Hydrograph and its components

HYDROGRAPH  AND ITS COMPONENTS

A hydrograph of a catchment produced by a storm is a graphical representation of discharge rate of a stream with respect to the time from the commencement of storm. It is an inverted U shaped diagram.

Components of Hydrograph :
Hydrograph generally contains the following three parts.
1. Rising Limb
2. Peak (or Crest) Segment
3. Falling (or Recession) Limb

Fig : Element of flood hydrograph

1. Rising Limb : It is the ascending curved portion of the hydrograph. The rising limb rises slowly in the early stage of the flood but more rapidly toward the end portion. The shape of rising limb depends on duration and intensity distribution of rainfall. This is because in early stages the losses is more and water reaches to the stream faster.

2. Peak Segment (or Crest Segment) : Peak segment is shown by inverted U in the hydrograph. This is the part which is taken as matter of interest by hydrologists. Peak of hydrograph occurs when all parts of basins contribute at the outlet simultaneously at the maximum rate. Depending upon the rainfall-basin characterstics, the peak may be sharp, flat or may have several well defined peaks.

3. Falling Limb (or Recession Limb) : Recession Limb represents the withdrawal of water from the storage built up during the early phase of hydrograph. It extends from the point of inflection at the end of the crest to the beginning of the natural groundwater flow. The shape of recession limb depends upon basin characteristics only and independent of the storm.

Hydrological Cycle

Fig : Hydrological Cycle

     The water in the earth is in balance over time, but despite this, it is continuously move from one reservior to another in a cycle, such as from river to ocean, from ocean to atmosphere and from atmosphere to the river/ocean again.


        Hydrological cycle is the cycle of water in the periphery of earth. More preciously it can be termed as the phenomenon with which moisture from the atmosphere comes down to the earth and later moves back to the atmosphere and goes on for ever.


        Hydrological cycle mainly follow 3 steps
a. movement of water from atmosphere to the earth's surface though precipitation (e. g. rain, snow, hail, frost etc)
b. surface and subsurface movement of water through stream and rivers towards lake or ocean. and,
c. evaporation of the water from ocean to the atosphere and so on...

Process of Hydrological Cycle :

Water evaporates from the ocean, river and the land surface to become part of atmosphere. Water vapour evaporates is transported and lifted in to the atmosphere until it condenses and precipitates on the land or the oceans. Precipitated water may be intercepted by vegetation, become overland flow over the ground surface, infiltrate in to the ground, flow through the soil as sub surface flow and discharge into stream as surface runoff. Much of the intercepted water and surface runoff returns in to the atmosphere through evaporation. The infiltrate water may percolate deeper to recharge groundwater, later emerging in springs or seeping in to streams to form surface runoff and finally flowing out to the sea or evaporating into the atmosphere as the hydrological cycle continues.

Components of Hydrological Cycle :

1. Evaporation : It is the process by which water from water bodies or land changes its state to gases state and transferred in to atmosphere. When water is heated, its surface molecules becomes sufficiently energized to break free the attracting force that bind them together and evaporates to the atmosphere.  It is also termed as loss of water from the surface of earth to the atmosphere.

2. Transpiration : Transpiration is also a process of loosing water to the atmosphere, but in this case, water is lost from plant's tissue (i.e. plant leaves, stem). Every day an actively growing plant transpires 5 to 10 times as much water as it can hold at once. The amount of water lost to atmosphere through transpiration is very less as compared to evaporation.
           Loss of moisture from earth due to evaporation and transpiration combined is called evapo-transpiration. In Canada 40% of precipitation is lost due to evapo-transpiration. This factor is used widely for calculating water requirement for crops in irrigation.

3. Precipitation : It is the process of fall of moisture from atmosphere to the land surface by any means or any form like rain, snow, sleet, hail, drizzle etc. Moisture moves around the earth in the form of cloud propelled by air current. For instance when they move over mountain, they become saturated with water and falls downs as rain or snow depending on temperature of surrounding air.

4. Interception : Before the precipitation reaches the earth surface some part is blocked by high buildings, plants/trees and absorbed which is known as interception. Intercepted water do not contribute in surface runoff, rather it is evaporated.

5. Percolation : Some part of moisture that fall on the ground moves downward into the soil through cracks, pores and joints in soil and rock. It reaches the water table and becomes ground water.

6. Surface runoff : The part of water that reaches the stream channel and finally to the ocean by varieties of path above the surface of the earth is known as surface runoff.

7. Sub surface runoff : The infiltrated water which flow laterally through the unsaturated soil to the stream channel is called subsurface runoff.

8. Evaporation