HYDROGRAPHS

Introduction:

A plot of the discharge in a stream plotted against time chronologically is called a hydrograph. Depending upon the unit of time involved, we have:

1. Annual hydrographs showing the variation of daily or weekly or 10 daily mean flow over a year. 
2. Monthly hydrographs showing the variation of daily means flow over a month. 
3. Seasonal hyfrographs depicting the variation of the discharge in a particular season such as the monsoon season or dry season. 
4. Flood hydrographs or hydrographs due to a storm representing stream flow like due to a storm over a catchment. 

Each of these type has particular applications. Annual and seasonal hydrographs are use in:

• Calculating the surface water potential of a stream. 
• Reservoir studies, and
• Drought studies 

Factors Affecting Runoff Hydrograph:

Physiographic factors:

1. Basin characteristics:

• Shape 
• Size
• Slope 
• Nature of the valley
• Drainage density 

Climatic factors:

1. Storm characteristics: precipitation, intensity, duration
2. Initial losses
3. Evapotranspiration 

Components of hydrographs:

Rising Limb:

The rising limb of a hydrograph, also known as concentration curve represents the increase the increse in discharge due to the gradual building up of storage in channels and over the catchment surface. The initial losses and high infiltration losses during the early period of a storm cause the discharge to rise rather slowly in the initial period. As the storm continues, more and more flow from distant parts reach the basin outlet. Simultaneously the infiltration losses also decrease with time. Thus, under a uniform storm over the catchment, the runoff increases rapidly with time. As indicated earlier, the basin and storm characteristics control the shape of the rising limb of a hydrograph. 

Crest segment:

The crest segment is one of the most important parts of a hydrograph as it contains the peak flow. The peak flow occurs when the runoff from various parts of the catchment simultaneously contribute ammounts to achieve the maximum amount of flow at the basin outlet. Generally, for large catchment, the peak flow occurs after the cessation of rainfall, the time interval from the centre of mass of rainfall to the peak being essential controlled by basin and storm characteristics. Multiple peaked complex hydrographs in a basin can occur when two or more storm occur in succession. Estimation of the peak flow and its occurrence is important in flood flow studies. 

Recession Limb:

The recession limb, which extends from the point of inflection at the end of the crest segment to the commencement of the natural ground water flow represents the withdraw of water from the storage built up in the basin during the earlier phase of the hydrograph. The starting point of the recession limb i.e., the point of inflection represents the condition of maximum storage. Since the depletion of storage takes place after the cessation of rainfall, the shape of this part of thr hydrograph is independent of storm characteristics and depends entirely on the basin characteristics. 

Thr storage of water in basin exists as (i) Surface storage, which includes both surface detention and channel storage (ii) Interflow storage (iii) Ground storage 

Unit hydrographs:

It is defined as the hydrograph of direct runoff resulting from one unit depth(1 cm) of rainfall excess occuring uniformly over the basin and at a uniform rate for a specified duration (D hours).  The term unit here refers the to a unit depth of rainfall excess which is usually taken as 1cm. 

The S-curve :

If it is desired to develop a unit hydrograph of mD, where m is a fraction, the method of superstition cannot be used. A different technique known as S curve method is adopted in such case, and this method is applicable for rational value of m. 

The S curve is also known as S hydrograph is a hydrograph produced by summation of a infinite series of D-h unit hydrographs spaced D-h apart. 

At any given time the ordinates of the various curves occuring at that time coordinate are summed up to obtain ordinates of the S curve. A smooth curve through these ordinates result in an S shaped curve called S curve. 

Synthetic unit hydrograph:

If the rainfall and runoff records are available, unit hydrographs can be derived. However, in the case of ungauged rivers, these data are not available. In some other cases, the data available may be scanty. For such catchment, unit hydrographs,  derived from basin characteristics relationship is known as a synthetic unit hydrograph. It is derived by Synder's method.