Water Scenarios: ‘The Unpredictable Flow’
Water, so critical to life on our planet, is everywhere, but unevenly so. Its ﬁckle nature – water falls and ﬂows when and where it chooses – hinders long-term planning for its availability and use. Globally, the water cycle provides about 40,000 cubic kilometres of renewable fresh water annually (Shiklomanov, 2000); but such a number is of little value to the water resources planner in a country or at the local level since the differences in rainfall, evaporation (from land and water) and transpiration (from plants)1 vary greatly by region and over time. Some regions experience heavy precipitation for parts of the year followed by long stretches of near total dryness (monsoonal and some temperate climates), while others such as Africa’s Sahel region have large multi-annual or multi-decadal variability. Only some parts of the temperate zone enjoy fairly stable and reliable precipitation and beneﬁt from low evapotranspiration.