1The Basin, with a catchment area of approximately 5 376 km^2, was located between latitudes 10°-11°N and longitudes 4°55-5°50 W (Figure 2.1), and involved the following countries:

• Burkina Faso to the North: the Agro-Pastoral Zones of Sindou (44 500 ha), Loumana (60 000 ha) and western Niangoloko (297 250 ha, of which only about 99 080 ha were located in the Basin), drained mainly by north-bank tributaries of the River Léraba s.s. (L.ss) and the Léraba Occidentale (L.oc), and by the Léraba Orientale (L.or);
• Côte d’Ivoire to the South: the Niellé, Diawala and Ouangolodougou Sub-Prefectures of Ferkessédougou Department, drained by south-bank tributaries of the L.ss and the L.oc;
• Mali to the West: Kadiolo Cercle, Région de Sikasso, drained by the western tributaries of the L.oc.

2That part of the Basin subjected to a mediumscale, longitudinal land utilization analysis, was located between latitudes 10°7-10°23 N and longitudes 5°01-5°13 W, in Burkina Faso and Côte d’Ivoire. It was drained mainly by L.ss, but also by L.oc and L.or in the North.

3The short stretch of L.ss used for local, largescale vegetation mapping was situated at approximately latitude 10°10 N and longitude 5°04 W, in Burkina Faso and Côte d’Ivoire.

Soils

4The ferralitic/ferruginous soils of the Basin were of two main types (Anon, 1976):

• category 8d: shallow to medium depth soils (<40 cm), gravelly and/or sandy to clayish in the savanna, with danger of erosion;
• category 12: deep soils (>100 cm), alluvial/clayish near the surface and clayish below, immediately adjacent to the large watercourses (broader on the deposition banks of the rivers and narrower on the erosion banks).

Climate

5Prior to the onset of the recent drought, which started in the early 1970s, the mean annual rainfall of the basin was from 1 100 to 1 400 mm. The rainfall statistics presented in Table 2.1, clearly show that in 1992 rainfall was still below the predrought average.

2.1 Location and Basic Geographical Features of the Upper Léraba Basin. The Inset Labelled “A” Denotes the Boundaries of the Medium-scale Land Utilization Assessment Area. BURKINA FASO / CÔTE D’IVOIRE LERABA BASIN / BASIN DE LA LERABA

6The wet season, from April/May to September/October, was characterized by a unimodal precipitation pattern. The mean annual temperature was about 27°C.

Hydrometry

7Hydrometric data collected at Léraba Bridge hydrology station during 1990 and 1991, are presented in Table 2.2, and are listed on a monthly basis.

Vegetation

8The Basin was located in the dry, Sudanian vegetation zone (Péron & Zalacain 1975; Vennetier 1978; Traoré & Monnier 1980), which was characterized by savanna/woodland mosaics.

9The principal climax vegetation types were, fire-tolerant savanna woodland, floodplain grassland, and fire-tender, riverine forest. Where the woodland joined the floodplain grassland, and around the rocky upland plateaus, there were densifications of the vegetation, known as ecotones. A vegetation profile taken across the main Léraba valley is presented in Figure 2.2, to illustrate the relationships between these different vegetation communities, in the absence of human interference. More precise descriptions of each of these main communities are given below.

Woodland Savanna

10The woodland savanna biotope occupied the largest surface area, and constituted the one most suitable for human settlement and cultivation.

11The most typical tree species of the undisturbed woodland, which rarely exceeded a height of 20 m, and seldom created a completely closed canopy, were Isoberlinia doka, I. dalzielii, Burkea africana and Detarium microcarpum. Other common species included Uapaca somon, U. togoensis, Erythrophleum guineense, Pterocarpus erinaceus, Afzelia africana, and Daniella oliveri. Species of economic importance and thus common in cultivated areas, included Borassus aethiopum, Butyrospermum parkii and Parkia biglobosa. In the northern part of the area it was not uncommon to find Ziziphus mauritiana and species of Acacia, which were more typical of the Sahelian savanna. Likewise in the south of the area some species that were typical of the more southerly Guinea savanna were also encountered, e.g. Uapaca togoensis, Lophira lanceolata and Cussonia barteri.

12Perennial grasses included species of Hyparrhenia, Cymbopogon, Ctenium and Loudetia, while annuals included species of Andropogon, Pennisetum, Eragrostis and Ctenium. On the top of arid, rocky plateaus the dominant grass was Sporobalus pectinellus.

Floodplain Grassland

13Where the ground sloped downwards below the 280 m contour line, the woodland was replaced by shallow depressions, which constituted the floodplain grasslands. Those grasslands were most extensive along L.ss, but also occurred along L.or and L.oc. and some of their larger tributaries. Being prone to flooding in years of heavy rainfall, or to becoming marshy during years of less heavy rainfall, those grasslands were generally devoid of tree/shrub cover (the tree D. oliveri and some fragmented parts of the riverine forest were exceptions) and supported a very limited grass flora; Vetiveria nigritana being the only common grass species. Largely because of these factors, the floodplain grasslands were unsuitable for the cultivation of many crop species. However, they were suitable for rain-fed rice, e.g. along the lower L.or, and provided good pasture for livestock.

Riverine Forests

14The slightly higher ground that formed the banks of the larger rivers supported tall (up to at least 30 m), closed-canopy, evergreen forest. Just north of Léraba Bridge those forests were up to 165 m wide on the deposition banks of the river and as narrow as only a few meters on the erosion banks.

2.2. Schematic Vegetation Profile (Transverse Section) of the Southern Part of the Upper Léraba Basin

15The most common trees and shrubs found in the riverine forests of the main watercourses (especially L.ss.) were Berlinia grandiflora, Parinari polyanda, Ficus platyphylla, Khaya senegalensis, Syzygium guineense, Cola cordifolia, Carapa procera, Pentadesma butyracea, Adina microcephala and Mucuna pruriens.

16Smaller rivers and streams also supported riverine forests, but they were much less extensive and composed of a much smaller number of tree species.

Ethnography

17Traditionally, the greater part of the Basin was occupied by farmers of the Senoufo ethnic group, with some significant populations of Toussian and Gouin peoples in the extreme eastern part of the area. However, in recent years there had been an influx of other peoples from much further afield in Burkina Faso and Côte d’Ivoire. Lérabadougou village was a case in point. Although this village was dominated by the Senoufo, Toussian and Gouin, there were significant numbers of people of Bobo and Mossi origin (coming from as far away as Ouagadougou).

Agricultural Practices

18The most common subsistence crops were sorghum, maize, millet and yam. Secondary and/or localized crops included manioc, groundnuts, sweet potato and rain-fed rice. Since 1972, cotton had become an increasingly important cash crop, and the only one for which pesticides were used.

19Livestock included cattle, sheep, goats, pigs and fowl.

Onchocerciasis and its Control

20In the early 1970s, prior to the introduction of control measures, onchocerciasis was endemic throughout the Pilot Project area. The disease was hyperendemic (prevalence rate>60%) in the north of the Basin, associated with the headwaters of the L.or and L.oc tributaries and in the southeast along L.ss. Between these two important foci, it was mesoendemic (prevalence rate 40-60%) in the west of the L.oc valley, and around the L.oc/L.or/

2.3. Evolution of the Catch Per Unit of Effort of Fish (by Weight) at Léraba Bridge, 1975 to 1993

21L.ss junction. In all other parts of the basin it was hypo-endemic (prevalence rate <40%).

22In 1974 the OCP came into existence, and the following year onchocerciasis control, through large-scale vector control, commenced in the Pilot Project area and in adjacent parts of Burkina Faso, Côte d’Ivoire, Ghana and Mali. By 1988 (14 years after the commencement of vector control) onchocerciasis had been so successfully controlled in the Basin that it was possible to phase out vector control operations over the next two years.

23Regular monitoring of the aquatic environments of the OCP area since 1975 showed that, overall, vector control activities had had a very limited impact on the aquatic environments. However, at the Léraba Bridge monitoring station, there was no indication of a rapid recovery of the fish fauna following the cessation of vector control, according to the recorded values of the Catch per Unit of Effort (Figures 2.3 and 2.4). However, the relatively constant levels of the Coefficient of Condition and of the Species Richness over the period 1975 to 1993 (Figures 2.5 and 2.6) implied that the aquatic ecosystem was in a healthy State. These somewhat conflicting views were interpreted as suggesting that, although the ecosystem was in a State of general well-being, there were stress factors exerting an influence, which were not associated with OCP vector control operations. This State of affaire raised important environmental questions which eventually resulted in the decision being taken to conduct the Pilot Study.

2.4. Evolution of the Catch Per Unit of Effort of Fish (in Numbers) at Léraba Bridge, 1975 to 1993

2.5. Evolution of the Coefficient of Condition of the Principal Species of Fish at Léraba Bridge, 1975 to 1993

2.6. Evolution of the Species Richness of Fish at Léraba Bridge, 1975 to 1993