Previous Page Table of Contents Next Page


Influence of rainfall on the productivity of grasslands

L. DIARRA* and H. BREMAN"*

(*) L. Diarra, Centre Pédagogique Supérieur, Bamako Mali.

(**) H. Breman, Associate Expert, UNESCO.


Introduction
Conclusion
Bibliography


SUMMARY

Productivity in Mali depends particularly on the rainfall, which varies strongly from the south to the north and from one year to the next. An adequate inventory of the Malian grasslands is rather difficult because of their enormous expanse. This study is, however, a confirmation of the hypothesis that it is possible to estimate the productivity of these grasslands on the basis of the annual rainfall. Such estimations will be useful until the moment when there can be detailed information about the whole surface of pasturelands.

Introduction

The recent drought has once again confirmed that the Sahel is a zone with low and very variable rainfall. Stock breeding is quite possible, but the stocking rate of its rangelands changes greatly from one year to the next, depending upon the intensity of the rains. To plan stock breeding, and for that matter, productivity, the stocking rate of the pastures should be known. However, the wide expanse of Malian grasslands renders their complete evaluation difficult. Breman tried to evaluate the productivity of these pasturelands, taking into account rainfall variations, on the basis of data contained in four limited agrostological studies made in Mali. This study is an attempt to verify the correlation he found between rainfall and the productivity of the pasturelands.

In Mali, rainfall seems to be a decisive factor for productivity. The mean annual precipitation varies between 1,550 mm for the extreme south of the country and almost 0 mm for the north, while annual fluctuations are characterized by a standard deviation of 15 to 30 percent for the prospective breeding zone. In this study, we have limited ourselves to the zone with a mean precipitation of 500 mm (from Nara to the Mauritanian border) and 1,100 mm (Bamako). This region is particularly important because it is undergrazed In addition, it should be further studied because the existence of Glossina in the south of the country hampers the development of stock-breeding, and since water is not a limiting factor here as in the Sahel. Moreover, Breman only assessed its productivity by extrapolation, because available agrostological studies were made in areas with more or less high precipitation.

1. Zone of study

The rangelands were studied on the basis of an imaginary line stretching from Bamako to Nara. Bamako has a Soudanian climate with an average precipitation of 1,100 mm distributed from May to October. The average annual temperature is 28.40°C, reaching its maximum in April and its minimum in December. The annual potential evapotranspiration (PET) is 1,650 mm. It is in the Soudanian region, with a savannah characterized by trees like Vittelaria paradoxa, Parkia biglobosa, Bombax costatum, and Borassus flabellifere. The herbaceous cover there is type An 6 (Rattray, 1960) with, among other plants, Andropogon gayanus, Andropogon pseudapricus, and Sataria pallidefusca on sandy and sandy-clay soils; Elionurus elegans and Loudetia togoensis on laterite soils. These species disappear or become very rare around Mourdiah, where the mean annual precipitation is 565 mm. Gradually, the Sahelian zone is approached: an Acacia- and Comiphora-wooded steppe with type CE 7 herbaceous cover (Rattray, 1960). The main graminaceae found there among other plants are Cenchrus biflorus, Ctenium elegans, Eragrostis tremula, Pennisetum pedicellatum, and Dactyloctenium aegyptium on sandy soil; and Andropogon amplectans and Schoenefeldia gracilis on clay-sandy-clay soils. Except for Cenchrus and Schoene-feldia, all these species are again found in Bamako. In Nara, one is already in the heart of the Sahel. The average annual rainfall is a little under 500 mm. The annual potential evapotranspiration (PET) is 1,600 mm. The average annual temperature is 31 °C, reaching its maximum in June and its minimum in January.

2. Method of study

The productivity of the rangelands is assessed on the basis of the biomass of the herbaceous stratum at the end of the growth season. To evaluate it, the vegetation was cut at ground level on an area of 12 X 1 m² The samples were dried under the sun and the dry weight was expressed in ton/ha/year. The samples were, in principle, taken every 50 km on the Bamako-Nara road. At the two extremes and in the middle of the line, more than one sample was taken. Each time, efforts were made to choose the most representative areas.

The necessary rainfall data were provided by the National Meteorological Department. The rainfall for sites without rain-gauges was assessed on the basis of data from nearby stations.

3. Results

Observations made after the 1974 rainy season will enable us to ascertain whether the productivity/ rainfall correlation suggested by Breman is indeed valid for zones external to those described in the studies used. Three successive years of study (from 1972) on the overall pasturelands in Bamako will make it possible to verify his hypothesis that local variations in precipitation influence productivity in the same way as inter-regional variations which determine productivity at different places. This hypothesis is founded on the relative uniformity of the vegetation.

3.1. Productivity and inter-regional variation in rainfall

Rainfall in the Malian Sahel was heavier in 1974 than during the last few years, though it was deficient, with the deviation varying from 200 mm north of the zone of study to 50 mm in the latitude of Kolkani. On the other hand, Bamako and its surrounding areas had more than normal rainfall, with the surplus reaching 250 mm in some areas. It has proved impossible to verify the rainfall/productivity correlation by making a simple comparison of observed and assessed productivity. Theoretical and observed data have been gathered in Table 1. The observation sites have been shown on it according to their average annual precipitation.

Fig. 1 is a direct confrontation between the yields observed and those suggested by the average rainfall/productivity correlation for the different areas. There are big differences between the two. However, this is no longer the case if the yields observed are squared with the rainfall on the sites during the year of study, as shown in Fig. 2. There the observations seem to support the correlation suggested for rainfall and productivity. The only serious doubts that exist as to the validity of this correlation relate to the ³ 1,000 mm rainfall. This is not surprising, however, because Breman only had the figures for Yanfolila (mean annual rainfall = 1,300 mm) when drawing the curve between 600 and 1,500 mm. The productivity was measured in 1970, when the rainfall was only 1,078 mm (IEMVT, 1971), and was therefore underestimated; thus, the curve for the high rainfall should be corrected. This has been done in Fig. 3, where it has been shown that our observations in general do tally with the rainfall/productivity correlation.

3.2. Productivity and local rainfall variation

The preceding paragraph suggest though not explicitly, that local rainfall variations influence productivity in the same way that inter-regional variations determine productivity at different places. Thus it is suggested that the curve on Figs. 1 to 3 not only represent the productivity/average rainfall correlation, but also the relationship between productivity and effective rainfall.

Table 1 - Rainfall/productivity correlation between Bamako and the Mauritanian border

Stations (annual rainfall in mm)

Rainfall in 1974 in mm

Productivity in ton/ha/year

Theoretical under normal rainfall

Observed in 1974

Standard deviation

500

350

1.8

0.9

0.3

700

500

2.4

2.0

0.6

800

650

2.6

2.3

0.4

850

800

2.7

2.8

0.9

950

950

2.9

2.9

0.9

1,000

1,000

3.0

3.5

0.8

1,100

1,320

3.2

4.5

1.2

Table 2 - Influence of rainfall variation from one year to the next.

Year

Precipitation in mm

Productivity in ton/ha/year

Observed

Theoretical

1972 *

700

2.5

2.5

1973

868

2.8

2.8

1974

1,127

4.9

3.4

* According to Mob. L. Bah, Internal Report of the CNRZ.

Figure 1

Productivity in relation to mean annual rainfall on observation sites. (The curve drawn is the correlation suggested by Breman. The observations made in 1974 are shown with their standard deviations.)

Figure 2

(Productivity in relation to effective rainfall on study sites.) (Relationship suggested by Breman; o 1974 observations with their standard deviations.)

Fig. 3

This hypothesis was proved at only one point, in Bamako, on the rangelands of the Centre National de Recherche Zootechnique, where there are several vegetation groups. Table 2 shows average production of the rangelands and precipitation from 1972 to 1974.

At first sight, the results do not appear very encouraging, especially those of 1974. However, we have pointed out that the correlation found by Breman underestimates the productivity of stations with more than 1,000 mm rainfall. After the correction made in Fig. 3, it is expected that productivity under 1,127 mm rainfall will be 3.9 t/ha. This value is still smaller than the one found (4.9). But the productivity of zones that were flooded during a certain period of the year and which had very high productivity is included in this value. Without these flooded zones, the average productivity of the Sotuba grazing lands would be 3.6.

Conclusion

It seems possible to estimate the productivity of Malian grasslands on the basis of annual rainfall. According to the latitude, rainfall variation produces the same effect as the variation in any given place. Further studies are necessary to give more weight to these conclusions.

Bibliography

1. BELIME, E.L. - Les travaux du Niger. Paris, Martin-Mamy/Crouan et Rogues, 1940.

2. BOUDET, G. - Désertification de l'Afrique tropicale sèche, Adamsonia (ser. 2: 505-524) 12: 1972.

3. CLAUDIA, J. - Etude Phytoécologique du Hodna. Algérie. M.A.R.A./F.A.O., 1973.

4. I.E.M.V.T. - Etude Agrostologique pour la création d'une Station d'embouche dans la région de Niono, n° 29, le Mali, 1970.

5. I.E.M.V.T. - Esquisse Pastorale et Esquisse de Transhumance de la région du Gourma, le Mali, 1971.

6. I.E.M.V.T. - Aménagement du berceau de la race N'Dama dans le cercle de Yanfolila, n° 36, le Mali, 1972.

7. I.E.M.V.T. - Projet de Développement de l'Elevage dans la Région de Mopti, n° 37, le Mali, 1972.

8. Mali. - Annuaire Statistique 1970, Service de la Statistique générale de la Comptabilité Nationale et de la Mécanographie, 1970.

9. Mali. - Rapport sur l'Etude Sectorielle concernant l'Approvisionnement en eau du pays. D. Balfour and Sons, 1973.

10. RATTRAY, J.M. - The Grass Cover of Africa, F.A.O., 1960.


Previous Page Top of Page Next Page