Influence of soil protection techniques on runoff, erosion and plant production on semi-arid hillsides of Cabo Verde

A combination of human, climatic, geomorphological and pedological factors has led to extensive degradation of the soils of the Cabo Verde Islands. To survive in fragile conditions, the stabilisation of the farming systems and the maintenance of high and sustainable yields have become absolute priorities for farmers. The project described here is part of a programme of water resource optimisation on the island of Santiago in a semi-arid mountainous area near Godim. The objective of this study was to evaluate the influence of two conservation techniques (light mulching with maize (Zea mays L.) haulms and hedging with bushes and grass) on runoff, erosion and phytomass production under natural rainfall. The main data were collected on five runoff plots of 100 m2 and 10 pairs of 4 m2 plots scattered over the hillslope. Although, rainfall erosivity was higher than in Sahelian areas, runoff occurred only for rainfall higher than 40 mm and intensities over 40 mm/h. Soil losses reached 12 mm/year on bare plots, 48 mm with traditional crops, 3.1 mm with hedges and less than 0.03 mm under mulching on 50% slopes. This study confirmed that mulching alone, or associated with the plantation of hedges, reduces runoff and erosion on the steep slopes of the Cabo Verde mountains. However, the ability to improve the use of the water resources by grain-producing crops in semi-arid regions was questionable. Plant material for mulching was only available and efficient during the wet years. The efficiency was therefore irregular and depended on the abundance of rainfall, weed production and the degree of satisfaction of plant water and nutrient requirements at the critical moment when the crops are in flower. Hedges of Leucaenaleucocephala (Lam.) De Wit or Cajanuscajan (L.) Huth (Congo pea), without ditches or banks, planted on the flat and mulched could contribute to the supply of fodder in the dry season as well as blocking rivulet runoff and erosion in the Sahelian area.