The concept of the hydrological cycle forms the basis for the hydrologist’s understanding of the sources of water at or under the Earth’s surface and its consequent movement by various pathways back to the principal storage in the oceans. Two of the greatest problems for the hydrologist are quantifying the amount of water in the different phases in the cycle and evaluating the rate of transfer of water from one phase to another within the cycle. Thus measurement of the components of the cycle is a major objective of the engineering hydrologist and scientific hydrologist.