A characterization of rice-growing environments in West Africa

Agriculture, Ecosystems and Environment, 33 ( 1991 ) 377-395

377

Elsevier Science Publishers B.V., Amsterdam

A characterization of rice-growing environments in West Africa W. Andriesse ~ and L.O. Fresco 2 The Winand Staring Centrefor Integrated Land, Soil and WaterResearch (SC), P.O, Box 125, 6 700 AC Wageningen(The Netherlands) ZDepartment of Tropical Crop Science, Agricultural University Wageningen, P.O. Box 341, 6 700 AH Wageningen(The Netherlands) (Accepted for publication 8 June 1990)

ABSTRACT Andriesse, W. and Fresco, L.O., 199 I. A characterization of rice-growing environments in West Africa. Agric. Ecosystems Environ., 33: 377-395. Building on past classifications, this article proposes a comprehensive characterization of rice-growing environments in West Africa on the basis of ecological and agronomic parameters. Using climate, soils, toposequence, land types and rice cropping system as classificatory principles, 18 different environments are distinguished and their characteristics are summarized in three sets of tables. Current constraints to rice production in each of the environments and the implications for rice research are briefly highlighted, as well as the need for field-level verification of the characterization.

BACKGROUND

Rice ranks among the six major food crops of West Africa. Over the last decade, the growing demand for rice has been met mainly through higher imports. To counter this trend, increased indigenous rice production is being promoted by national and international agencies. So far, however, increased production has been achieved largely through expansion of the cultivated area, while rice yields per unit area have not risen significantly and fall much behind average world production levels (West Africa Rice Development Association (WARDA), 1988). Apart from factors governing the economic viability of rice production, the limited impact of rice research and the poor performance of West African rice seems to be mostly due to the complexities of the rice-growing environments in the region. Broadly, rice-growing environments in West Africa have been grouped as uplands, inland swamps, irrigated humid, irrigated Sahelian and mangrove environments (WARDA, 1988 ). In fact, the application of a strict differentiation between uplands and inland swamps is not realistic as these environments occur as a continuum of gradually changing ecological conditions (soils, 0167-8809/91/$03.50

© 1991 - - Elsevier Science Publishers B.V.

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W. ANDRIESSE AND L.O. FRESCO

hydrology) down along the slopes of the undulating landscape of the West African plains and plateaus. These changing conditions, in turn, affect the possibilities for (rice) cultivation, as is reflected by the current cropping systems. In West Africa, ecological conditions vary considerably. At the macro-level, climate, geology and soils account for major differences between agro-ecological zones, whereas conditions also differ at the meso-level, within each zone, according tothe position of the rice field along the toposequence. Even within the rice field, factors like the heterogeneity of soil preparation or weed infestation result in micro-level variations. The large number of different ricegrowing environments, and the difficulty of differentiating between them, has been recognized as an obstacle to concentrating rice research effectively and has led to various attempts at description and classification (Kilian and Teissier, 1973; Buddenhagen, 1978; WARDA, 1980; International Rice Research Institute (IRRI), 1984; Raunet, 1985; Bunting, 1987; Gigou, 1987). In general, these efforts at classification have been limited to a qualitative description of a restricted number of factors such as climate, soil type, source of water and water management. IRRI's classification seems insufficiently detailed for West African conditions, while the regional classifications pay little or no attention to the cropping system or to landscape position. Furthermore, there is little agreement between these existing classifications and no single system is generally accepted in the region. This paper aims to elaborate upon previous work through a comprehensive and interdisciplinary characterization of West African rice-growing environments according to ecological and agronomic criteria. Geographically, it puts emphasis on those areas in West Africa where the upland/inland swamp continuum is prominent. This excludes most of the low-rainfaU areas, i.e. the Sahel and Sahara zones. The implications of this characterization for the development of improved rice cropping systems are discussed briefly. In view of the general absence of site-specific yield and production figures for rice in West Africa, the present characterization remains of a qualitative nature. The data used in this paper involve ecological and agronomic data from secondary sources. Climatic data are derived from previous agro-ecological studies and classifications of West Africa (Cocheme and Franquin, 1967; TAMS/Comit6 Interafrican d'Etude Hydrologique, 1976; Ojo, 1977; Food and Agriculture Organization ( FAO ), 1978; Virmani et al., 1980; Hekstra et al., 1983). A broad agro-ecological zonification of West Africa is shown in Fig. l, while the characteristics of these zones, including their estimated areas, are summarized in Table 1. It goes without saying that in reality the distinction between the agro-ecological zones is gradual rather than absolute. Lithologic information on the area has been compiled from national and regional geologic studies and maps at scales between 1 : 500 000 and 1 : 2 rail-

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0 0-90 90-165 165-270 > 270

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lion (Furron, 1963; Barrere and Slansky, 1965; Blanchot et al., 1973 ). Landforms and soils have been obtained from national maps at various scales, most of which are incorporated in the West Africa sheet of the FAO/Unesco Soil Map of the World (FAO, 1977). Additional general data on West African soils and their potential for (riee) cultivation were summarized from Ahn (1970), Jones and Wild ( 1975 ), Kowal and Kassam ( 1978 ), Moormann and Veldkamp (1978), Dabin and Maignien (1979) and Greenland ( 1981 ). Aggregation of physical data (Table 2 and Fig. 1 ), with the aim of estimating the occurrence and extent of inland valleys in West Africa, has been carried

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TABLE 2 Main characteristics and extent of major land regions in West Africa (Fig. 2) and relative area of valley bottoms Land Description region/ map symbol Coastal plains and terraces Cp 1 Beach ridges, (minor) river floodplains, tidal flats and slightly dissected coastal terraces Cp 2 Strongly dissected older coastal terraces with distinct dipslopes and steep scarps Alluvial plains Ap 1 (Major) river floodplains and lacustrine plains Interior plains Ip 1 (Slightly) dissected peneplains with inselbergs and mesas; over basement complex formations Ip 2 Slightly dissected peneplains, locally with mesas and basalt cones, locally strongly dissected over sedimentary formations Plateaus PI 1 Slightly dissected plateaus, with inselbergs, hill ridges and mesas; over basement complex formations PI 2 Dissected plateaus, with steep scarps and valleys; over sedimentary formations Desert lands DI 1 Undulating sandy desert land of shifting dunes (erg) DI 2 Undulating coarse- to medium-textured desert land, over sedimentary formations; partly stony DI 3 Rocky desert land; partly fiat, partly steep (hamada) Highlands H1 1 Strongly dissected mountain ranges and high plateaus; not differentiated Lakes Total study area

Slope (%)

Area (estimated) Total map unit

Valley bottoms

(103 km 2)

(%)

(%)

0-5

120

1.9

5-15

115

1.7 15-25

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3.3

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RICE-GROWINGENVIRONMENTSIN WESTAFRICA

3 81

out previously, in the framework of related research (Hekstra et al., 1983; Andriesse, 1986 ). In Table 2, the percentages of areas occupied by valley bottoms form a measure of the prominence of the upland/inland swamp continuum. Agronomic data were obtained from various experimental sources and studies of farm-level constraints (Abifarin et al., 1972; Huke, 1976; Buddenhagen, 1978; Moormann and van Breemen, 1978; Ruthenberg, 1980; Richards, 1985; Grist, 1986; Gupta and O'Toole, 1986; IITA, 1986; Stoop, 1987). CENTRAL CONCEPTS

The characterization of rice-growing environments presented here is based on two central concepts: toposequence and rice cropping system. Toposequence land type or catena, the hill to valley continuum that determines the landscape position of the rice field, has been elaborated with respect to rice by Moormann et al. (1977), Moormann and van Breemen ( 1978 ) and Veldkamp (1979). Along the toposequence, three major physio-hydrographic positions are distinguished, representing the source of water for cultivation: pluvial, phreatic and fluxial (Fig. 2). Pluvial rice cultivation depends on precipitation on the land under consideration. Excess water not stored in the soil is discharged by run-off or by percolation. Pluvial water is the main source of water on the crests and the upper and middle slopes of the toposequence. Phreatic water (groundwater), is the

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major source of water on the lower slopes of the catena, at least during the rainy season when the groundwater in these positions is at relatively shallow depths, within the reach of plant roots. This is caused by lateral substratum l o w (interflow) from the higher parts. Fluxial land receives its water mainly from surface flow, i.e. run-on and flooding by streams. Fluxial lands are, naturally, situated in the lowest parts of the landscape, i.e. the valley bottoms and river and coastal floodplains. Rice cropping system refers to land, labour, capital inputs and management as they affect the production of rice on a given field. A cropping system is a component of a farming system and is thus influenced by other cropping and livestock systems managed by a farm household. In West Africa, a distinction must be made between shifting, pluvial rice cropping systems and permanent, wet rice cropping systems. Almost always, farming systems cut across environments as farmers occupy fields at different toposequence positions and adjust their crop and varietal choices to these. In shifting, pluvial rice cropping systems, rice is grown as a first crop after clearing of the forest in shifting or fallow systems without any fertilizer. It is regularly intercropped with maize and sorghum, and followed by other crops, often annuals like cassava, but sometimes also low-input perennials such as oil palm. This cropping system occurs on the upper part of the toposequence. The farming system often comprises other cropping systems without rice and rice may not be the main food crop, depending on food habits. In permanent, wet rice cropping systems, rice is grown more or less continuously on the same field, generally under waterlogged or flooded conditions. There is no real fallow and rice is the main crop, although non-rice crops may be grown, off-season, on the same field. Outside irrigated areas, fertilizers or manure are rarely applied. This type of rice cropping system is found on the lower slopes of the toposequence, in the valley bottoms and in the river and tidal floodplains. The farming system, however, also includes upland fields and rice may have to compete for labour and other resources with these. RICE-GROWING ENVIRONMENTS

For the three agro-ecological zones subject to this study (i.e. the Equatorial forest, Guinea savanna and South-Sudan savanna zones), the main characteristics of the soils and the rice cropping systems occurring in the various physico-hydrographic positions along the toposequenees of the inland valleys in West Africa are shown in Tables 3, 4 and 5. These tables include assessments of the main limitations to (rice) production. Additionally, for each of the major agro-ecological zones, a similar characterization is given for the specific settings of the river floodplains and the coastal plains.

RICE-GROWING ENVIRONMENTS IN WEST AFRICA

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The Equatorial forest zone In the Equatorial forest zone (Table 3 ), soils along the toposequence range from well drained and (very) deep, coarse- to medium-textured, strongly weathered Ultisols and Oxisols (Soil Survey Staff, 1987) on the upper and middle slopes, to imperfectly and poorly drained, deep and coarse-textured Ultisols, Inceptisols and Entisols in the lower slopes and valley bottoms. Mostly, these soils are gravelly and have very low inherent fertility (low pH, base saturation and cation exchange capacity) and are characterized by high aluminium saturation of the exchange complex and potassium deficiency. Phosphate fixation is a common problem in the uplands. Rice, cultivated on the lower slopes and in valley bottoms, frequently suffers from an excess of ferrous iron in soil solutions and from low nitrogen levels due to N losses by alternating nitrification and denitrification. Coarse soil texture in the valley bottoms causes tillage problems. In this agro-ecological zone, pluvial rice is currently grown as part of shifting cultivation systems, either as a single crop in rotation or intercropped with other food crops such as maize, cassava, yam and minor crops such as green vegetables (e.g. Amaranthus spp. ). The occupational period is limited to ~ 3 years. In the lower parts of the catena and in the valley bottoms, rice is cultivated more permanently and is usually not intercropped. Vegetables, sweet potato or taro may be grown, however, in close proximity to rice on mounds or on bunds and may be planted prior to or after the main rainy season, depending on the water-holding capacity of the soil and the depth of the groundwater table. Low solar radiation in this agro-ecological zone, combined with relatively high air humidity, promote the incidence of pests and diseases, particularly blast (Pyricularia oryzae), in pluvial rice. The Guinea savanna zone In the Guinea savanna zone (Table 4), soils on the upper and middle slopes are well drained, coarse to medium textured and generally gravelly (ironstone). They tend to be less deep than in the Equatorial forest zone due to the more frequent occurrence of petroplinthite a n d / o r rock. These soils are also somewhat less weathered and comprise Alfisols and Ultisols. (Soft) plinthite is common in the subsoils of the lower slopes. In areas with relatively rich parent materials such as shales, siltstones and intermediate and basic rock formations, medium-textured soils prevail, in the uplands as well as in the valleys. Low water retention is a main limitation to cultivation, in particular in coarse-textured and moderately deep or shallow soils. Low inherent fertility forms a limitation too, although not as prominently as in the Equatorial forest zone. In this ago-ecological zone, rice may occur on all parts of the catena, but

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its frequency in the fallow rotations on the higher positions is low as a result of limited water retention. The relatively high population densities in this zone have caused a replacement of the traditional shifting cultivation systems by more or less degraded fallow systems, leading to an overexploitation of the natural resource base (Ruthenberg, 1980). In the lower part of the toposequence, single cropping of wet rice is becoming more common. On the lower slopes and in the valley bottoms, the rice crop may be followed by pulses (cowpea) or vegetables.

The South-Sudan savanna zone In the South-Sudan savanna zone (Table 5 ), soils on the upper parts of the toposequence are increasingly shallow, generally overlying petroplinthite. Soil weathering and depletion have not reached the advanced stage of the Equatorial forest zone or even the Guinea savanna zone. Therefore, soils are generally somewhat more fertile and include Alfisols, Inceptisols and Entisols. Moreover, a replenishment with Ca, Mg and K from Harmattan dust takes place in large areas of this zone. Structure degradation and surface sealing are major limitations to cultivation, both for dryland and for irrigated crops. In the valley bottoms, salinity may affect crop performance. In this agro-ecological zone, rice is unlikely to be found on the higher pluvial parts of the catena, with the exception of a few areas with deeper soils of high water-holding capacity. In the valleys, rice may be the most important crop if water control is provided in some way. Sequential planting of maize, followed by a transplanted rice crop, can also be found. Double cropping of wet rice is not only exceptional because of the unavailability of (irrigation) water, but also because of extremely high temperatures in the dry season and low temperatures in December-February. Furthermore, the absence of an intensive rice-growing tradition and related management skills have thus far prevented the spread of double cropping.

DISCUSSION

Constraints and potential of the different rice-growing environments In the Equatorial forest zone and in parts of the South-Sudan savanna zone where fallows are shortening, erosion and declining soil fertility become serious constraints (Stoorvogel and Smaling, 1990) and lead to clearing of new land. Also, weed infestation is an increasing menace. These constraints can only be overcome if the systems move away from the present use of low external inputs. Soil conservation practices should be introduced, in combination with fertilizers and weed control. Breeding for Fe tolerance may also be a

RICE-GROWING ENVIRONMENTS IN WEST AFRICA

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requirement. In the humid tropics of West Africa, however, permanent upland cultivation of food crops is unlikely to be sustainable on most soils unless a deep-rooting perennial crop is included in an alley cropping or plantation pattern. In contrast, in the lower parts of the catena and especially in the valleys, intensification offers a realistic possibility in the near future with modest investment once water control is achieved through bunding, levelling and, where needed, drainage. Contour bunding, in particular, seems promising as was shown in Sierra Leone (Oosterbaan et al., 1987 ). Notwithstanding limitations to potential production due to low solar radiation levels, these ricegrowing environments present an enormous underutilized potential. Improved cropping systems applicable to the Guinea savanna zone are similar to those in the Equatorial forest zone, but the potential for rice will often be limited by water availability. As in the Equatorial forest zone, contour bunding in the lower part of the toposequence and in the valley bottom may alleviate this problem at field level. At the level of the valley catchment, small dams or head bunds may provide additional (irrigation) water from the storage reservoirs raised behind them. In the lower parts of the toposequence, parallel forms of intensification could be envisaged, as in the more humid environments. If cultivars adapted to the specific seasonal and diurnal temperature fluctuations of the northernmost part of the South-Sudan savanna zone are used, the rice-growing environments in this zone hold considerable promise for high yields and double cropping of irrigated rice, or for rice followed by other crops such as vegetables. In practice, however, the infrastructural and institutional requirements of water control facilities prove to be the main limitations and these are unlikely to be overcome in the short term. In this zone, as well as in the Guinea savanna zone, rice production may be hampered by conflicts between farmers and cattle holders over water resources in valley bottoms.

The value of the characterization The combined use of ecological and agronomic parameters, and the introduction of the concepts of toposequence and rice cropping system, in characterizing rice-growing environments allow a far more accurate description than in previous attempts. The results confirm the diversity and complexity of the conditions under which rice is grown in West Africa. In particular, the results show that the existing broad agro-ecological classifications of the West African region, which are mainly based on the length of the rainy season, are inadequate because they do no justice to the various ways in which rice is actually grown. In West Africa, the length of the growing season is a function of toposequence as much as of rainfall pattern. This conclusion is supported by Smaling et al. (1985a, b), who estimated that the length of the growing period in two locations in Nigeria and Sierra Leone

392

W. ANDRIESSE AND L.O. FRESCO

varied from 160 days on the uplands to a maximum of 350 days in the valley bottoms in the Guinea savanna zone (central Nigeria), and from 240 to 360 days in corresponding positions in the Equatorial forest zone (Sierra Leone ). The qualitative characterization proposed here now requires validation through detailed, interdisciplinary field studies in order to develop a comprehensive and quantitative framework. A first step will have to be the development of a detailed and consistent database. Such a database would require field information on soil characteristics and on vegetation, as well as on cropping systems, including measurements of crop production in different cropping systems on individual land types. Minimum sets, at least, of laboratory data will be required to characterize soils according to the international classification systems (Soil Survey Staff, 1987; FAO/UNESCO/International Soil Reference and Information Centre (ISRIC), 1988 ). Climatic data from published sources and from actual observations will have to complement each data set. The calculation of potential rice production levels for each of the 18 environments described in Tables 3, 4 and 5 would allow a comparison with available experimental results in order to obtain quantitative assessments of the factors limiting production.

The dynamics of rice cropping systems in West Africa Rice cropping systems are not static: they change over time in response to population growth and related changes in technology and management. In West Africa, almost without exception, the rice cropping system forms but one element of farmers' land use and in many cases rice is only a minor crop in the farming system (Ay et al., 1984). Health hazards (e.g. bilharzia and river blindness), as well as the occurrence of excessive bird damage, play an important role in this respect. Farmers' strategies are based on spreading economic and ecologic risks by balancing the resource inputs in the various environments with the outputs of the cropping systems. Often, this leads to a complex cropping pattern of many different (rice) cultivars at different sites (Richards, 1985 ). Any change in rice cultivation technology can be expected to affect other elements of the farming system as well. Therefore, in developing rice technology a careful characterization of the ecological and socio-economic conditions must be carried out, and technology testing sites must be selected so as to be representative for larger areas. Cropping strategies can markedly influence yields so that, contrary to expectations, average valley bottom yields of rice are lower than upland yields. This can be explained by differences in planting densities (higher in uplands ), planting techniques (direct seeding in uplands, transplanting in valley bottoms) and weeding (hardly any in valley bottoms). In systems based on shifting cultivation, increasing population pressure re-

RICE-GROWING ENVIRONMENTS IN WEST AFRICA

393

duces the length of the fallow and increases the number of years a field is cropped. This affects the restoration of soil fertility, weed incidence and, sometimes, the presence of pathogens. Farmers respond to these problems in several ways. Rice (and other crops like yam) may be replaced by less demanding crops, in particular cassava, or by crops with a higher market value. Also, intercropping arrangements may be simplified in an attempt to maintain rice yields. The negative effects of decreased fallow periods may further be countered by bringing more land into cultivation, either marginal land such as the upland slopes or hitherto unused hydromorphic soils suited to rice growing. Alternatively, home gardens may gain importance. The latter two options open avenues to permanent land use. Levi (1976) has documented this process in Sierra Leone by showing how the proportion of the rice area in swamps is directly related to the "dependency ratio", a measure of population pressure, and to the length of the fallows. Also, the number of intercrops in rice fields seems to be inversely related to population pressure. It appears, however, that because of the high labour demands, the option of opening new rice fields on hydromorphic soils is preceded in many cases by the extensification of land use and by the reduction of intercropping. This development could' have disastrous environmental consequences, in particular in the higher rainfall zones. The implication is that an expansion and intensification of permanent wet rice cultivation on hydromorphic soils constitutes a high priority for research and policy. The response to growing population pressure in permanent wet rice cropping systems lies in the intensification of land use, through the application of biochemical inputs or through double or multiple cropping. Important conditions are, of course, that economic conditions are favourable and medical problems, such as bilharzia and fiver blindness, are solved. As rice responds well to inputs and because of the possibility of including additional crops after rice, it can be expected that this crop will play an increasingly important role in the cropping systems in West Africa. ACKNOWLEDGEMENTS The authors wish to express their thanks to N. van Breemen, F.R. Moormann, E.M.A. Smaling, W.A. Stoop and R.F. van de Weg for their valuable comments on an earlier draft of this paper. REFERENCES Abifarin, A.O., Chabrolin, R., Jacquot, M., Marie, R. and Moomaw, J.C., 1972. Upland Rice Improvement in West Africa. In: Rice Breeding. International Rice Research Institute, Los Baflos, The Philippines, pp. 625-635. Ahn, P.M., 1970. West African Soils. Oxford University Press, London, 332 pp. Andriesse, W., 1986. Area and distribution. In: A.S.R. Juo and J.A. Lowe (Editors), The Wet-

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lands and Rice in Subsaharan Africa. Proceedings of an International Conference, 4-8 November, 1985, at IITA, Ibadan, pp. 15-30. Ay, P.T., Gebremeskel, A., Jalloh, M. and de Vries, C.A., 1984. Draft report on the agro-socioeconomic survey of 2 sites in Sierra Leone. Wetland Utilization Research Project, West Africa. Phase II, Pre-implementation Phase. ILRI, Wageningen/KIT, Amsterdam. 115 pp. Barrere, J. and Slansky, M., 1965. Notice explicative de la carte grologique ~ 1:2,000,000 de rAfrique occidentale. Mem. Bur. Rech. Grol. Min. No. 29, Paris, 120 pp. Blanchot, A., Dumas, J.P. and Papon, A., 1973. Carte grologique de la partie mrridionale de l'Afrique de l'ouest ~ 1 : 2,000,000. Bur. Rech. Grol. Min., Paris. Buddenhagen, I.W., 1978. Rice Ecosystems in Africa. In: I.W. Buddenhagen and G. Persley (Editors), Rice in Africa. Academic Press, London, pp. 3-10. Bunting, A.H. (Editor), 1987. Agricultural Environments; characterization, classification and mapping. Proceedings of the Rome Workshop on Agro-ecological Characterization, Classification and Mapping, 14-18 April 1986, at CAB International, Wallingford, 335 pp. Cocheme, J. and Franquin, P., 1967. A study of the agroclimatology of the semiarid area south of the Sahara in West Africa. FAO/UNESCO/WMO Interagency Project on Agroclimatology. Tech. Rep., F.A.O., Rome, 325 pp. Dabin, B. and Maignien, R., 1979. Les principaux sols d'Afrique de rOuest et leurs potentialitrs agricoles. Cah. ORSTOM Srr. Prdol., XVII: 235-257. FAO, 1977. FAO/Unesco Soil Map of the World, 1:5 000 000. Vol. VI, Africa. Unesco, Paris, 299 pp. FAO, 1978. Report on the Agro-ecological Zones Project. Vol. I, Methodology and Results for Africa. World Soil Resour. Rep. 48, F.A.O., Rome, 158 pp. EAO/UNESCO/ISRIC, 1988. Revised Legend of the FAO/UNESCO Soil Map of the World (1:5.000.000). World Soil Resour. Rep. 60. F.A.O., Rome, 119 pp. Furron, R., 1963. Geology of Africa. Oliver and Boyd, London, 377 pp. Gigou, J., 1987. L'adaptation des cultures dans le centre de la C6te d'Ivoire. Agron. Trop., 42: 1-12. Greenland, D.J. (Editor), 1981. Characterization of Soils in Relation to their Classification and Management for Crop Production: Examples from Some Areas of the Humid Tropics. Oxford University Press, London, 446 pp. Grist, D.H., 1986. Rice. 6th Edn. Longman, London, 599 pp. Gupta, P.C. and O'Toole, J.C., 1986. Upland rice - A global perspective. International Rice Research Institute, Los Bafios, The Philippines, 360 pp. Hekstra, P., Andriesse, W., de Vries, C.A. and Bus, G., 1983. Wetland Utilization Research Project, West Africa. Phase I, The Inventory. Vol. II, The Physical Aspects, Voi. IV, The Maps. International Institute for Land Reclamation and Improvement (ILRI), Wageningen, 78 pp. Huke, R., 1976. Geography and climate of rice. In: Climate and Rice. Proceedings of a symposium, 23-27 September 1974, at International Rice Research Institute, Los Bafios, The Philippines, pp. 31-50. IITA, 1986. Annual report and research highlights 1985. International Institute of Tropical Agriculture, Ibadan, 144 pp. IRRI, 1984. Terminology for Rice Growing Environments. International Rice Research Institute, Los Bafios, The Philippines, 35 pp. Jones, M.J. and Wild, A., 1975. Soils of the West African Savanna. CAB Tech. Commun. 55. Commonwealth Agricultural Bureau, Harpenden, 246 pp. Kilian, J. and Teissier, J., 1973. Mrthodes d'investigation pour l'analyse et le classement des bas-fonds dans quelques rrgions de l'Afrique de rouest. Propositions de classification d'aptitudes de terres/l riziculture. Agron. Trop., 28:156-171. Kowal, J.M. and Kassam, A.H., 1978. Agricultural Ecology of Savanna; a Study of West Africa. Clarendon Press, Oxford, 403 pp.

RICE-GROWING ENVIRONMENTS IN WEST AFRICA

395

Levi, J.F.S., 1976. Population pressure and agricultural change in the land-intensive economy. J. Dev. Stud. 13: 61-79. Moormann, F.R. and van Breemen, N., 1978. Rice: Soil, Water, Land. International Rice Research Institute, Los Bafios, The Philippines, 185 pp. Moormann, F.R. and Veldkamp, W.J., 1978. Land and rice in Africa: Constraints and Potentials. In: I.W. Buddenhagen and G. Persley (Editor), Rice in Africa. Academic Press, London, pp. 29-43. Moormann, F.R., Veldkamp, W.J. and Ballaux, J.C., 1977. The growth of rice on a toposequence. A methodology. Plant Soil, 48: 565-580. Ojo, O., 1977. The Climates of West Africa. Heinemann Educational, London/Ibadan, 218 pp. Oosterbaan, R.J., Gunneweg, H.A. and Huizing, A., 1987. Water control for rice cultivation in small valleys of West Africa. In: ILRI, Annual Report 1986. ILRI, Wageningen, pp. 30-49. Raunet, M., 1985. Bas-fonds et riziculture en Afrique. Approche structurale et comparative. Agron. Trop., 40:181-201. Richards, P., 1985. Indigenous Agricultural Revolution Ecology and Food Production in West Africa. Hutchinson, London, 192 pp. Ruthenberg, H., 1980. Farming Systems in the Tropics. 3rd Edn. Oxford University Press, London, 424 pp. Smaling, E.M.A., Dyfan, T. and Andriesse, W., 1985a. Detailed soil survey and qualitative land evaluation in the Rogbom-Mankane and Matam-Romangoro benchmark sites, Sierra Leone. Wetland Utilization Research Project, West Africa, Phase II, Preimplementation Phase. ILRI/ Stichting voor Bodern Kartering (STIBOKA), Wageningen, 56 pp. Smaling, E.M.A., Kiestra, E. and Andriesse, W., 1985b. Detailed soil survey and qualitative land evaluation of the Echin°Woye and the Kunko benchmark sites, Bida area, Niger state, Nigeria. Wetland Utilization Research Project, West Africa, Phase II, Pre-implementation Phase. ILRI/STIBOKA, Wageningen, 35 pp. Soil Survey Staff, 1987. Keys to Soil Taxonomy. Soil Management Support Services, United States Department of Agriculture (USDA); Soil Management Support Services (SMSS) Tech. Monogr. No. 6. Cornell University, 280 pp. Stoop, W.A., 1987. Variations in soil properties along three toposequences in Burkina Faso and implications for the development of improved Cropping Systems. Agric. Ecosystems Environ., 19: 241-264. Stoorvogel, J.J. and Smaling, E.M.A., 1990. Assessment of soil nutrient depletion in sub-Saharan Africa: 1983-2000. Vol. I, Main Report. Report No. 28, Winand Staring Centre, Wageningen, 137 pp. TAMS/CIEH, 1976. Savanna Regional Water Resources and Land Use Project. Vol. 5, Savanna Resources. Comit6 Interafricain d'Etude Hydrologique, Ouagadougou, 139 pp. Veldkamp, W.J., 1979. Land evaluation of valleys in a tropical rain area; a case study. Thesis, Agricultural University Wageningen, 266 pp. Virmani, S.M., Reddy, S.J. and Bose, M.N.S., 1980. A handbook on the rainfall climatology of West Africa: data for selected locations. Inf. Bull. No. 7, International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), Patancheru, 52 pp. WARDA, 1980. Classification of types of rice cultivation in West Africa. WARDA, Monrovia, 23 pp. WARDA, 1988. Strategic Plan 1990-2000. WARDA, Monrovia, 66 pp.