Runoff Hydrograph: Meaning, Components and Factors | Geography

In this article we will discuss about:- 1. Meaning of Runoff Hydrograph 2. Components of Hydrograph 3. Factors Affecting the Shape 4. Base Flow Separation.

Meaning of Runoff Hydrograph:

Hydrograph is a graphical or tabular presentation of instantaneous runoff/discharge rate against time. Sometimes, it is also known as storm hydrograph, flood hydrograph or simply hydrograph. A hydrograph presents the total runoff (direct + base flow) occurring at a given time. It also shows the distribution of total runoff with respect to time at a certain point of measurement. All hydrographs have three characteristics regions viz.,- rising limb, crest segment or peak point and falling limb.

These characteristics regions are shown in the schematic diagram of the hydrograph (Fig. 2.6). The hydrographs are mainly in two types, i.e., – single peaked and multi-peaked. The multi-peaked hydrograph is also known as complex hydrograph.

The occurrence of single or multi-peaked hydrograph depends on rainfall characteristics, complexity of watersheds and their peculiar interactions. For example an isolated rainfall yields the single peaked hydrograph, while complex storm (varying rainfall intensity) yields the complex hydrograph.

Components of Hydrograph:

1. Rising Limb:

It is also known as concentration curve, is the ascending portion of hydrograph. Its slope steepness depends on the rise of discharge due to gradual building of storage in drainage channels as well as over the watershed surface. The shape of rising limb is dependent on the storm and watershed characteristics, both.

In general, the shape of rising limb is being concave upwards and rises slowly in the early stage of the flow, but as the storm continues and more and more flow from distant apart reaches to the outlet of watershed, the rising limb rises very rapidly up to the peak point of the hydrograph. The time base of hydrograph is fixed by the duration of outflow. In a simple hydrograph, the extent of rising limb is comparatively shorter than the falling limb, as a result the area below this limb is less to that of the falling limb.

2. Crest Segment:

This segment is one of the very important parts of the hydrograph, as it contains the peak flow. It is extended from the point of inflection on the rising limb to a similar inflection point on the falling limb. The peak flow occurs, when various parts of the watershed yield the runoff simultaneously to the outlet. Generally, in large watersheds the peak flow occurs, when rainfall gets stop. The time interval from centre of mass of rainfall to the peak is controlled by the storm and watershed characteristics. Hydrographs of some watersheds resulted from a single and relatively short duration rainfall, have two or more peaks. Multi-peak, i.e. complex hydrographs can also occur, when two or more storms occur in a close succession.

3. Falling Limb:

It is the descending portion of hydrograph, is also known as recession limb. The falling limb is extended from the point of inflection at the end of crest segment to the commencement of natural ground water flow. It represents the withdrawal of water from the storage build up in the watershed during initial phase of hydrograph. The point of inflection on the falling limb of the hydrograph indicates the stage, when rainfall has been stopped and channel flow is due to storage made over the watershed.

The shape of falling limb is dependent only on the physical features of the channel; and is independent of the storm characteristics. Generally, falling limb is in convex shape due to continuous decrease in runoff volume. Variation in areal rainfall distribution minutely affects the shape of recession curve. Unusually high rainfall intensity results the rapid recession, while delayed recession is due to concentration of rainfall in upper portion of the basin.

Factors Affecting the Shape of Hydrograph:

The shape of hydrograph is dependent on the runoff volume and time to peak of the watershed.

Various factors that affect the shape of hydrograph can be broadly grouped into following two groups:

1. Climatic factors; and

2. Physiographic factors.

Each of the two groups of factors, affecting the shape of hydrograph, further contain host of the factors; the important ones are listed below:

1. Climatic Factors:

These are mainly the storm characteristics, given as:

i. Types of precipitation

ii. Intensity of rainfall

iii. Duration of rainfall

iv. Direction of rainfall

v. Magnitude of rainfall

vi. Other factors, like initial losses and evapotranspiration.

2. Physiographic Factors:

Include the basin as well as channel characteristics, are given as under:

Basin characteristics:

i. Shape

ii. Size

iii. Slope

iv. Nature of the valley

v. Elevation

vi. Land use pattern; and

vii. Soil characteristics of the basin.

Channel Characteristics:

i. Cross-section of the channel

ii. Roughness of the channel

iii. Storage capacity, and

iv. Drainage density etc.

Generally, the climatic and physiographic factors control the rising limb and peak segments, while recession limb is only by the physiographic factors. The time base of hydrograph is mainly influenced by the watershed characteristics i.e., physiographic factors.

Base Flow Separation from Hydrograph:

A runoff hydrograph represents the cumulative runoff resulted from surface and sub-surface (base flow) runoff. The surface runoff or direct runoff hydrograph is obtained from the total storm hydrograph by separating the base flow. The separation of base flow is an arbitrary manner, unless a large flow from the antecedent storm is available. Due to this reason the errors made in the base flow separation are taken as negligible.

A simple hydrograph, which is not affected by the rainfall, prior to or subsequent to the period of observation, any one among following three methods, can be used:

1. Straight Line Method:

This method consists of drawing a straight line from the beginning of the surface runoff to an arbitrary point on the recession limb, representing the end of the direct runoff. In Fig. 2.7, it is shown by the line a. b, in which point ‘a’ represents the beginning of direct runoff, is identified by the view of sharp change in the runoff rate.

The arbitrary point ‘b’ is roughly located at the time N = 0.84 A^0.2 days after the peak of the hydrograph, in which ‘A’ is the watershed area (km²) and N is in days. The accuracy of N depends on careful study of a number of isolated storm hydrographs. This method of base flow separation is the simplest among all three methods.

2. Method-II:

In this method the base flow curve existing prior to the commencement of surface runoff, is extended till it intersects the straight line drawn from the peak of the hydrograph. This point is joined to the arbitrary point (b), simply by a straight line. In Fig. 2.7, it is shown by straight line ac and cb. The area below this line of hydrograph represents the base flow, while the area above the line is noted as direct runoff. This method is most suitable and widely used for base flow separation.

3. Method-III:

This method is based on the use of composite base flow recession curve. In this method, the base flow recession curve, after depletion of the flood water is extended back­ward till it intersects the straight line, drawn from the point of inflection.

In Fig. 2.7, it is shown by joining the point a to point f and point f to b by smooth curve. This method is appro­priate, particularly when ground water contri­bution is expected to be significant and reaches the stream, quickly.