The surface of the continental united states has an area of about 8 x 10^6 km^2 and an average elevation of about 500 m (above sea level). the average yearly rainfall is 75 cm. the fraction of this rainwater that returns to the atmosphere by evaporation is 2/3; the rest eventually flows into the ocean energy of the water-earth system associated with that flow could be fully converted to electrical energy, what would be the average power? (the mass of 1 m^3 of water is 1000 kg.)