Hydrologic Fundamentals

Abstract

Hydrology is the science that describes the occurrence of water on earth, its areal and temporal distribution, and its quantitative and qualitative aspects. It is estimated that approximately 97% of all the water in the world -1.2 × 10¹⁸ m³—is in the oceans. The remaining 3%, or about 4 × 10¹⁶ m³, is fresh water that may be usable for irrigating lands. More than ³/₄ of the fresh water cannot be developed and utilized with the current technology: 3 × 10¹⁶ m³ is frozen into polar ice caps and mountain glaciers, and more than 4 × 10¹⁵ m³ is estimated to lie in aquifers at depths exceeding 800 m below the ground surface, leaving only about 4.4 × 10¹⁵ m³ that could be used each year for the benefit of mankind. The available amount is divided between surface water (rivers, streams, freshwater lakes), groundwater (to a depth of 800 m below the ground surface), and atmospheric water (atmospheric moisture, clouds, etc.) as follows:

Surface water	0.13 × 1015 m3
Groundwater	4.27 × 1015 m3
Atmospheric water	0.01 × 1015 m3
Total	4.41 × 1015 m3