Introduction to Hydrograph

Fig: Single Peaked Hydrograph

   Hydrograph or Flood hydrograph is the graphical representation of the instantaneous rate of discharge of a stream plotted with respect to time at particular point. In simple words, hydrograph are the graphical drawing used to show how the water flows in a drainage basin respond to a period of rain.

The hydrograph may have a single peak or multiple peaks, depending upon the nature of the storm . If the subsequent storm does not occur before the direct runoff of the first storm ceases, such storm is called isolated storm and it results single peak hydrograph. And if next storm(rainfall) occur before the direct runoff of the first storm ceases then such storm is called complex storm and it results multi-peaked hydrograph.

The beginning of Hydrograph starts when storm starts. As the storm(rain) is commenced, water starts flowing toward the natural stream (river) from the catchment. Slowly the discharge of the river increases (shown as rising limb in the hydrograph) as water is added to the stream from the catchment. Time come when the water from farthest point of the catchment reach the river. In this period river reaches its maximum discharge (shown as peak flow in the hydrograph). Now as the storm ceases and addition of water to the river through storm stops, the remaining water from the catchment still flows to the stream, adding less discharge to the stream than before. Hence discharge of the river slowly decreases, as only the remaining flow from the catchment of previous storm enters the river (shown as falling limb or recession limb in the hydrograph). And slowly base flow is attained and the river carries same discharge as before the storm had arrived.

*Note: The detailed parts and points of single peaked hydrograph are explained in the next post. Please keep following.

History of Hydrology

Hydrological Cycle is a continuous water circulatory system. It is present in the world since time immemorial but human being felt its presence later on.

Most of ancient civilization was flourished on the banks of river. This mean civilization flourished where there was assured source of supply of water meeting various demand of human beings. During 2000 B. C. , many ground water developments were made through wells. In early period, Aristotle and Plato put forward the concept of underground sea below the surface of the earth responsible for the arability of ground water. Leonardo Da Vinchi and Bernard Pallissy were among the first to give the physics based theory of infilteration. Hindu scriptures indicate the existence of the knowledge of hydrological cycle in the Vedic period. Chanakya 300 B.C. , in the Arthasastra, describes the first description of the rain-gauge and its use. Most of the present day science of hydrology has been developed since 1930.

Precipitation

Precipitation is all form of water that reaches the earth surface form atmosphere. Precipitation can be in solid form or in liquid form. The usual forms of precipitation are rainfall, snow, hail, frost, dew etc

Forms of precipitation :

1. Rainfall : Precipitation  in form of water of size later than 0.5mm is called rainfall. The maximum size of raindrop is about 6mm. Any drop larger than this size tends to breakup into drop of smaller size during its fall from the cloud. On the basis of its intensity rainfall can be classified as

       Type                          Intensity
1. Light Rain               trace to 2.5 mm/hr
2. Moderate Rain        2.5mm/hr to 7.5 mm/hr
3. Heavy Rain              grater than 7.5 mm/hr

2. Snow : Snow is another important form of precipitation consisting ice crystal usually combined to form flukes. Snow is white in color and fluffy while touching and falls in cold region where usually temperature is low for most part of year and where temperature goes below 0'C.

3. Drizzle : A fine sprinkle of numerous water droplets of size less than 0.5mm and intensity less than 1mm/hr is known as drizzle. In drizzle, drops of water are so small that they appear to float in air.

4. Glaze : When rain or drizzle comes in contact with cold ground at around 0'C, the water drop freeze to form an ice coating called glaze and freezing rain.

5. Sleet : It is frozen rain drops of transparent grains which forms when rainfalls through air at sub freezing temperature. In Britain, sleet denotes precipitation of snow and rain simultaneously.

6. Hail : It is showery precipitation in form of irregular pallets of size more than 8mm. hail occurs in violent thunderstorm in which vertical current very strong.

7. Frost or dew : Consolidation that occurs at the ground surface at night,

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