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"Hydraulic radius = cross sectional area / wetted perimeter. Hydraulic radius is a measure of the efficiency of the the river channel. The higher the hydraulic radius, the more efficient the river channel is. The more efficient the river is, the more energy the water will have to move downstream (so as hydraulic radius increases, velocity increases) carry load (so as hydraulic radius increases, the river's competence and capacity increases) increase the rate of erosion (in the upper course, as hydraulic radius increases, there is a higher rate of vertical erosion, so gradient increases; further downstream where the river is closer to base level, as hydraulic radius increases, there is a higher rate of lateral erosion)."

As the world's population increases, temperature rise and societies develop, will water become the "credit crunch" of tomorrow?

Parts of India are on track for severe water shortages, according to results from Nasa's gravity satellites. The Grace mission discovered that in the country's north-west - including Delhi - the water table is falling by about 4cm (1.6 inches) per year. Writing in the journal Nature, they say rainfall has not changed, and water use is too high, mainly for farming.