Hydrologic Cycle


Hydrologic Cycle
Man Made Impacts on Hydrologic Cycle


Hydrologic Cycle

It is the science, which deals with the occurrence, distribution and movement of water on the earth includes that in the atmosphere and below the surface of the earth.

Hydrologic Cycle: -

            Water occurs in the atmosphere in the form of vapour, on the surface as water, snow or ice and below the surface as ground water occupying all the voids within a geologic stratum. Except for the deep ground water, the total water supply of earth is in constant circulation from earth to atmosphere, and back to earth. The earth’s water circulatory system is known as the hydrologic cycle. Hydrologic cycle is the process of transfer of moisture from atmosphere to the earth in the form of precipitation, conveyance of precipitated water by streams and rivers to ocean and lakes etc. and evaporation of water back to the atmosphere.

The hydrologic cycle consists of the following processes:


            The water from the surface of Ocean, river, and lakes and also from moist soil evaporates. The vapors are carried over the land by air in the form of clouds. Transpiration is the process of water being lost from the leaves of the plants from their pores. Thus the total evaporation (E) inclusive of transpiration consists of   (i) surface evaporation (ii) water surface evaporation from surface of river, lakes, ocean (iii) evaporation from plants and leaves and (iv) atmospheric evaporation.


            It may be defined as the fall of moisture from the atmosphere to the earth surface in any form. It may be in two forms; (a) Liquid precipitation i.e. rainfall   (b) Frozen precipitation consists of snow, hail, sleet, freezing rain.

3 RUN OFF (R):

            Run off is that portion of precipitation that is not evaporated. Where moisture falls to the earth surface or precipitation a part of it is evaporated from the water surface, soil and vegetation and through transpiration by plant, and the remainder precipitation is available as run off which ultimately runs to the ocean through surface or sub-surface stream. Thus run off may be classified as follows:

a). Surface run off:      

Water flows over the land to reach the streams & river, which ultimately discharge the water to the sea.

b). Sub-Surface run off:

A portion of precipitation infiltration into surface soil and depending upon the geology of basins runs as sub-surface run off and reaches the stream & river.

c). Ground water flow:

It is the portion of precipitation, which after infiltration, percolates down and join the ground water reservoir, which is ultimately connected to the ocean.

Thus the hydrologic cycle may be expressed by the following simplified equation:

Precipitation (P) =Evaporation (E) + Run off (R).

provided adjustment is made for the moisture held in storage at the beginning and at the end of the period..



Man made impacts on hydrologic cycle:


Man influences the hydrologic cycle in various ways. The impact on the global scale is little because of the inability of Man to control the energy distribution on the earth and global climate, which are the main forces in governing the hydrological cycle. But Man has scored a good success in manipulating the cycle by controlling the processes like run – off, evaporation, precipitation and infiltration of water on a local sacle.

            The deforestation is sometimes carried out in the catchment or watershed areas of the basins to increase the water yield by augmenting the run – off. But this, unfortunately, decreases the infiltration of water into the soil, as there is lack of the obstacles and organic matter, which facilitate capturing of the surface moisture. The direct heavy rain on the bare surface may lead to the pulverization of surface materials, which block the soil pores and reduce infiltration.

            The agriculture field where the soil is not covered throughout the year also promote run – off. Deforestation increases the soil water because of the reduction in the consumption of water by the plants. In some areas, where heavy precipitation is associated with steep slopes, floods may occur with the decrease of time lag between precipitation and run – off. The percolation and run – off depends mainly on the soil types. In sandy or permeable soils, though, percolation is more, the upper loose profile of soil is lost with run – off along with the nutrients. This process is called soil erosion. In the clays or less permeable soils, soil saturation may be caused with development of anaerobic conditions, particularly in the cold and humid regions.

            In the areas where precipitation is low with marked seasonality and variability with higher evaporation rates, the manipulation of water cycle may result in the development of desert conditions. The run – off is increased by the removal of vegetation at the expense of infiltration, the water left in the soil evaporates, and the underground water is tapped beyond the amount  of recharge. All this leads to the gradual scarcity of water in the area. As the water table goes down, the wells are further deepened.  Ground water becomes non – renewable resource as the withdrawal increases beyond the recharge. Rivers and streams get dried up fast because of the lowering of water table . Continued cropping and grazing increase  the risk of wind erosion, and symptoms of drought become pronounced. Eventually, the productive areas turn into deserts.

            We are now trying for the change in course of flowing waters and diverting them to the areas of need, which may cause enormous ecological problems. The creation of dams for storage of flowing waters is also of ecological concern. This increases the natural evaporation of water by increasing the surface area of water. Dams can induce earthquakes as has been reported in several parts of the world. The downstream quality of water is affected and there may be some ecological consequences where the river meets sea. We try to augment the flow of rivers in lean seasons by increased melting of ice covers on the mountains, which may also alter the ecology and microclimate of the area.

            Another important aspect of interference with the hydrological cycle is artificial rain, which is carried out by cloud seeding. Our success in this field is limited to only at small scale. If later some effective methods are developed and applied to a large area, it may result in disastrous flooding in one place and serious drought in another. 






          1. Irrigation And Water Power Engineering – B.C.Punmia.

2. Hydrology - H.M.Raghunath.

3. Water Pollution ( Causes, Effects and Control) – P.K. Goel.