Hydrograph Analysis : Components, Elements, Direct Runoff, Base flow, Effective Rainfall, Rainfall Excess of Unit Hydrograph

A hydrograph is the graphical representation of the instantaneous rate of discharge (Q) of the stream plotted with respect to time (t).

The response of a catchment area to a rainfall input as precipitation.

  1. Components of Hydrograph  

a. Rising Limb

It is the ascending portion of the hydrograph. Initially, due to losses, discharge rise slowly and rises rapidly at the end portion.

b. Peak/Crest

i. The maximum rate of flow.

ii. Peak of the hydrograph occurs when all portions of basins contribute at the outlet simultaneously at the maximum rate.

c. Descending Limb

i. Falling Limb

ii. It represents the withdrawal of water from the storage built up in the basin during the earlier phase of the hydrograph.

iii. It extends from the point of inflection at the end of the crest to the beginning of natural groundwater flow.

iv. It is affected by basin characteristics only and independent of the storm.

  2. Elements of Hydrograph  

Here, Band C is a point of inflection and BC is the crest segment.

i. Time peak

The time-lapse between starting of the rising limb to the peak.

ii. Time Lag

The time interval between the center of mass of rainfall hyetograph to the center of mass of runoff hydrograph.

iii. Basin Lag

The time period between the center of the hyetograph and the peak discharge is called a basin lag. This basin lag depends upon the catchment and storm characteristics.

iv. Time of concentration

The Time taken by a drop of water to travel from the remotest part of the outlet is known as the time of concentration.

v. Timebase of Hydrograph (T_b):

The Time between starting of the runoff hydrograph to the end of direct runoff due to the storm.

  3. Direct Runoff and Base Flow  

The part of precipitation that occurs quickly as flow in the river is direct runoff.

Direct Runoff = Surface runoff + Subsurface runoff

The part of runoff that receives water from the groundwater storage is called base flow.

  4. Factors affecting Hydrograph  

The factors affecting the hydrograph are as follows:

a. Shape of Basin

In the fan-shaped basin, the time taken for water to reach the outlet from remote parts is almost similar. so, it is distributed over a short time and has a high peak. In the elongated basins, the time taken to reach output from remote parts is different. So, runoff is continued over a long time and has a low peak. With more runoff, the discharge will be more.

b. Size of Basin

In the small basins, Overland flow occurs whereas in a large basin, channel flow is predominant and a longer time is needed to deliver runoff to the outlet, and peak discharge will be less for the large basins.

c. Slope of Catchment

The slope of the catchment is more, infiltration less, runoff more, and discharge will be high.

d. Drainage density

We know,

Drainage density = Total channel length / Total drainage area

The more the drainage density, the runoff will be and the discharge gets peak value.

e. Land Use

More forest, less runoff due to high resistance to flow, and less discharge at the outlet will be less.

In the Built-up area, a runoff will be more due to less infiltration and discharge will be maximum.

  5. Baseflow Separation  

It is necessary to separate the hydrograph into direct runoff and baseflow.

There are three methods of separation of base flow.

a. Method 1 

– Straight-line method

– In this method, we assume that base flow is constant.

A horizontal line segment AB is shown. A is the beginning of direct runoff and b is the ending of direct Runoff.

The area below AB gives base flow and above the line, AB gives effective rainfall/direct runoff.

b. Method 2

In this method, the tangent line is drawn at the beginning of runoff i.e. at A, and extends this tangent to intersect with the coordinate down at peak point C.

After drawing the line AD joins D to B and the area below ADB gives the base flow.

c. Method 3

In this method, a tangent line is drawn at the ending point of direct runoff extends this tangent to intersect with a line drawn at the point of inflection C. After drawing line BC join C to A, and the area below ACB gives base flow.

  6. Rainfall Excess  

If the initial loss is subtracted from total rainfall, the remaining portion of rainfall is called rainfall excess.

It is also called supra-rain

Rainfall excess  = Total rainfall – ( Initial loss and infiltration loss)

  7. Effective Rainfall  

Effective rainfall is the portion of rainfall that causes direct runoff. As the direct runoff includes both the surface runoff and interflow, the effective rainfall is slightly greater than the rainfall excess