Marie-Claire lepape
Associated expert FAO Project CCP/CVI/002 BEL
Potential evapotranspiration or pet
Geology, relief and soils
Vegetation
The Cape Verde Islands lie in the Atlantic Ocean, some 450 km from the west coast of Africa, 1400 km south of the Canary Islands, between 14°40' and 17° 15' north and 22°40' and 25°25' west.
They are made up of 10 main islands and 8 smaller ones which, depending on their position in relation to the prevailing northeast wind, are divided into "leeward or "barlavento" islands (Santo and Antao, San Vicente, San Nicolau, Santa Luzia, Sal, Boa Vista, and other small islands).
The islands are included in the vast arid climate zone which stretches from West Africa to the Red Sea, continuing in Asia. The climate of these areas is determined by the movements of three air masses.
a) The trade winds from the northeast, originating from the Azores anticyclone. These prevail throughout almost the entire year, but bring no rain.b) The harmattan, a hot, dry wind from the continent, occurring occasionally from October to June. Despite its low frequency and short duration it can be dangerous and may scorch the vegetation, especially at the end of the rainy season.
c) The monsoon from the southwest. This originates from areas of high pressure lying between the African and American continents. This mass of hot moist air comes into contact with the northeasterly trade winds and the harmattan, forming a border line called the Inter-tropical Front or ITF. The irregular and periodic movements of this front determine the rainfall regime. The front only reaches the Cape Verde Islands for a short period from July to October, called the rainy season.
Rainfall is greater than the arid appearance of these islands would lead one to believe, but it is highly irregular. Since 1968 the Cape Verde Islands have been through an exceptional period of drought, in which rainfall has not reached even 40% of average values (1968–1973). These average values are fairly low: 200 to 300 mm per year in low-altitude areas (below 300 to 400m), but can reach 600 to 1000 mm in higher areas.
The probability of rain in August and September is nearly 100%, 65% in July, 85% in October and 55% in November (Dittrich, personal communication).
Precipitation is unfortunately concentrated over a period lasting only a few days, 15 to 20 days per year in arid areas, 45 to 55 days per year in high-altitude areas.
The rains are hard and violent, falling in a few hours with very great intensity. Thus, at Agua de Caldeira, rainfall of 488 mm was recorded in 24 hours.
The variability coefficient1 of annual precipitation in the low-lying arid areas may be higher than 60%. It is lower than 35–40% in high-altitude areas. Thus, at Praia (at an altitude of 70m) the variability coefficient is around 80%2.
1 
= standard deviation of annual precipitation
R =average annual rainfall
n = (
/ R) 
100
n =variability coefficient
2 Praia: 70 m altitude.
Data on relative humidity are not very accurate and can only serve as indications. Over a period of ten years (1951–1960), average relative humidity at Praia varied from 53% in March to 74% in October.
Fog plays a major role in high-altitude areas. Slopes facing north-northeast between 500 and 1000 m are covered with a cloud layer for a large proportion of the year, especially during the dry season, reducing evapotranspiration and providing a fair amount of moisture.
The average annual temperature recorded at Praia over 10 years (1951–1960) is 24°6, with slight variations from year to year. Monthly temperatures are also regular, average range being 4°7. Absolute minimum temperatures are not lower than 4°.
A remarkable feature of the climate is the presence of wind throughout almost all the year (the prevailing wind is from the northeast). Their force is from moderate to strong. At Monte Verde in 1963 it ranged from 5 m/sec to 16 m/sec. These winds play an important part in evapotranspiration.
It is difficult to ascertain the exact PET values, since a great deal of dates are missing. However, Cunha, in his study "Balanco hydrologico na Isla de Santiago" uses the Penman formula to determine a PET of 2359 mm/year for the low-altitude Praia area, 1793 mm for Santa Catarina, an area of medium altitude. By using other more simple formulas the PET of a few sites has been estimated at between 1700 and 2500 mm/year.
The Cape Verde Islands are volcanic in origin, consisting of basaltic rocks and features associated with them, such as pyroclasts; breccia, lapilli and tuffs. Some of the islands, such as Maio, Sal and Boa Vista, have had no volcanic activity for a long time, so that erosion has levelled down the original relief. These islands are relatively flat, with summits approaching 300–400 m.
The other islands have had more recent eruptions, so that their relief is extremely jagged, with summits reaching 1392 m in Santiago, 1979 m in Santo Antao, 1300 m in San Nicolau and 2800 m in Fogo.
The soils vary considerably but are generally young and coarse, of average to shallow depth. Lithosols, litholithic soils, alluvial and colluvial soils, red paraferralitic soils, isohumic soils and vertisols are found.
The flora of the Cape Verde Islands include some 650 indigenous or naturalized species of which one third are endemic, and 150 cultivated species. The severe climate, particularly in recent years, the continually expanding population, the search for new land, firewood and timber, together with the excessive number of goats, are the factors responsible for the critical state of the vegetation in the islands. The arid lowlands are severely degraded, and the most palatable species to livestock have been eliminated. Only a few plants with a low forage value remain:
Aristida adscensionis, Aritida cardosoi, Tripogon minimus, Chloris pycnothrix, Aerva persica, heliotropium undulatum, Zygophyllum simplex, Acanthospermum hispidum, Eragrostis spp. are found, and a few other which are more sought after: Cenchrus ciliaris, Dichantium annulatum, Botricholoa pertusa. In areas with a more favourable climate are found Panicum maxium, Desmanthus vergatus, Hyparrhenia hirta, Andropogon gayanus, Trifolium spp. and Echium stenosiphon. A few shrubs also remain, often widely scatterd: Faidherbia albida, Parkinsonia aculeata, Acacia nitotica, Ziziphus mauritiana, Prosopis juliflora, Tamarindus indica and Ficus gnaphalocarpa.
Browse plants have an important role to play in Cape Verde. They may occupy vast areas (achadas) in the arid lowlands, which represent 50 to 60% of the total areas of the islands and which are primarily suitable for pastoral utilization, apart from a few pockets of good land fit for cultivation. Their economic importance is considerable, since to a large extent their trees and shrubs are crucial for animal production consisting of meat and milk and derived products. They also produce firewood and timber.
At Santiago there are a few old colonies of Prosopis juliflora and Parkinsonia aculeata, these two species having given the most positive results, being well adpated and highly resistant to drought. The age of these plants is difficult to ascertain since there are no documents on the subject.
On Maio Island 600 ha of browse plants are used to supply the Calhata Centre Zootechnique, which ranges 300 Karakul sheep and a hundred or so goats. These plantations consist of Parkinsonia aculeata (220 ha planted 20 years ago), and a mixture of Prosopis juliflora and Parkinsonia aculeata (280 ha planted 10 and 15 years ago, together with 100 ha of Prosopis juliflora planted in 1979. Since 1978 a considerable effort has been made to regenerate grazing land on the islands of Santiago and Maio. The main species used have been as follows: Prosopis juliflora and Prosopis chilensis, Parkinsonia aculeata, Acacia nilotica, Atriplex halimus and A. nummularia, Cajanus cajan and Leucaena leucocephala, with varying results.
Studies of the biomass of these browse plants have just taken place, so that it is still too early to establish their relative significance. We shall limit ourselves to providing a few indicative values.
Faidherba albida: used for its leaves, pods and wood. The production of pods, measured by M. Gonzales over six trees of the Ribeira de Trinidade, amounted to 75 kg of pods per tree in 1967 and 55 kg per tree in 1968. The subjects were mature and well developed.
Parkinsonia aculeata: used for leaves, pods and wood. The spacing used in existing plantations is either 4 by 4 m or 10 by 10 m, without any great difference in performance being noticeable. Average individual production (circumference of 44 cm at 10 cm from the ground for a tree aged 14 years) was as follows:
Robust wood (non-chlorophyllic): 20.00 kg of DM
Chlorophyllic shoots
3.20 kg of DM
Leaves
0.70 kg of DM
Flowers:
0.17 kg of Dm
Fruit (estimated):
0.50 kg of DM
i.e. 24.47 kf of DM per tree.
Given spacing of 4 by 4 m, i.e. 625 trees/ha, the deciduous biomass is 812 kg of DM or 325 FU, or an average annual carrying capacity of one sheep/ha.
As it became clear that the Parkinsonia needed to be regenerated for subjects aged 20 years (the majority of the stumps were rotten), a trial in which the trees were cut monthly 10 cm from the ground was launched in May 1979. The shoots at 9 months had an epigeal biomass of 10 kg DM of chlorophyllic branch wood and 1.5 kg DM of leaves. Annual production would therefore be 1245 kg of Dm/ha. Allowing for a utilization rate of 60%, real consumable production obtained would be 750 kg/ha or 300 FU ha/year, or again an annual carrying capacity of one sheep per ha/year.
The current utilization method for Parkinsonia consists of pruning, a highly expensive operation. Grazing the new clumps of shoots directly would be more economic, but more difficult to manage since it would be necessary to ensure that not all the shoots were consumed by the passing animals, so as to preserve a sufficient regeneration capacity for the tree. It is thus appropriate for the shoots to reach 1.50 to 2 m in height before first utilization takes place, so that at least 23% of the browse will be out of reach. The interval at which cutting back should be applied and the intensity of utilization should be determined by experimentation.
Prosopis juliflora: used for its wood and pods, the leaves being unpalatable to livestock. An initial study of the biomass begins this year on the island of Maio. The trees are 14 years old and show remarkable development, due to the presence of groundwater near the surface.
The ligneous biomass is probably about 200 kg/tree, i. e. 50 t/ha, the trees being 20 m apart. Pod production is probably in the order of 10 to 15 kg/tree/year for adult plantations, i.e. 250 to 400 kg of DM/ha/year, or carrying capacity of 0.6 to 0.8 sheep. These values would certainly be lower on other less favourable sites.
The utilization of Prosopis juliflora for browse consists of picking up the ripe pods falling to the soil and storing them in large silos. They are consumed during difficult periods (dry season) or when peak feeding requirements arise, for example at the end of gestation, or during lambing and lactation.
Trial introductions of species from the arid zones were undertaken in September 1978 at Santiago, on two sites in the lowlands representing average arid conditions (see list of species in the annex). The plants were simply planted in a hole 2 by 2 m, without any special system for collecting water at the foot of the plants. It is still too early to draw any conclusions, but it can be said that species which withstand the conditions on these two sites have good chances of developing where techniques for accumulating water at the foot of the plant will be used (planting on partitioned banks, for example).
In December 1979 the survial results on the least favourable site were as follows:
>90% for Parkinsonia aculeata;
>75% for Acacia peuce, A. victoriae, A. astringens, A. tortilis, Prosopis juliflora, P. chil ensis, P. glandulosa and Atriplex
nummularia;
>50% for Acacia acuminata, A. tetragonophylla, A. ansura, A. pendila, A. notabilis, A. sowdeni, A. stenophylla, Eucalyptus tarrafal (local adaptation), Pittosporum phylliraeoides, Casuarina dielsiana, C. decaisneana, Conocarpus lancifolius, Schinus mope (Maudou, 1979).
Since the dry-season grass production is low and irregular, browse species alone, owing to their deep root system, will be able to offset the irregularity of the forage grass production, which is totally dependent on the hazards of a freakish climate. The future of animal production therefore depends to a great extent on the development of browse plants, since the prospects for irrigated forage crops are extremely limited.
List of species introduced in the arid zones.
Acacia peuce
Eucalyptus camaldulensis (local variety) cathechiste
Prosopis tamarugo
Acacia tortilis
Conocarpus lancifolius
Pittosperum phylleraeoides
Casuarina dielsiana
Eucalyptus camaldulensis tarrafal (local variety)
Casuarina decaisneana
Brachychiton populneum
Eucalyptus angulosa
Acacia victoria
Acacia microbatrya
Schinus molle
Acacia cyanophylla
Cassia occidentalis
Acacia notabilis
Parkinsonia aculeata
Prosopis glandulosa
Acacia acuminata
Acacia aneura
Prosopis verdyosa
Casuarina cristats
Acacia tetragonophylla
Prosopis velutina
Prosopis chilensis
Prosopis juliflora (Trindade)
Angophora melanoxyllon
Acacia astringens
Prosopis julifdora (Trindade, spineless)
Acacia pendula
Acacia ligulata
Atriplex nummularia (local variety Trindade n° 4)
Eucalyptus ochorphloia
Acacia cyclops
Brachychiton rupestre
Acacia oswaldii
Acacia stenophylla
Acacia sowdini
