Mozambique country paper
Wetlands for agricultural development

Introduction

Over much of the world, the combination of population growth, rising food demands and severe economic stress contributes to increased pressure on the renewable natural resource base. In some regions, particularly in Africa, this is aggravated by drought. Wetlands are especially vulnerable to such pressures and many of them are lost because their full value to the society is not taken into account in the planning process (Dugan, 1991).

Wetlands are ecosystems, which have recently caught attention, though the need for their sustainable development deserves further concern. The importance of wetlands is related to their potential to retain large volumes of water, which can be used for system maintenance and for dry season agriculture. They provide a habitat for different migratory species, such as birds, and are a refugee for some wildlife during the droughts (Chenje et al, 1996).

Agricultural production in Mozambique is extremely dependent on the rainfall pattern and the climatic uncertainty contributes to wide variations in crop production. According to the modified Thornthwaite method (Reddy, 1984), 80% of the country is classified as semi-arid tropics, constituting the dryland agricultural belt (Figure 1). Despite the predominant arid and semi-arid climate of Mozambique, its geographic and physiographic conditions contribute to the occurrence of a very diversified wetland system. Wherever there are wetlands there are people, mainly small farmers and fishermen. This close association between people and wetlands draws attention to the remarkable strategic importance of these ecosystems in the rural economy and the need for an effective planning, management and conservation strategy.

Classification of the wetland systems in Mozambique

According to the Ramsar Convention (1971) the definition of wetlands considers a very wide range of inland, coastal and marine ecosystems, including lakes, floodplains, freshwater marshes, peatlands, estuaries and mangroves (Dugan, 1991).

F. Gomes, J. Mafalacusser, M.R. Marques 
National Institute for Agricultural Research 
Rui Brito 
Faculty of Agronomy and Forestry Engineering 

Mozambique is a coastal country, crossed by many rivers running to the Indian Ocean and particular attention should be paid to the potential of wetlands for ecosystem maintenance and agricultural development. According to Chabwela (1991) all the five wetlands ecosystems occur in Mozambique: (a) Marine system, (b) Estuarine system, (c) Riverine system, (d) Lacustrine system and (e) Palustrine system (Figure 2).

Marine system: It is the largest and the most important wetland system of the country (Chabwela, 1991), extending along the coast for about 2 500 km in length alongside the coast. The most distinctive feature is the almost continuous fringing of beach ridge complexes and beach plains and coral reefs. The swales between the beach ridges are generally swampy. Present beaches, sand splits and bars, tidal flats or non-tidal swamps are included in this system.

Estuarine system. Particularly well developed in the deltas of Zambezi, Pungoé and Save rivers. Mangrove swamps seems to dominate this ecosystem, being almost continuous along the open coastline in the north and centre, but less common in the south (Hughes and Hughes, 1992). Estimates of the total area occupied by mangroves is about 396 000 ha (Saket and Matusse, 1994). The estuarine system is important for artisanal fishing, salt extraction, and shrimp culture.

Riverine system. This is the largest inland wetland in Mozambique and it includes the floodplains and swamps, following the main drainage system. Floodplains and swamps are important environments for livestock production, fisheries, wildlife and agriculture. They comprise the following main river systems from south to north (Hughes and Hughes, 1992):

• Rivers of Maputoland
• Limpopo system
• Inharrime river and the interior of Inhambane Province
• Save, Gorongosa, Buzi, and Pungoé rivers
• Lower Zambezi river
• Rivers of Zambezia Province
• Rivers of the Northeast Coast

In terms of human impact and utilization of these riverine swamp and floodplain systems, the most affected are those in the Zambezi River basin and those south of it. The northern river basins have been subjected to less pressure (Hughes and Hughes, 1992).

In the Zambezi River, the dams of Kariba and Cabora Bassa reduced downstream peak season flows, but increased the dry season flows (Hughes and Hughes, 1992). As a result of these regulations the flood regime on the lower Zambezi is now greatly reduced, erratic, and mainly out of season. This has affected the prawns capture rates in the Sofala Bank (Gammelsrod, 1992). There are also evidences suggesting that the wetlands of the Zambezi Delta have come through severe changes (DNFFB, 1997), due to the regulation of the river flows by these two major hydraulic structures. The Zambezi delta is one of the areas where the reduction of mangroves forest has been quite severe in the past few years (Saket, 1994).

Agricultural development, sometimes involving irrigation, can be found in the lower section of the Zambezi River basin. Overgrazing and burning has been a problem in many areas. It has been a very serious problem in the protected Zambezi Wildlife Utilization Area where the new controlled

flood regimes favour the spread of wildfire and the reduction of pastures (Hughes and Hughes, 1992).

Upstream impoundment also affected the annual flows of the Revué/Buzi, Save, Limpopo, Incomati, Umbeluzi and Maputo rivers, decreasing the availability of water during the dry season. These are international rivers, covering most of the semi-arid zone of the country. It is also in these river basins that most irrigated areas are located: 67% in the Limpopo, Incomati, and Umbeluzi river basins, 25% in the Buzi, Pungoé, and Zambezi river basinss and 8% among the other river basins (Mihaljovich and Gomes, 1986)].

Palustrine system: This includes coastal lakes, lagoons, swamps, springs, peatlands and dambos. They may occur along rivers, lakes or coastal area, or in the form of seepage or spring (Chabwela, 1991). Their conditions have considerable effect on human settlement, being intensively used for livestock production, dry season agriculture, domestic water supply, fishing and hunting.

Peatlands and dambos are of enormous importance to small-scale agriculture. Peat soils are common in the south, where semi-arid climatic conditions predominate. Their importance is associated with water availability all year round, as well as easy workability. Dambos are mainly concentrated in the central and north high rainfall areas and are a common feature of headwaters of most streams.

Lacustrine System: It comprises the interior lakes: Niassa, Chiuta, Chilwa and Amaramba along the border with Malawi. The system also includes the dams of Cahora Bassa (Zambezi River), Chicamba Real (Revué River), Massingir (Elephants River), Corumana (Sabié River) and Pequenos Libombos (Umbeluzi River). In terms of human utilization and impact the lakes are used for fishery, and their banks for cattle grazing and agriculture.

Wetlands for small scale agriculture in Southern Mozambique

The South of Mozambique is characterized by arid and semi-arid climates together with light/coarse textured soil, having low available water content (AWC). Here, according to Reddy (1986), 50% of the area has soils with an AWC less than 100 mm/m and 25% less than 50 mm/m. This exacerbates the risk of drought and most of the area has a chance of dryland crop failure of over 50%.

The mean annual rainfall decreases from 800-1 000 mm near the coast, to less than 400 mm in the interior, mainly concentrated during the rainy period between October and April (Reddy, 1986). This pattern of rainfall distribution makes the coastal zones the most heavily populated and with a high land use intensity (Snijders, 1986), in spite of the low fertility of these soils, which results in very low yields.

To face these adverse conditions, farmers usually cultivate the lowland areas where the residual soil water content can be used for crop growth. Main limitations are:

• high investments for drainage and flood protection
• bad soil structure associated to low infiltration rates
• risks of salt intrusion due to tidal fluctuation and lowering of the water table.

In the lower areas, yield losses are mainly due to flooding and excessive soil water during the rainy season, while in the upland areas, they are mainly due to large soil water deficits which can occur throughout the year. According to the wetland classification, most of these lowland areas belong to the Palustrine system. They play a very important role for household income and food security of thousands of families.

Peat soils and hydromorphic sandy soils

Hydromorphic soils are found in areas of water seepage, at the footslopes (lower slopes) of the scarps which form the transition between the beach ridges, degraded beach ridges, coastal margins and the lower areas, such as flat marshy zones, swampy depressions, swales and tidal flats. These soils are saturated with water either permanently or during most of the year. This causes a lack of oxygen, slowing the breakdown process of organic matter by bacterial activity. Although some mineralization is caused by anaerobic bacteria, this is much less than under dryland conditions (Bleeker, 1983). A strong accumulation of organic matter gives rise to an organic peat horizon, which can vary from 0.4 to 1.0 m depth.

These organic (peat) soils are called machongos and they are in general very fertile and continuously wet. They receive fresh water all year round through seepage from the surrounding dune areas with high infiltration and high recharge rates. They also present a very good soil structure for plant growth with high water holding capacity, high soil aeration, and easy workability. These characteristics make these soils specially recommended and attractive for small-scale agriculture. When subjected to drainage, machongos are intensively used for small-scale agriculture. However, excessive drainage can contribute to mineralization of the peat, resulting in soil acidification.

The hydromorphic sandy soils are less rich in organic matter than the machongos, but they also have a watertable close to the soil surface. Due to the similarity of these soils, in terms of occurrence, wetness/drainage, land use, and vegetation, farmers also commonly call them machongos.

The importance of machongos for small-scale agriculture in southern Mozambique has been recognized for a long time. Several machongos have been studied in Inhambane province (Ripado, 1950; Casimiro, 1971), and between 1951 and 1957 a total of 2 800 ha were identified, surveyed (scale 1 : 10 000), developed and distributed among 4 200 farmers in Gaza province (Monteiro, 1957).

Several other areas of peat soils and hydromorphic sandy soils were also studied at different levels of detail (Table 1).

Main occurrence and characterization of peat soils and hydromorphic sandy soils

Using the digitized soil map of Mozambique at 1 : 1 000 000 scale (INIA, 1995), its associated soil database and GIS facilities (ILWIS, v 1.41), two major soil units with hydromorphic characteristics, identified as peat soils and hydromorphic sandy soils, were selected (Table 2). Their occurrence in Southern Mozambique is mapped at 1: 4 500 000 scale (Figure 3). Some of the areas identified as hydromorphic sandy soils in Figure 3 might also be peat soils. Lack of information available does not allow an accurate distinction between the two soil units.

TABLE 1
Some of the soil studies covering peat soils and sandy hydromorphic soils in Southern Mozambique

TABLE 2
Main occurrence of peat soils and hydromorphic sandy soils (locally called machongos) in Southern Mozambique

1. Symbol used in the national soil map legend (1: 1 000 000) to represent a unit of soil.

2. Associated with aquic conditions. The presence of these conditions is indicated by redoximorphic features.

These soils occur in lowland areas as swampy coastal plains and lagoons, separated from the sea by beach ridges and between the 1 000-600 mm rainfall isohetes. Their proximity to the sea determines a strong presence of marine sediments and a high risk of salinity and/or sodicity.

Organic soils (peat)

Most peat soils occur in lowland areas, under poorly drained and swampy conditions in the vicinity of the coast and in some delta areas. The dominant vegetation is Phragmites sp. and Juncus sp. Many are very young soils characterized by little or no soil formation. Thick layers of black to very dark grey-brown, raw to well decomposed peat, peat clay and clayey peat, alternating with one or more mineral horizons are most typical. Within one soil profile it is often possible to find individual peat layers in various stages of decomposition. Soil reaction varies between very acid

alternating with more alkaline peat soils close to the coast and mangrove areas. These soils have moderate to high permeability and run-off is absent. The water table is found between the surface and 0.5 m depth.

Hydromorphic sandy soils

These are poorly drained or swampy soils characterized by sandy textures related to wetness. These soils show a slight darkening caused by the presence of organic matter in the A horizon which merges gradually into undifferentiated structureless sand. Hydromorphic sandy soils are relatively common in southern Mozambique, with their occurrence being extended to swales, swampy depressions, lower and flat areas between beach ridges and degraded beach plains along the coast. Similar to the organic soils, the water table is very close to the soil surface.

Management and use of `Machongos'

Water management is the key issue in the use and management of machongos. Shallow drains, combining the functions of drainage and irrigation, and maintenance of groundwater levels both in wet and dry season, are usually dug by hand. While drainage is not so problematic in hydromorphic sandy soils, it can be very dangerous in peat soils. Due to a high accumulation of organic matter, and because they are nearly saturated with water for most of the year, peat soils usually have an accentuated acid reaction (pH values between 3.8 and 5.5). This is due to the release of hydrogen ions (H⁺) during the decomposition of organic matter, which is strongly associated with increasing mean air temperatures and aeration of the soil when drained.

Under improved drained conditions, main cultivated crops are rice, maize, beans, and vegetables. Rice is considered the main crop to be grown on these soils due to its rooting system which is well adapted to waterlogged conditions and to its growing cycle during the hot and rainy season, when the peat soil is likely to be flooded. Yields of 1.9-2.8 tons/ha for rice were recorded by some farmers in the machongos of Gaza province (Monteiro, 1957). Similar observations in farmer's fields, have shown yields around 20 tons/ha for lettuce and sweet potato, and 10 to 15 tons/ha for cabbage, onions, carrots and tomatoes during the dry season (INIA-DTA, in prep). These soils are, however, quite fragile due to the absence of the mineral component. Mismanagement of peat soils can lead to its degradation and permanent loss for agriculture.

The Manguenhane case study (1996-1997)

During a recent detailed soil survey in Manguenhane some samples of peat collected at different soil depths were subjected to a drying process in order to study the impact of drainage on soil properties (Mafalacusser et al, 1997). The intention was to simulate excessive field drainage in a peat soil and to monitor the changes in pH values. Two cases were considered:

1. the swampy and uncultivated valley floor (soil unit Ft₂), with soils characterized by having 1.0 m thick peat layer, overlying one or more mineral horizons under swampy conditions (water table < 0.25 m deep)
2. the presently drained and farmed (banana, maize, and residues of rice) valley floor (soil unit Fa) with soils characterized by having a peat topsoil of 0.4 m depth, overlying sandy deposits. The water table is at 0.5 m depth as a result of improved field drainage.

Acidity (pH -H₂0) of each soil sample was measured in the field with a digital pH-meter (EL 513-055). The soil sample was later dried in a forced air-drying oven at 70^oC for 48 hours. Soil acidity and the organic matter content was then determined. Results are shown in Figure 4a and 4b.

The swampy peat soil (Ft₂) has shown a pH value around 7 for the whole 1.2 m deep soil profile. As the water was removed from the soil sample, oxidation of organic matter took place releasing ions of hydrogen (H⁺), and decreasing the soil pH value by 2.5 units (Figure 4a). The reaction was quite evident for the first soil horizon (0.0-0.8 m) where the organic matter content varied from 35 to 44%. The subsoil (<2% organic matter content) also presented an acid reaction but to a lesser extent than in the upper soil layer. This was due to differences in organic matter content.

The drained and cultivated peat soil (Fa), presented field pH values around 6.0 in the first 0.35 m soil depth, suggesting a topsoil that was already acid (Figure 4b). This was probably caused by the field drainage as can be seen by the depth of the water table (~0.5 m). Dried soil samples presented values of pH similar to those measured in the field. Differences were only noticed from 0.5 to 1.0 m soil depth, with pH values decreasing from 7 to 6, when the water was extracted from the soil sample.

Conclusions of the case study and recommendations

Based on the above results some basic principles for a sustainable management of these soils (Monteiro, 1957; Dikshoorn et al, 1988) were confirmed by the Manguenhane case-study (Mafalacusser et al, 1997):

• Shallow drainage must be encouraged in order to prevent the water table from lowering to low more than 0.2-0.40 m depth; drain spacing and depth should be determined for each case, according to the physical conditions of each site. Soil subsidence results from the mineralization but also from the shrinkage of the organic material. These factors should also be considered when reclaiming these soils for agricultural use. Drains should be dug by hand and artisanal gates can be used to control the water table at farm level.
• Irrigation should be as frequent as possible in order to maintain high water content in the upper layers and decrease soil temperature specially during the hot season. The water should flow from the top to the bottom in order to leach the salts downwards and avoid salinization.
• Mechanization is limited in these soils due to its low carrying capacity. A technology based on intensive use of manual tools should be recommended.
• Land clearing should avoid burning vegetation and crop residues as it also destroys the peat topsoil; burning should occur outside the machongo area or in selected places with high water table levels to avoid its spread. Ashes can then be spread on the soil surface to improve the soil nutrient status.
• Crop selection and rotation should consider the conditions of these soils: shallow water table and soil acidity. Most suitable crops are rice during the rainy season, and vegetables (with shallow rooting system and with tolerance to acidity) during the dry season. Depending on the depth of the water table, crops such as maize and beans can also be considered.
• Flooded rice cultivation will contribute to peat layers reestablishment and maintenance. Cultivation in mounds or hills permits the farming of some crops (for example sweet potato) while the soil is still poorly drained.
• Mulching with crop or vegetation residues will reduce the effect of solar radiation in the oxidation and decomposition of peat layers.
• There are always risks of floods during the wet season. If not taken into account, all investments will be lost. Intensive maintenance of the drainage system is crucial for the development of these wetlands.
• Peat soils are also important for keeping a certain ecological habitat. Impact assessments of reclamation for agricultural use should always be studied and weighed.
• Non agricultural uses of these soils are also possible. Reeds, natural vegetation of unreclaimed machongos, are important materials for construction. Fishing, grazing, hunting and soil compost are other rural activities possible in these wetlands.

Research needs

The extreme importance of expanding and developing the knowledge of the wetlands natural resources basis in Mozambique is also recognized. For this reason other research topics should also be included. Characterization, classification, assessment and monitoring of wetlands in Mozambique are, in our opinion, issues deserving more attention.

Bibliography

Bleeker, P. 1983. Soils of Papua New Guinea. CSIRO and ANU Press. Camberra.

Brito, R. and Cambule, A. 1995. Notas para o Curso de Gestão de Machongos. Inhambane 7 a 9 de Dezembro de 1995.

Casimiro, J.F. 1969. Alguns solos do Chibuto (Macaluane, Macontene, Chaimite). Comunicações n^o 36. IIAM. Moçambique.

Casimiro, J.F. 1971. Os solos das baixas do Inhassune (Panda, Homoíne, Inharrime e Inhambane). Comunicações n^o 61. IIAM. Moçambique.

Chabwela, H.N. 1991. Wetlands. A conservation Programme for Southern Africa. A report document, Vol.II. SADC and IUCN.

Chenje, M., Chivas, M., King, A.S., and Laisi, E. 1996. Water in Southern Africa. A report by SADC, IUCN, and SARDC. Eds: Munyaradzi Chenje and Phylis Johnson.

Chidumayo, E.N. 1992. The utilization and status of dambos in Southern Africa: A Zambian case study. In: Wetlands Conservtion Conference for Southern Africa. Eds. T.Matiza; H.N. Chabwela. Gland: IUCN.

DNFFB. 1997. Recursos Florestais e Faunisticos do Norte de Sofala. Volume 1. Plano estratégico de maneio integrado 1997- 2000. MAP.

Dungan, P.J. 1991. Wetlands Management: a critical issue for conservation in Africa. In: Proceeding of the SADC Wetlands Conservation Conference for Southern Africa. Eds. T.Matiza and H.N. Chabwela.

Dykshoorn, J.A., Marques, M.R., Serno, G., Gomes, F., and Brito, R. 1988. Estudo Pedo-hidrológico do vale do Infulene - parte Sul da via rápida da Moamba. Parte A: Solos; Parte B: Agrohidrologia. Série Terra e Água do INIA, Comunicação n^o 58.

Eshweiler, J.A. 1986. As possibilidades para agricultura de uma parte da baixa costeira (Distrito de Maputo). Parte A: Aspectos Pedológicos - Avaliação da Terra para Aptidão Agrícola. Série Terra e Água do INIA, Nota Técnica n^o 50A.

Faculdade de Agronomia. 1981. Estudo Hidro-pedológico de Bobole. Projecto de Marracuene. Volume III.

FAO. 1989. Nature and Management of Tropical Peat Soils. FAO Soil Bulletin n^o 59. Rome.

Gammelsrod, T. 1992. Improving Shrimp Producion by Zambezi Regulation. Ambio vol. 22 n^o 2.

Gomes, F. 1986. As possibilidades para agricultura de uma parte da baixa costeira (Distrito de Maputo). Parte A:Aspectos Agrohidrológicos - Possibilidades de Drenagem. Série Terra e Água do INIA, Nota Técnica n^o 50B.

Hughes R.H., and Hughes J.S. 1992. A directory of African Wetlands. Mozambique. IUCN.

INIA. 1995. Carta Nacional de Solos (escala 1: 1 000 000). Série Terra e Água do INIA, Comunicação n^o 73. Maputo, Moçambique.

INIA-DTA (in preparation). Crop yield data monitoring in the machongo of the Infulene Valley from 1988-1990.

Kauffman, J.H. and Konstapel, C.D. 1980. Os solos do Vale do Infulene. Avaliação preliminar da aptidão para horticultura. Comunicações do INIA, Série Pedologia n^o 5. Maputo, Moçambique.

Konstapel C.D., van der Laan, J.C. and Noort, L.F. 1983. Possibilidades de aproveitamento do Vale do Infulene. INIA e Faculdade de Agronomia. Maputo.

Mafalacusser, J.M., Ussivane, A.M., and Langa, A. 1997. Estudo sobre o Aproveitamento Hidro-Agrícola do Vale de Manguenhane, Mandlakaze, Gaza. Série Terra e Água do INIA, Comunicação n^o 87.

Mihajlovich and Gomes. 1986. Áreas de rega: Inventário e Possibilidades Futuras. Série Terra e Água do Instituto Nacional de Investigação Agronómica. Comunicação n^o 53. Maputo. Moçambique.

Monteiro J.F.S. 1957. Resgate dos Machongos do Sul do Save. Repartição Técnica de Agricultura, Secção de Hidráulica Agrícola. Provincia de Moçambique.

Reddy, S.J. 1984. General Climate of Mozambique. Serie Terra e Água do INIA, Comunicação no 19-a. Maputo, Moçambique.

Reddy, S.J. 1986. Agroclimate of Mozambique as relevant to dry-land agriculture. Serie Terra e Água do Instituo Nacional de Investigação Agronómica, Comunicação no 47. Maputo, Moçambique.

Ripado, M.F.B. 1950. Os `Machongos"das regiões de Inharrime e Inhambane (Contribuição para o seu estudo). Separata do Documentário "Moçambique" n^o 62.

Saket, M., and Matusse, R. 1994. Estudo da determinação da taxa de deflorestamento da vegetação mangal em Moçambique. MAP, DNFFB.

Saket, M. 1994. Report on the updating of the exploratory national forest inventory. Ministry of Agriculture and Fisheries, Department of Forestry. Forest Inventory Unit.

Snijders, F.L. 1986. Land use inventory of Mozambique. Série Terra e Água do INIA. Comunicação n^o 43. Maputo. Moçambique.