HYDROLOGIC CYCLE Vijay P Singh and Sharad K Jain Abstract The hydrologic cycle is the endless circulation of water between the earth, the oceans, and the atmosphere, and is fundamental to the study of the waters of the earth. The cycle entails a huge turnover of water and is driven by the energy of sun and gravity. It has profound influence upon the landscape and earth’s climate. The major components of the hydrologic cycle are precipitation, evapotranspiration, interception, infiltration, overland and channel flow, and ground water flow. Quantitatively, the hydrologic cycle is represented by a mass balance or continuity equation. The relative significance of the terms of this equation depends upon the space and time scale. Human activities and the hydrologic cycle are interactive and influence the earth’s climate. Introduction The Hydrologic Cycle is a fundamental concept in hydrology and is amongst a number of cycles known to be operating in nature, such as the carbon cycle, the nitrogen cycle, and other biogeochemical cycles. The National Research Council (NRC, 1982) defines the hydrologic cycle as “the pathway of water as it moves in its various phases to the atmosphere, to the earth, over and through the land, to the ocean and back to the atmosphere”. This cycle has no beginning or end and water is present in all the three states, viz., solid, liquid, and gas, in the cycle. A pictorial representation of the hydrological cycle is given in Fig. 1. Fig. 1 Pictorial representation of the hydrological cycle (Source: Singh, 1992). The hydrologic cycle, also known as the water cycle, is a concept which considers the processes of motion, loss, and recharge of the earth's waters. It connects the atmosphere and two storages of the earth system: the oceans, and the landshpere (lithosphere and pedosphere). The water that is evaporated from the earth and the oceans enters the atmosphere. Water leaves the atmosphere through precipitation. The oceans receive water from the atmosphere by means of precipitation and from the landsphere through streamflow and ground water flow. The only way that water goes out of oceans is through evaporation. The landsphere receives water through precipitation. The water leaves this sphere through evaporation, transpiration, evapotranspiration, streamflow, ground water flow. Evaporation and precipitation are the processes that take place in the vertical plane while streamflow and ground water flow occur mostly in the horizontal plane. The exchange of water among the oceans, land, and the atmosphere has been termed as ‘the turnover’ by Shikhlomanov (1999). This turnover affects the global patterns of the movement of ocean waters and gases in the atmosphere, thereby greatly influencing climate. Besides, water is a very good solvent and hence geochemistry is an integral part of the hydrologic cycle. The hydrologic cycle is, thus, the integrating process for the fluxes of water, energy, and the chemical elements (NRC, 1991). Usually, rain and snow are considered as the purest form of water although these may also be mixed with pollutants that are present in the atmosphere. During the journey on earth, many chemical compounds are mixed with water and consequently the water quality undergoes a change.