”|
M. CHIMOWA and C. NUGENT
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This paper presents an analysis of some of the data used by the Fisheries Unit of AGRITEX concerning the small dams of Zimbabwe. Some useful data on the distribution of these small dams is presented, both in tabular and map form. The analysis of a large database of dams, of which the only geographical reference was the national grid reference, was made possible using GIS (Geographical Information System) software called ARC/INFO. This was used to overlay the grid references on top of a map of administrative boundaries of Zimbabwe. From this, information on Province, District and land classification were extracted for each dam.
The information presented is only a first small step in the analysis of the data, and future work will include extensive verification of the existing database, as well as integrating much more detailed data about the dams and their fisheries.
The GIS approach
Fisheries Unit is a specialist technical support unit, based in Harare. The staff are expected to provide their knowledge and expertise to farmers and the agricultural field service throughout the country. Acquiring and analyzing information is an important function of the Unit. It is by accumulating, analyzing and archiving this information, which is obtained over years, that the Unit can acquire sufficient knowledge about the sector to be in a position to efficiently advise, assist and to guide development.
Many important aspects and constraints of fisheries in Zimbabwe have a geographical link: e.g. rainfall, temperature, altitude, water availability, agro-ecological zones, hydrological zones. Other aspects of importance to developing fisheries e.g. demography, road infrastructure, other farming activities, towns(markets). are also mapped on thematic maps, or are quantified in various databases. AGRITEX and its Fisheries Unit collect data on fish farming and dam fisheries in various forms, usually filing it or entering it on a database on the basis of location at various different levels: province, district, ward, point.
This rather varied array of information lends itself to being organized and exploited in a Geographical Information System or GIS. The GIS computer system is able to combine maps (of administrative boundaries, or thematic information such as agroclimatological zones), with general information from various sources such as population (source CSO) or rainfall (Met. services), as well as with databases of fisheries information (such as fish farmers, pond productivity, dam water conditions.). From these combinations of georeferenced information, it is possible to make analyses useful for the Department.
Fisheries Unit has begun the development of a simple GIS with the intention of improving the management and systematic acquisition of essential fisheries related information. Use of the GIS will provide the Fisheries Unit with opportunities to clearly define which types of data are important, and to make data gathering more efficient. They will be in a position insist on a minimum quality of the data, and to analyze and present certain types of information in an accessible way to both collaborators and farmers. The overall framework for the fisheries GIS is presented in figure 1.
AGRITEX, as a department, is developing a similar approach for agricultural information in general. Fisheries GIS is just a part of this overall approach, and indeed it will only reach its full potential when the main GIS, with its extra resources, is fully operational and can provide a much richer choice of digitized maps and various appropriate data layers.
The Dam Database
Initial work in Fisheries Unit has centred around building up a number of databases (see figure 1), of which the largest and most comprehensive is a database containing details of ‘all’ the dams in Zimbabwe. This is based on a database developed by the Department of Water Development, and which has been adapted and extended with a number of new fields to suit work on a fisheries GIS.
At present it contains about 9800 entries, which includes a number of probable duplicates and omits about 600 entries for which it is not yet possible to provide a geographical reference. Most fields in the database are incomplete, many with little information; it is hoped that these first steps to use the information will provide the impetus to both Departments over the coming years to complete a full set of data.
This report presents some facts about dams in Zimbabwe after preliminary analyses using both GIS and traditional database approaches. Kariba, being a singular exception in most ways, is not included these analyses of the smaller dams.
Figure 1: GIS-ASSISTED MANAGEMENT OF AGRITEX FISHERIES DATA - A SCHEMA
FIGURE 2.: Location of 9818 dams in Zimbabwe.
Figure 2 gives a good visual impression of the uneven distribution of dams across the country. Individual dam locations can not all be distinguished due to overlap of points in areas where the density of dams is highest, especially in the large scale commercial farming areas (see also figure 4).
Table 1: No. and capacity of dams per Province in Zimbabwe
|Province||Total number of dams||% No.||Total capacity|
|Number of dams|
The same information is presented in Table 1, which details the number of dams to be found in each Province. Total dam water capacity is also given in Table 1, although these figures are not definitive since this field is not yet complete for all dams, and the figures still have to be carefully verified. Nevertheless, this field gives some interesting information on the types of dam to be found in the different Provinces. Matabeleland South, dry cattle country, has a very high density of dams (23% of the national total), but these represent only 11% of the total water capacity; most of the dams in this area are relatively small and are built for livestock watering. The figures for Masvingo Province are the reverse, and this reflects a small number of relatively large dams supplying irrigation water to the low veld (Mutilikwe, Manjirenji, Manyuchi, Siya).
Table 2: No. and capacity of dams per Province in Zimbabwe
|Province||Total number of dams||Total
|Number of dams|
|>1 000 000|
|<1 000 000|
The most important fisheries will be centred around the largest dams, and Table 2 and Figure 3 indicate the distribution of the largest dams (>1 000 000 m3) across the country. Table 2, in particular clearly shows that most of the dams are very small, less than 100 000 m3 each; those dams with no capacity data are included in this latter category, but at least 60% of all the dams are certainly less than 100 000 m3. Again it should be emphasized that the exact figures will only be available after detailed verification of the capacity data. Table 3 gives a detailed breakdown of the numbers of dams by administrative district; sample maps giving the distribution of dams in the pilot districts of the project are found below.
FIGURE 3: Location of the largest dams in Zimbabwe (greater than 1000000 m3)
Table 3: Number and capacity of small dams by District
|DISTRICT||NUMBER OF DAMS||DAM CAPACITY th.m3|
It is of particular interest to know how the dams are distributed among the different types of land occupation (communal, commercial etc..). Details have been obtained by overlaying a plot of all the dams in the database on top of a map of land classification.
Clearly the density of dams is much higher in the commercial areas than in most communal areas (see figures 4.1. and 4.2.). Similar basic information is presented in Table 4. for all the major land classification types. The actual figures for resettlement area dams will not be available until an up-to-date map of resettlement farms is digitized; at present most resettlement farm dams are still included in the category of large scale commercial farming (LSCFA).
FIGURE 4.1. : Location of dams in the communal areas of Zimbabwe.
Table 4: Numbers and capacities of dams by land classification in Zimbabwe
|Land Classification||Total number of dams||No.|
|Number of dams||Number of dams||Number of dams||Number of dams|
|>1 000 000|
|<1 000 000|
|PARKS, FORESTS etc..||87||1%||1863936||23%||11||3||23||50|
FIGURE 4.2.: Location of 1686 communal area dams out of a total of 9818 dams.
FAO project OSRO/ZIM/202/SWE “Restocking of dams in communal farming areas” has set out to restock the communal dams which exceptionally dried out during the drought of 91/93, in order to quickly re-establish the fisheries for the benefit of the local communities. Planning of a logistical exercise of this nature could benefit from the use of GIS. The GIS was not ready in time to contribute to planning, but could do so in the second phase beginning in November 1993.
FIGURE 5.1.: Combined image of dams stocked and those remaining unstocked - July 1
Figures 5.1., 5.2., and 5.3. show the current information on dams that have been reported to AGRITEX district offices as having dried out during the drought. These figures also display the 383 dams that have already been stocked during the first phase of the operation to restock dams, which continued until July 1993. The breakdown of stocked dams by Province is given in Table 5. Some provinces have received relatively more attention than others, which will be targeted more intensively in the second phase during the 1993/4 season.
FIGURE 5.2.: Dams reported as dried up to Jan 93.
FIGURE 5.3.: Dams restocked by OSRO project to July 93.
Table 5: Dams restocked by Province - OSRO/ZIM/202/SWE
|PROVINCE||NUMBER OF DAMS STOCKED|
The maps presented in this document are necessarily small in order to fit a national image on an A4 page. However, the GIS is able to zoom in on any given area, to give the operator as detailed information as he requires. The following figures are images created of district boundaries, giving the distribution of dams in those districts chosen for initial pilot activities.
Figure 6: Location of pilot zones for small dam fisheries development
|Figure 7.1: Mount Darwin district||Figure 7.2: Murehwa (UMP)|
|Figure 7.3.: Mudzi district||Figure 7.4.: Mutoko district|
|Figure 7.5.: Murehwa district||Figure 7.6.: Mwenezi district|
The data reviewed above gives a first clear picture of the distribution of the different sizes of dams in Zimbabwe. As expected, the majority of dams have been constructed in the commercial farming areas. In contrast, these dams tend to be small, especially in cattle ranching areas, and it appears that the overall capacity figures for commercial and communal areas are comparable. However, the figures for communal areas include some large dams built also for commercial irrigation on neighbouring farms. Careful verification of the data in the data fields for full supply capacity and full supply area is now urgent. Close contact and collaboration should be maintained with the Department of Water Development if such a verification is carried out.
The analysis of the data was only possible by using the GIS software; this linked the information from the grid reference data field in the database, to the digitized maps of administrative boundaries and land use classification. Grid reference data will be the most useful form of georeference for dams, and should continue to be used in preference to the incomplete location coding system of the original database. This has demonstrated the potential of the GIS to organize the large quantities of data in the dam database, and to present this in a useful way for the technical specialists and planners of the department. In the short term, the same basic information will be linked to digitized data on
For the short/medium term the FAO consultant Dr. G. Meaden has made proposals for the development of the GIS for fisheries related data. These will be adopted and developed in conjunction with AGRITEX's plans for a departmental GIS for all land use planning data.
“Dam Database”. Ministry of Lands, Agriculture and Water Development, Department of Water Development.
Consultants reports. Dr. G. Meaden. FAO/ZIM/88/021.