APES: Chapter 9: Water: Hydrologic cycle and Human Use9.1 Water- A Vital ResourceAll major terrestrial biota, ecosystems, and humans depend on fresh waterFresh water:water that has a salt content of less than 0.1% (1000ppm). Over 97% of Earth’s water is salt water of oceans and seasOf the 2.5% fresh water, two thirds is built up in polar ice caps and glaciersFreshwater is a continuously renewable resource because the solar powered hydrologic cycle resupplies the small percentage of fresh water that is available. Regions with abundant precipitation support lush forest ecosystems; other regions have hardly any rainfall and are deserts as a result. In dryer regions, with growing human populations, conflicts are increasingly arising between human needs and those of natural ecosystems. Countless ecosystems are under stress or are already dead because of diversions of water for human usesHydrologists: experts on waterWater shortages increase conflicts and public health problems, reduce food production, and endanger the environmentA considerable number of countries are now satisfying their needs only by withdrawing groundwater faster than it is replenished, thereby depleting their supply for future generations. These trends are not sustainable. 9.2 The Hydrologic CycleEarth’s water cycle, also called the hydrologic cycle, consists of water rising to the atmosphere through either evaporation or transpiration and returning to the land and oceans through condensation and precipitation. Water cycle (hydrologic cycle):the movement of water from points of evaporation, through the atmosphere, through precipitation, and through or over the ground, returning to the points of evaporation. Modern references to the hydrologic cycle distinguish between water vapor and liquid water as green water or blue water, respectively. Evaporation, Condensation, and PurificationA weak attraction known as hydrogen bonding tends to hold water molecules together. Below 32Fthe kinetic energy of the molecules is so low that hydrogen bonding is enough to hold the molecules in place with respect to one another, and the result is ice. At temperatures above freezing, but below boiling, the kinetic energy of the molecules is such that hydrogen bonds keep breaking and reforming with different molecules. The result is liquid water. As the water molecules absorb energy from sunlight or an artificial source, the kinetic energy they gain is enough to allow them to break away from other water molecules entirely and enter the atmosphere. This is a process we know as evaporationand the water molecules are saidto be in the gaseous state. Evaporation is the process whereby molecules leave the liquid state andenter the vapor or gaseous state.