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There are two primary types of fresh water resources, groundwater (the term used for underground water) and surface water (rivers, lakes, and streams). Although fresh surface water is only a very small part of the earth's total water inventory, it is a very important part and is widely used for many purposes. The majority of a typical general hydrology course is devoted to the study of surface water hydrology. The diagram at the left shows information about the relative amounts of different types of the earth's water resources.
It is worth noting at this point that there is a fundamental difference between managing water resources and managing some other natural resources, such as fossil fuels for example. Water does not get consumed as it is being used. The total inventory of water on and in the earth remains the same. It may be converted from one form to another, as for example fresh water to salt water, ice to liquid water, or water vapor in the air into stored water in plant matter through photosynthesis. In fact fresh water is continually being regenerated by a major component of the water cycle, which is evaporation from the oceans and precipitation over land. Fossil fuels on the other hand are consumed through their conversion to energy. The fossil fuels will not be regenerated unless conditions on the earth become once again as they were millennia ago, when the fossil fuels were produced from organic matter.
If water does not get consumed through its use, why are we concerned with managing our water resources? Quite simply, there is too much water at some places at some times, resulting in flooding, and there is too little water at other places at some times, resulting in drought or water shortage. Also, there is concern about contamination of our water resources through the way that we use them.
