Quantifying the Human Appropriation of Fresh Water by African Agriculture

Human appropriation of renewable freshwater (HARF) is a measure for the influence of human activities on the global water cycle. It describes the fraction of accessible water that is directly used by human-dominated systems. We present a comprehensive model-based analysis of HARF for crop production on the African continent. Our analysis includes the components evapotranspiration from cropland ("green" water fluxes) and water used for cropland irrigation (crop-related fraction of "blue" water fluxes). Model experiments were conducted for two scenarios with a time horizon of 2050, taking into account the combined effects of land-use change, climate change, and technological change. For the year 2000, evapotranspiration from Africa's rainfed cropland (green water flux) is estimated to be 1085 km super(3)/yr, whereas the abstraction of water for irrigation purposes from the renewable water resources (blue water flux) is estimated to be approximately 180 km super(3)/yr. According to the model experiments, between 2000-2050, an area between 1.25 million km super(2) and 1.56 million km super(2) of natural biomes will be converted to cropland. Consequently, for 2050, evapotranspiration from rainfed cropland is substantially greater than in 2000, ranging from 1870-2040 km super(3)/yr, depending on the scenario, and irrigation abstraction increases up to 194-330 km super(3)/yr. These findings point out the significant role of water appropriated for rainfed crop production in the continental water cycle, in contrast to the sum of water appropriated for irrigation. Furthermore, they suggest that it would be worthwhile to look for opportunities to reduce the amount of water evaporated and transpired from cropland to increase the "water productivity" of cropland. Finally, they indicate that under the given scenarios, this additional production is very likely to come at the cost of the extent of natural biomes and their associated ecosystem services.