Of the estimated 1.4 billion km3 of water in the world (Maidment, 1992), only about 2.5 per cent of that is freshwater, with just a fraction actually accessible by humans. In most places, water does not arrive where, or when, it is needed. Only about 40 per cent of all rainfall enters our river systems, equating to some 40,000km3 of water per year.