Modifying land management in order to improve efficiency of rainwater use in the African highlands.

摘要

Water scarcity and drought in Africa are often in the news. The widespread tendency to relate farmers' notion of drought to changes in the occurrence of dry spells is misguided: several recent studies have yielded little evidence of an increase in the length and/or frequency of such spells. The farmers' concept of drought is contextual and an indirect result of land degradation. Plant production suffers because water is not available due to deteriorated physical properties of soil. Farmers' perception of drought refers to the Green Water Use Efficiency (GWUE), i.e. the fraction of rain that is used for plant transpiration. GWUE in Africa is remarkably low: in sub-Saharan Africa, only 15% of the terrestrial rainwater is used by plants for the production of food, fodder and fibre. Although a millet crop grown under traditional circumstances uses only 50 mm in transpiration, the crop frequently suffers from agricultural drought due to excessive losses of rainwater. A range of land management practices is available to help improve GWUE. They can be classified according to their function. Hedgerow barriers and terraces can mitigate runoff; infiltration rates below hedgerows in Kenya were found to be three to eight times higher than where the crop was grown. Mulch that triggers soil fauna can improve water availability; mulching with straw from a local perennial grass in Burkina Faso doubled the water use efficiency from 1 kg mm-1 without fauna to 2 kg mm-1 with fauna. Water harvesting and water-nutrient synergy can improve water use; in case permeable barriers combined with the use of compost Sorghum yield in Burkina Faso was 2.3 times higher than in the control plots and the plots with the barriers only. Green water deserves more attention from policy makers, planners, land users, water engineers and managers.