Tlll

The Earth is an interacting system of matter and energy, that as part of its functioning produces phenomena like volcanoes, glaciers, mountain ranges, oceans, and continents. The energy that keeps this system going is on one hand the internal heat (from radioactive decay) that drives plate tectonics, and on the other hand solar energy that maintains ocean and atmosphere circulation and helps to drive erosion.

In the previous chapter we have discussed the internal processes (radiogenic heat production, mantle convection) that drive plate tectonics and crustal dynamics. Geologists call this complex of internal processes the Tectonic System. They refer to the complex of solar powered external processes (atmosphere and ocean circulation, erosion) as the Hydrologic System, because water and its properties are the key ingredient.

THE HYDROLOGIC SYSTEM includes the entire cycle (hydrologic cycle) of water movement on the Earth (be it on the surface or in the air). The driving force behind this cycle is the energy input from the sun.

THE HYDROLOGIC CYCLE as shown in above diagram shows the various pathways of (1) water to the oceans (rivers, glaciers, precipitation); (2) water into the atmosphere by evaporation (from falling rain, rivers & lakes, soil, the oceans, transpiration by plants); and (3) onto the landmasses (by rain, snow). Water movement/transport occurs through movement of clouds, by rivers, ocean circulation, groundwater flow, and evaporation.

The bulk of the water is contained in the oceans, which contain about 30000 times more water than atmosphere and continents combined, cover approximately 70% of the Earth's surface and are on average 3800 m deep. The remainder of the water is found in ice caps & glaciers (3%), groundwater (1%), and rivers and lakes (0.01%). The latter two reservoirs constitute the terrestrial fresh water supply. Thus, only a very small fraction of the overall water supply is suitable and available for human...