As discussed in Chapter 4 and illustrated in Figure 4.1, close to 50% of the solar energy intercepted by the Earth is absorbed at the surface. Approximately half of this energy, 78 W m– 2, is used to evaporate water, mainly from the ocean. What this means is that evaporation of water accounts for as much as a third of the total solar energy absorbed by the Earth (atmosphere plus surface). The atmosphere has a limited ability to retain this water. Evaporation is balanced in close to real time by precipitation. A portion of this precipitation reaches the surface in regions elevated with respect to sea level— in mountainous locations, for example. It is endowed in this case with what we refer to as potential energy (Chapter 4). This potential energy can be stored (in lakes or dams, for instance), or it can be released, converted to kinetic energy (directed motion) as the water flows downhill on its return to the ocean. And along the way, energy can be captured and channeled to perform useful work. An early application involved exploiting the power of running water to turn a flat stone, one of two that constituted the apparatus used to grind grain, the other remaining stationary during the grinding process. The Domesday Book records that by AD 1086 as many as 5,624 water mills were operational in England south of the River Trent, deployed not just to grind grain but for a multitude of other tasks, including, but not confined to, sawing wood, crushing ore, and pumping the bellows of industrial furnaces (Derry and Williams 1960). Later, running water would provide the motive force for the textile industry that marked the beginning of the industrial age in North America, specifically in New England (Steinberg 1991; McElroy 2010). The most important contemporary application of water power involves the generation of electricity, the bulk of which is obtained by tapping the potential energy stored in high- altitude dams, a lesser fraction from the kinetic energy supplied by free- flowing streams (what is referred to as run- of- the- river sources).