ABSTRACT
Freshwater is vital to human existence. While water is abundant on Earth, much of it is saline, and unsuitable for human consumption. Of the only 2.5% of the Earth’s water that is fresh, roughly two-thirds is stored as ice, and almost a third is stored deep beneath the surface as groundwater. The most readily accessible stocks of freshwater are those at the land surface, including for example, rivers, lakes, and in human-constructed reservoirs. These account for perhaps a quarter of a percent of all fresh water (Shiklomanov 2000). Fortunately, these stocks are renewed on timescales of days to years, as fresh water is evaporated from the oceans and precipitates over land, before returning to the ocean as river discharge. Over the course of a year, about 36,000 km3 of fresh water will pass through terrestrial fresh water stores (Syed et al. 2010).
