Hydrologic Cycle

Estimates have been made of the quantities of water that are stored in various global compartments. By far the largest quantity of water occurs in the deep lithosphere, which contains an estimated 27 × 10¹⁸ tons (27-billion-billion tons) of water, or 94.7% of the global total. The next largest compartment is the oceans, which contain 1.5 × 10¹⁸ tons, or 5.2% of the total. Ice caps contain 0.019 × 10¹⁸ tons, equivalent to most of the remaining 0.1% of Earth's water. Although present in relatively small quantities compared to the above, water in other compartments is very important ecologically because it is present in places where biological processes occur. These include shallow groundwater (2.7 × 10¹⁴ tons), inland surface waters such as lakes and rivers [0.27 × 10¹⁴ ton], and the atmosphere [0.14 × 10¹⁴ tons]).

The smallest compartments of water also tend to have the shortest turnover times, because their inputs and outputs are relatively large in comparison with the mass of water that is contained. This is especially true of atmospheric water, which receives annual inputs equivalent to 4.8 × 10¹⁴ tons as evaporation from the oceans (4.1 × 10¹⁴ tons/yr) and terrestrial ecosystems (0.65 × 10¹⁴ tons/yr), and turns over about 34 times per year. These inputs of water to the atmosphere are balanced by outputs through precipitation of rain and snow, which deposit 3.7 × 10¹⁴ tons of water to the surface of the oceans each year, and 1.1 × 10¹⁴ tons/yr to the land.

These data suggest that the continents receive inputs of water as precipitation that are 67% larger than what is lost by evaporation from the land. The difference, equivalent to 0.44 × 10¹⁴ tons/yr, is made up by 0.22 × 10¹⁴ tons/yr of runoff of water to the oceans through rivers, and another 0.22 × 10¹⁴ tons/yr of subterranean runoff to the oceans.

The movements of water in the hydrologic cycle are driven by gradients of energy. Evaporation occurs in response to the availability of thermal energy and gradients of concentration of water vapor. The ultimate source of energy for virtually all natural evaporation of water on Earth is solar electromagnetic radiation. This solar energy is absorbed by surfaces, increasing their heat content, and thereby providing a source of energy to drive evaporation. In contrast, surface and ground waters flow in response to gradients of gravitational potential. In other words, unless the flow is obstructed, water spontaneously courses downhill.