Types of Hydrograph

There are many types of Hydrograph based on the uses like Unit hydrograph

Unit Hydrograph :

The Unit Hydrograph (UH) is the simplest but at the same time a very powerful tool for hydrological analysis in general and flood forecasting. A unit hydrograph is a hydrograph of surface runoff that would result at a given point in a stream from unit rainfall excess occurring in unit time uniformly over the catchment area above that given point.

S curve  represents the maximum rate at which an effective rainfall intensity of 1 cm/hr in D-hours duration can drain out from a catchment of area A km2 (SA). It is used to obtain a unit hydrograph of shorter duration from longer duration or vice versa.

• A unit hydrograph is a flow hydrograph
• A unit hydrograph is a hydrograph of surface runoff not total runoff
• The hydrograph of surface runoff results from the rainfall excess
• The rainfall excess represents total rainfall minus losses
• For any unit time period, the rainfall excess is assumed to occur uniformly over the catchment
• Typical unit time used in the unit hydrograph analysis are 1,2,3,6,8,12 which are multiples of 24

Factors affecting Unit hydrograph :

1. Rainfall distribution over the catchment
2. The physiography of the catchment: slope , shape, vegetation, soil type etc.

Storm Hydrographs :

Storm hydrographs are used to describe the response of streams and rivers to overland flow discharge. At any point selected, the flow verses time response to a storm will show a relatively rapid rise, a peak and a gradually declining recession back to the base flow fed by springs.

Spring Hydrographs :

A spring hydrographs illustrates how the discharge from a spring varies over time. There are three main components to the hydrographs rising limb , peak and falling limb. falling limb provides information on the configuration of the karst conduits within an aquifer system, as well as it’s transmissivity and volume.

Combining the hydrograph with the chemograph for parameters like Ph, specific conductance and chemical concentrations of a spring can yield estimations of the residence time of water within the aquifer and the volume of the aquifer.

Hydrograph of snow-affected regions :

Ground water hydrographs in snow-affected regions typically have two falling and rising limbs.

REFERENCES:

• Flood Estimation by Unit Hydrograph by Dr. Anil Kumar Lohani (National Institute of Hydrology)
• Water Resources (https://www.sciencedirect.com/referencework/9780122274107/encyclopedia-of-physical-science-and-technology)
• Caves and Karst (https://doi.org/10.1016/B978-0-12-409548-9.12437-6)