The Impact of Desertification as Revealed by Mapping

Abstract

Maps prepared for the United Nations Conference on Desertification are critically reviewed and parts of selected maps are illustrated.The World Map of Desertification, called for by the United Nations General Assembly, delineates areas at risk of desertification as determined by bioclimatic stress, the inherent vulnerability of the land, and pressure of land-use. Areas of very high desertification hazard are shown to be those arid and semiarid regions with very heavy pressure of land-use. The threat of desertification is shown to be highest beyond the margins of the full deserts, but the risk extends wherever drought marks the seasonal or periodic extension of aridity. The term ‘drylands’ is used to define areas in which this risk occurs.Alternative maps at global scale include the Climate Aridity Index map based on the Budyko Ratio, which is the ratio between annual net radiation and mean annual precipitation. This allows a more sensitive depiction of aridity gradients and reveals that larger areas are under drought-stress than were previously recognized.The map of Experimental World Scheme of Aridity and Drought Probability uses soil features as an integrative record of past and present environmental conditions, including ‘land aridity’, but is constrained by the concept of soil zonality, without sufficient consideration of topographic and inherited factors which determine soil properties.The map of the Status of Desertification in the Hot Arid Regions shows the existing degree of desertification as judged by the difference between former (in the pristine state) and existing productivity. This is revealed as increasing outwards from the desert cores into the semi-arid lands, but the map does not extend into the sub-humid zone. However, it usefully depicts desertification as an expression of human impact on the land. Only very restricted areas are shown as having undergone very serious or irreversible desertification.Global maps are too generalized to depict actual desertification in quantitative terms, but the continental scale of the unpublished Desertification Hazards Map of Africa North of the Equator allows this to be done, in addition to depiction of the degree of vulnerability to desertification. Areas shown as already degraded occur mainly in the arid zone, whereas those of very high vulnerability lie mainly in the semi-arid zones.The U.N. Conference on Desertification recommended the compilation of an atlas of desertification to depict the state of the world's drylands and to serve as a cartographic basis for planning combative programmes. Regional maps of this kind, as exemplified in the synoptic maps accompanying the Case Studies of Desertification presented to the Conference, range in scale from 1:1,000,000 to 1:100,000—depending on the type of desertification portrayed—with the smaller scales for pastoral lands and more detailed scales for irrigation projects.