The ecosystems that qualify as wetlands are varied and many. The definition of a wetland continues to be a subject of great debate and not any one of those put forward by various workers fits all situations to perfection. For the purposes of this paper however, a basic and simplistic definition of a wetland is preferred as "being that land which is subject to permanent or temporary waterlogging, resulting in land use that supports aquatic or semi-aquatic plant life cycles permanently or temporarily in its natural state".
In Zimbabwe such wetland ecosystems include dambos (mapani), flood plains, artificial impoundments and pans. All of them share the common factor of having excess water retained or passing through for a long enough period to influence the soil characteristics, land use and life forms that flourish within them.
Wetlands are an important resource which has however been neglected by research and policy makers due to the misconception of them either being wastelands vulnerable to cultivation or natural resources with no need for management. To-date there is a dearth of information regarding their nature, management and response to agriculturally related activities.
Zimbabwe lies within the tropics and covers an area of 396 000 km2 extending from 15º30'S to 22º30'S and from 25ºE to 33ºE . The country has three major regions distinguished on the basis
of elevation: Lowveld (below 900 m above sea level), Middleveld (900-1 200 m) and Highveld (above 1 200 m).
A wide range of geological materials occur in Zimbabwe and most of these are igneous and metamorphosed igneous rocks which occupy about 65 % of the land area. Of these, granites are the most dominant in the craton, accounting for 46% of the area. Materials of sedimentary or aeolian origin are also found mainly in the northern and northwestern parts of the country accounting for 25 % of the land area.
Complexes of metamorphosed basaltic and andesitic lavas and sediments, although less extensive in area than the other formations, are an important feature of the Zimbabwean geology. They give rise to the agriculturally important red soils and are a source of valuable minerals such as gold.
The Great Dyke, which is a unique feature of Zimbabwe's geology forms the central crest stretching for 540 km in a north-northeast direction and is made up of 4 elongated lopolith mafic and ultra-mafic rocks containing high grade ores of such minerals as chrome and nickel. The mafic rocks give rise to formations rich in ferromagnesian minerals and thereby giving rise to red and yellowish red clays in well drained positions. The ultra-mafic rocks give rise to soils characterized by the dominance of magnesium over calcium and often contain toxic levels of the heavy metals such as chrome and nickel (Nyamapfene, 1991).
Basalts extruded during the Jurassic period occur in two areas: north of the watershed and south of the watershed and only differ in their mineralogy. These basalts give rise to black vertisols and are different from the basaltic greenstones of the Gold Belt which give rise to red clayey soils referred to earlier.
The relatively high elevation of the country as a whole has a moderating effect on temperature resulting in most areas of the country enjoying lower temperatures than would be expected for its latitude. Altitude ranges from 500 m in the Lowveld to 2 400 m in the Highveld (Eastern Highlands) and this comes with a variation of temperature from hot to cool respectively for most of the year. Frost incidences do occur occasionally during winter in isolated locations particularly in the Highveld.
Most of the rainfall in Zimbabwe, which ranges from a few hundred millimetres to over 200 mm, is of the conventional type although the orographic type occasionally occurs in the eastern highlands due to the increased elevation. Rainfall decreases from east to west and to a limited extent from north to south.
Some close associations exist between soil and rainfall, rainfall and vegetation and soil and vegetation (Nyamapfene, 1985). These associations between soil, rainfall and vegetation were the basis for the division of the country into five Natural Regions (NR) which are largely agro-climatic but also take into account other relationships and physical factors that may affect agricultural production (Nyamapfene, 1991). Agricultural production potential decreases from NR I to NR V. NR I has the highest mean annual rainfall and the lowest daily evapotranspiration whilst NR V has the lowest annual rainfall but has the highest evapotranspiration. This results in greater soil moisture surpluses in NR I than NR V during an average rain season.
The variety of wetlands that exist in Zimbabwe has been identified, described and mapped in one form or the other by various institutions and individuals. These ecosystems are dynamic and hence details could change over a short period. A brief outline of each of these wetlands is presented below. Pointers for their characterization and classification in respect to the guidelines are drawn from the available information and experience and also presented. For the purposes of this paper, only aspects referring to dambos will be discussed in detail.
Flood plains are associated with the major drainage systems and tend to be well developed in low lying flat areas. Zimbabwe is located on a plateau and hence there are few small flood plains. These are found in the Zambezi Valley and around the Save-Runde confluence in the southeastern part of the country. Currently flood plains are being used for safari hunting and tourism.
Pans are depressions that collect and retain water from the surrounding uplands. They are generally saline due to the accumulation of salts brought by water that eventually evaporates. A few pans occur in the drier and hotter environments of the country which include the western districts (Tjolotjo Communal Areas and Hwange National Park) and the southern districts (Gonarezhou National Park and Mwenezi). Those pans that occur in human settlement areas are used for cattle grazing and watering, while those located in national parks are habitats for water fowl and visited by game and tourists.
There are few swamps in Zimbabwe and the notable ones are Tsamtsa and Kwazulu Swamps, both located in low rainfall areas in the southern and northeastern parts of the country. The Binga Swamp in the Goromonzi District is in danger of drying out due to excessive utilization pressure. These swamps are used for grazing and livestock watering (Matiza, 1994).
Artificial impoundments (dams)
There are over 8 000 man-made impoundments (Matiza, 1994), ranging from very small single farm units to very large ones covering several square kilometers. The large impoundments include Kariba, Mutirikwi, Chivero, Manyame and Mazvikadei Dams. Except for the Kariba Dam, which was constructed for hydroelectric power generation, all the other dams were constructed for either domestic water supply, irrigation and livestock watering or a combination of those. Fishing, recreational and tourist activities have generally developed around these impoundments.
Dambo is a Chichewa word that is used to describe a grassland in both Zambia and Malawi. It has now been adopted for use by the scientific community within the SADC region for purposes of uniformity. Locally these ecosystems would be referred to as `bani' (Shona) or `vlei' (adapted Africaans). Elsewhere such terms as `inland valley' (Sierra Leone), `mbuga' (Tanzania) and `fadama' (Nigeria) are used to refer to similar ecosystems. Included in this class are riverine dambos which are found along most of the country's major drainage systems. These are included on the basis that they are old dambos that have been dissected by water passing through over time and thereby forming streams.
Geology and geomorphology of dambos
In general, dambos are be valley bottoms or depressions that form natural drainage systems with or without a developed and distinct stream. They form the upper reaches of the drainage system below both main and sub-catchment areas of the country. Zimbabwe has 5 drainage zones and dambos are found in all of them, albeit with different shapes, sizes and distributions.
The majority of dambos have geological characteristics of the Basement Complex which is largely comprised of the igneous and the metamorphic rocks. A smaller but significant amount is associated with the Kalahari sands and Karoo sandstones. Both geologies give rise to sandy or sandy loamy soils. A very small percentage of the dambos is associated with the doleritic geologies that give rise to clayey soils. It is estimated that dambos cover 3.6% of the land area and that approximately 84 % of these occur on gneiss and intrusive granitic rock (Whitlow, 1984).
The highest concentration of dambos is found above the 700 in contour which coincides with areas covered by NR 1, 11, III and parts of IV. The occurrence of dambos decreases with the decrease in altitude through NR IV and V. Dambos in the central highlands and midlands are generally broader and have more gentle slopes than those found in either NR I and V. The processes of natural erosion have, over time, shaped both the catchment areas and dambos to their present condition which is generally referred to as undulating. Dambos in the other areas are associated with steep slopes because of the mountainous and broken terrain.
Rainfall has a major influence on dambo formation and development and hence their occurrence in Zimbabwe is closely related to the rainfall distribution patterns. Areas with the highest concentration of wetlands (Central plateau) closely resemble the areas that receive the highest rainfall on comparable geologies and topography. Dambo development is brought about by the movement and accumulation of soil aggregates, solutes, and organic matter from the catchment areas to the lower areas by water. The movement is either on the surface through run-off or subterraneously through seepage of excess water. The higher the rainfall is, the higher is the frequency of such movement and hence the higher the rate of dambo development.
In respect to rainfall, dambos in Zimbabwe could be classified as high, medium and low rainfall area dambos. Locations of such dambos would broadly coincide with the NR I and II (high), III (medium) and IV and V (low) respectively. Such a classification would have direct input into the management of water as the major variable. An alternative assessment could focus on the level of development of the dambo brought about by the action of rain water. Comparable dambos in the high rainfall areas are more advanced in their development than those in lower rainfall areas in respect to profile formation/destruction. In this respect terminologies such as developed, medium and young could be used for such classification.
Winter temperatures are generally low throughout the country but some areas are more prone to the incidence of frost than others. Frost causes damage to most crops and wetlands are more susceptible to its occurrence than uplands since they accumulate cold air pockets due to their hollow formations. Furthermore, dambos facing east and southeast (the direction of the trade winds) tend to be more susceptible than those facing other directions. Those facing other directions are sheltered from the cold winds and also benefit from the afternoon sun. A review of the frost incidences recorded over time by the National Meteorological grid sta tions could result in the categorization of wetlands into frost prone and frost free zones. This classification would provide input into the guidelines for crop selection and cropping patterns for wetland areas.
Wetland soils differ widely in their texture, depth, profile, nutrient status, stability, workability, etc., to the point that many variants of this are found within and between dambos in close proximity. The variations are in three dimensions: top to bottom of dambo, perpendicular from edge to centre of dambo and the diagonal from the edge to the centre and bottom. In view of this any attempt to classify dambos according to soil types can only be on broad terms. The broad classes that are common and identifiable are:
The above classification would be equated to the conventional soil classification systems for standardization. Furthermore, soil classification of this nature would be linked to the management and workability of these soils for both agricultural and non-agricultural purposes.
Classification according to the nutrient -status would broadly place sandy dambo soils derived from granitic material in a class of low fertility, while dambo soils derived from the clay forming parent materials would be placed in the class of fertile soils. This is however influenced by the location in respect to rainfall amounts which cause leaching of nutrients in and out of the system. The occurrence of toxicity from heavy minerals in soils derived from ultra-mafic rocks in the Great Dyke would need recognition and identification of wetlands affected by this constraint.
The hydrology within the dambo, like in a reservoir, is dependent on the processes that take place in the catchment area. Dambos receive incident rain, catchment run-off and seepage from catchments. The importance and significance of any of these input sources to the hydrological status of the dambo is variable and dependent on such factors as catchment size, infiltration rates both in catchment and dambo, dambo size, ratio of dambo size : catchment size, rainfall amount, timing of rainfall event in respect to season, location, etc. This makes it difficult to generalize.
In some situations where the dambos dry out, incident rain is critical for crop establishment during the beginning of the summer (rainy) season and then the run-off takes the major role to top up the profile in the early crop growth stages and eventually the seepage water maintains the wetness to complete the season and beyond. Some dambos remain wet throughout the season and in that case the role of each input source changes.
A useful classification of wetlands based on the hydrology would be the groundwater level (wetness) of the dambo at the critical stages of its utilization cycle. Critical stages could be the time of land preparation, crop establishment or crop harvest. Understandably this varies within dambos, between dambos and between seasons but an average picture could be drawn based on average expected seasons. Development of such a classification system would require monitoring of ground water levels over a number of seasons covering both the extremes as well as the average quality ranges. The results would then be linked with the preferred utilization system and management of both dambo and catchment which might be cultivation, grazing, fishery, etc. or their combination. An example of such classification for dambos intended for cultivation would have the following criteria:
Vegetation is a potential good indicator of the wetland condition in respect to hydrology and soil fertility and hence can be used as a quick indicator of the possible uses of a dambo. Examples in this are the `Mukute tree' which is an indicator of shallow groundwater tables and is often used for the siting of shallow wells. Similarly, particular sedges and grasses are associated with waterlogged conditions, acid soils, alkaline soils, fertile soils or infertile soils. In this respect grasses could be used as indicators of soil nutrient status and hence potential dambo utilization system. Palatability of grasses as indicated by them being preferred by grazing animals is also an indicator of them being nutritious. Pastures with such grasses are generally referred to as being `sweet' and are associated with fertile unleached soils. To use vegetation as a quantifiable indicator it would be essential to determine correlations of the relevant vegetational types with the parameters that they indicate.
Dambos are part of an environmental system operated and managed by farmers for their benefit and the impact of their utilization on the whole system should be such that it is economically viable and sustainable. Whilst dambos can provide water, food and increased incomes through increased productivity compared to uplands, this should not be done on the expense of the environmental condition. Excessive pressure on these ecosystems will result in their degradation since they have, albeit more but finite resources.
In this respect there is need to determine optimal production potentials for the different classes of dambos and thereby draw their utilization guidelines and limits. The ratio of cultivated to grazed dambos would need to be determined in the different agro-ecological zones, based on the prevailing socioeconomic environment. Pilot schemes based on farmer participatory research approaches at catchment level would provide valuable information.
Two wetlands, dambo 1 and dambo 2, are identified from the classification methodology using satellite imagery at an average accuracy of 96%, which is within the overall acceptable classification accuracy for land cover mapping in the Department of Natural Resources. Dambo 1 is associated with open textured sandy soils that are characterized by low moisture tension which results in low moisture content during the dry period. It mainly occurs in areas underlain by granite alluvial sediments. There is a close relationship in the occurrence of dambo 2 and dolerite dykes and metasediments.
River, streams and sponges are treated as separate natural resources. These are mapped and stored separately in the hydrological thematic layer of IRIS database. The data capture and classification for those, is derived from the 1 : 50 000 Surveyor General topographic map.
Satellite image analysis is used in the classification process. The satellite which is used is Landsat Thematic Mapper (TM). The two main seasons in Zimbabwe are considered to be the most important factor on selecting the appropriate date for satellite remotely sensed data suited for wetlands inventorying and mapping. These are the wet season (November to March) and the dry season (April to October). The dry season is regarded to be the most ideal as wetlands stand out as islands of high moisture in a relatively dry environment.Out of a total of 7 Landsat TM bands, TM bands 3,4,5 are used for the purpose of wetland classification. This combination has been found to be among the most ideal band combinations used in classifying wetlands (Maedel et al, 1996).
The classification process is conducted using the supervised classification technique based on known `training' sites. The selection of training areas is primarily based on the geological/underlying bedrock types. The assumption is that wetlands found on different geological formations have different moisture regimes because of the different moisture retention capacities they possess (Murwira 1997). The supervised classification technique uses the Maximum Likelihood Classifier (MLC) algorithm. MLC assumes Gaussian distribution of data. The classification process involves grouping the unclassified pixels in a satellite image into different specified classes basing on probability. Pixels are assigned to their most likely classes basing on highest probability values (Lillesand and Kiefer, 1987).
The supervised classification is followed by field verification. Hand-held Trimble Geographical Positioning Systems with an accuracy of plus or minus five metres are used to enhance accuracy in the field checking. In case of Mashonaland East Province, 89 randomly selected areas were verified. The data collected on the ground checking and that of the MLC is compiled into a contingency table to assess the accuracy.
The final classification is geometrically corrected to the UTM system using the Integrated Resource Information System (IRIS) of the Department of Natural Resources. Digital spatial database is generated from the Surveyor General's 1 : 50 000 topographic maps. The geometrically corrected product is then used to produce wetlands maps at a scale of 1 : 50 000.
The inventory and mapping of wetlands has been done in Mashonaland East Province by the IRIS Unit in the Research and Technical Branch of the Department of Natural Resources. The latter is part of the overall IRIS programmes. IRIS is an environmental information system that is used to manage the natural resources and environment (at present in Mashonaland East Province). IRIS is based on the Inventory, Monitoring & Assessment and Prescribed Management (IMAP) data model. Geographical Information Systems (GIS), remote sensing techniques and orthodox manual techniques are used in IRIS . The GIS software that is used is ARC INFO. The attribute data are managed by Microsoft Excel software.
Research and Development
It is estimated that Zimbabwe is endowed with 1.28 million hectares of wetland areas and 20% of these are in the communal areas. Prior to colonization and the enactment of the Land Apportionment Act, indigenous farmers cultivated wetlands and were successfully producing crops as rice, maize, cleus esculentus and vegetables. Evidence of wetland cultivation is available from notes by early travelers, folklore and remnants of ridge and furrow landforms.
The utilization of these ecosystems is now governed by the Natural Resources Act and the Water Act, which seek to protect them from degradation particularly through cultivation. The enactment of these restrictive pieces of legislation was a reaction of the Government to the mismanagement of these ecosystems by settler farmers who sought to cultivate wetlands conventionally for purposes of growing crops as maize, wheat and tobacco. Since these crops do not grow well in areas with high water tables, drainage lines were installed, which caused the drying up of wetlands, soil erosion and the siltation of rivers and dams. Wetlands were then classified as being unsuitable for cultivation and are currently designated as grazing areas. Only in very special but rare cases is permission granted for a farmer to cultivate wetlands by the Natural Resources Board which is the authority with the responsibility to administer the legislation.
Currently wetlands and their related catchments continue to degrade at an alarming rate and the cultivation of wetlands is increasing without the granted permission from the Board. Such encroachment through cultivation, in the absence of technical guidance or traditional experience which has since been forgotten by many, can only result in increased damage.
Research has trailed behind mainly because of the misconceptions, ignorance, fear to destroy wetlands and the restrictive legislation imposed by the administrators. Very limited research was done on wetlands in the past and the need for gathering information on the management of these ecosystems is obvious and long overdue. It must be realized that, whilst the pieces of legislation are restrictive, their relaxation in the absence of appropriate utilization guidelines based on sound and proven scientific principles would be an act of irresponsibility which could worsen the current situation.
Current researchers have the task to develop an understanding of the processes that operate within the ecosystems (catchment-wetland-stream) and thereby formulate management techniques that enhance the conservation and utilization of these natural resources for the benefit of communities. It is imperative to conduct these studies in a systems approach with the full participation of farmers whereever possible or appropriate.
Results from recent/current research have shown the following:
Researchers on wetlands
Research on wetlands is being carried out by a number of institutions and individuals in Zimbabwe. Some of these institutions include the Lowveld Research Stations, the Agronomy Institute, the Horticultural Research Institute and the Chemistry and Soil Research Institute in the Department of Research and Specialist Services. Others include the Department of Natural Resources, several departments at the University of Zimbabwe and IUCN.
Research in the Department of Research and Specialist Services is coordinated by the Lowveld Research Stations and is focused on developing an understanding of the processes in the wetland ecosystems and on formulating management options for the conservation and utilization of wetlands on a sustainable basis. Current research is being carried out on-station at Makoholi Experiment Station and The Horticultural Research Centre and on-farm at pilot sites in Seke and Guru Communal areas. Monitoring is being carried out on a wide range of physical, chemical, biological and socioeconomic factors that are relevant to the management of wetlands.
Research in the Department of Natural Resources is carried out by the Research and Technology Branch (RTB). There are two main researches that have been carried out by RTB: inventorying / mapping of wetlands and the application of environmental economics. Research in the area of the application of satellite remotely sensed data for inventorying and mapping was carried out in Mashonaland East Province, while research in the application of the environmental economics in management of natural resources was carried out in the Mashonaland Central Province. The research focused on the application of natural resources valuing technique to enhance policies that affect the management of wetlands.
Currently research is being carried out in the area of soil moisture regimes and spectral variations in the wetlands. The research is expected to enhance inventorying and mapping capabilities of the wetlands as well as the vegetation that is associated with them. Landsat TM satellite imagery, a spectroradiometer and global position systems are being used in the research.
It is expected that results from these and other studies will be useful in the review of the current legislation.
Some of the constraints in the research of wetlands in Zimbabwe are:
Future research is being directed at analyzing appropriate methodologies of utilizing dambos with the aim of assisting the Natural Resources Board (NRB) and Department of Natural Resources (DNR) in the process of exempting the communities and or individuals who might want to utilize wetlands. Currently the NRB under the Natural Resource Act can allow individuals or communities to utilize the wetlands after the DNR officers have conducted field analysis of the wetland to be utilized. The capacity for this analysis needs to be sharpened and supported with scientific and technical information generated through research.
Some of the research needs are:
Wetlands are a valuable natural resource that has not received the attention that it deserves for far too long and is currently degrading at an alarming rate which might soon get beyond control, This is despite the fact that this resource offers the most obvious opportunity for a `green revolution' for this region - if only we can all see it that way.
Lillesand T.M. and Keifer R.W. 1997. Remote Sensing and Image Interpretation. John Wiley and Sons, Toronto.
Maedel, J., Murtha, P. and Mocre, K. 1996. Assessment of digital data for wetland identification in the Cariboo/Chilcotin Region of British Columbia. In: Proceedings for the 26th International Symposium on Remote Sensing of Environment, The 18th Symposium of Canadian Remote Sensing Society: Information Tools for sustainable Development, March 25-29 Vancouver B.C. Cananada pp233-244.
Matiza, T. 1994. Overview in Wetland Ecology and Priorities for Conservation in Zimbabwe: Proceedings of a Workshop on Wetlands in Zimbabwe. Matiza, T. and Crafter S.A. (Editors), IUCN.
Mharapara, I.M. 1994. A Fundamental Approach to Dambo Utilization. In: Proceedings for a Workshop on Dambo Farming in Zimbabwe: Water management, cropping and soil potentials for smallholder farming in the wetlands. pp 1-8. Owen, R., Verbeek, K., Jackson, J., Steenhuis, T. (Editors).
Mkwanda, R. 1995. Towards A Sustainable Management Strategy For Wetlands in Zimbabwe. In: Proceedings for a Workshop on Dambo Farming in Zimbabwe: Water management, cropping and soil potentials for smallholder farming in the wetlands. pp 83-87. Owen, R., Verbeek, K., Jackson, J., Steenhuis, T. (Editors).
Mkwanda, R. 1997. Wetlands Inventories and Environmental Management. In: Proceedings of the Second National Wetlands Workshop (in press).
Murwira, A. 1997. Application of satellite remotely sensed data (Lands at Thematic Mapper (TNM)) in the mapping of wetlands in Mashonaland East Province, Zimbabwe (in press).
Nyamapfene, K. 1991. Soils of Zimbabwe. Nehanda Publishers.
Whitlow, J. R. A Survey of Dambos in Zimbabwe. Zimbabwe Agricultural Journal 81-4.