 In Mauritania the isohyet indicating annual rainfall of 150 millimetres—considered the minimum for pastoralism—has shifted southward about 100 kilometres to a point well south of Nouakchott.During the 1980s, the desert was advancing southward at an estimated rate of six kilometres a year.Each major climatic zone had shifted southward, and in some cases near-desert conditions had reached the banks of the Senegal River.