Hydrologic cycle

HYDROLOGIC CYCLE

Water occurs on the earth in all its three states, viz. liquid, solid and gaseous, and in various degrees of motion. Evaporation of water from water bodies such as oceans and lakes, formation and movement of clouds, rain and snowfall, stream flow and groundwater movement are some examples of the dynamic aspects of water. The various aspects of water related to the earth can be explained in terms of a cycle known as the hydrologic cycle.

Figure 1 is a schematic representation of the hydrologic cycle. A convenient starting point to describe the cycle is in the oceans. Water in the oceans evaporates due to the heat energy provided by solar radiation. The water vapour moves upwards and forms clouds. While much of the clouds condense and fall back to the oceans as rain, a part of the clouds is driven to the land areas by winds. There they condense and precipitate onto the land mass as rain, snow, etc.

A part of the precipitation may evaporate back to the atmosphere even while falling. Another part may be intercepted by vegetation, structures etc., from which it may be either evaporated back to atmosphere or move down to the ground surface.

A portion of water that reaches the ground enters the earth's surface through infiltration. It enhances the moisture content of the soil and reaches the groundwater body. Vegetation sends a portion of ground water to the atmosphere through the process of transpiration. An example of transpiration is when a plant absorbs water in its roots.

The precipitation reaching the ground surface after meeting the needs of infiltration and evaporation moves down the natural slope over the surface. It further moves through a network of guides, streams and rivers to reach the ocean.

The groundwater may come to the surface through springs and other outlets after spending a considerably longer time than the surface flow. The portion of precipitation which by a variety of paths above and below the surface of the earth reaches the stream channel is called runoff. Once it enters a stream channel, runoff becomes stream flow.

 

1. WORLD WATER BALANCE

The total quantity of water in the world is estimated to be about 1386 million cubic kilometers (M km3). About 96.5% of this water is contained in the oceans as saline water. Some of the water on the land amounting to about 1% of the total water is also saline. Thus, only about 35.0 M km3 of fresh water is available. Out of this about 10.6 M km3 is fresh water and the remaining 24.4 M kmis contained in frozen state as ice in the polar regions and on mountain tops and glaciers.

An estimated distribution of water on the earth is given in Table 1.

TABLE 1 ESTIMATED WORLD WATER QUANTITIES

 

2. APPLICATIONS OF HYDROLOGY IN ENGINEERING

Hydrology finds its greatest application in the design and operation of water-resources engineering projects, such as those for (i) irrigation, (ii) water supply, (iii) flood control, (iv) water power, and (v) navigation. In all the above projects, hydrological investigations for the proper assessment of the following factors are necessary:

1. Capacity of storage structures such as reservoirs.

2. Magnitude of flood flows to enable safe disposal of the excess flow.

3. Minimum flow and quantity of flow available at various seasons.

4. Interaction of the flood wave and hydraulic structures, such as reservoirs and bridges.

The hydrological study of a project should be conducted before structural design studies. Many major projects in the past have failed due to improper assessment of the hydrological factors. Some typical failures of hydraulic structures are,

(i) Overtopping and consequent failure of an earthen dam due to inadequate spill way capacity.

(ii) Failure of bridges and culverts due to excess flood flow.

(iii) Inability of a large reservoir to fill up with water due to over estimation of the stream flow. The above failures are often called hydrological failures.

 

3. PRECIPITATION PROCESS

The term precipitation denotes all forms of water that reach the earth from the atmosphere. The usual forms are rainfall, snowfall, frost and dew. Of all these, only the first two contribute for significant amounts of water.

For precipitation to form, the following attributes are required:

(i) atmosphere must have moisture.

(ii) presence of sufficient nuclei to aid condensation.

(iii) good weather conditions for condensation of water vapour to take place.

(iv) products of condensation must reach the earth.

Under proper weather conditions, the water vapour condenses over nuclei to form tiny water droplets of sizes less than 0.1 mm in diameter. The nuclei are usually salt particles or products of combustion and are normally available in plenty. Wind speed facilitates the movement of clouds while its turbulence retains the water droplets in suspension. Water droplets in a cloud are somewhat similar to the particle in a colloidal suspension.

Precipitation results when water droplets come together and coalesce to form larger drops that can drop down. A considerable part of this precipitation gets evaporated back to the atmosphere. The net precipitation at a place and its form depend upon a number of meteorological factors, such as the weather elements like wind, temperature, humidity and pressure in the volume region enclosing the clouds and the ground surface at the given place. Rain is the major principal form of precipitation in India.

 

4. EVAPORATION PROCESS

Evaporation is the process in which liquid changes to the gaseous state at the free surface. It is below the boiling point through the transfer of heat energy.

Consider a body of water in a pond. The molecules of water are in constant motion with a wide range of instantaneous velocities. An addition of heat causes this range and average speed to increase. When some molecules possess sufficient kinetic energy, they may cross over the water surface. Similarly, the atmosphere in the immediate neighborhood of the water surface contains water molecules within the water vapour in motion. Some of them may penetrate the water surface. The net escape of water molecules from the liquid state to the gaseous state constitutes evaporation.

Evaporation is a cooling process where the latent heat of vaporization (about 585 cal/g of evaporated water) must be provided by the water body.

The rate of evaporation is dependent on,

(i) vapour pressures at the water surface and air above

(ii) air and water temperatures,

(iii) wind speed

(iv) atmospheric pressure

(v) quality of water

(vi) size of the water body

 

5. SUBSURFACE WATER

Study of subsurface flow is equally important since about 30% of the world's fresh water resources exist in the form of groundwater. Further, the subsurface water forms a critical input for the sustenance of life and vegetation in dry zones.

Since subsurface water is a significant source of water supply, various aspects of groundwater dealing with the exploration, development and utilization have been extensively studied. Researchers from different disciplines, such as geology, geophysics, geochemistry, agricultural engineering, fluid mechanics and civil engineering have undertaken the studies.

Wells form the most important mode of groundwater extraction from underground. They find an extensive use in water supply and irrigation engineering practices.