Introduction

Global population drives the water requirements for domestic use and in particular food, which determines close to 90 percent of the total water requirement of a person ( Savenije, 2000 ). With changing diets the water requirement may even become much higher (with meat requiring even more water ( Hoekstra and Chapagain, 2007 ; Liu and Savenije, 2008 ). If we compare the water requirements and the water resources of a country, many countries in sub-Saharan Africa are considered water scarce. However, such analysis considers only “blue” water and not “green” water ( Savenije, 2000 ). “Blue water” is the water which occurs in aquifers, rivers and lakes. In sub-Saharan Africa, 70-90 percent of the exploited “blue water” is used by irrigated agriculture. Increasing the agricultural production by increasing the irrigated areas is becoming more and more difficult, whereby suitable areas are becoming less available and “blue water” resources are diminishing. However, ninety percent of the agricultural land in sub-Saharan Africa is rainfed ( Rockström, 2000 ), which, in fact, uses “green water” ( Falkenmark, 1995 ). “Green water” is the water that is returned to the atmosphere through transpiration and is the most under-valued resource ( Savenije, 2000 ). Improving the productivity of rainfed agriculture in sub-Saharan Africa appears the best strategy. However, using this “green water” for agriculture may have an impact on the available “blue water” resources, by changing rainfall partitioning.