# Components of hydrograph pdf

Read this article to learn about the definition, assumptions, limitations and advantages of unit hydrograph theory! It is a typical hydrograph of direct runoff which gets generated from one centimeter of effective rainfall falling at a uniform rate over the entire drainage basin uniformly during a specific duration. Effective rainfall is that portion of rainfall which fully contributes towards direct runoff.

Therefore, unit hydrograph can also be defined as the hydrograph of a drainage basin which gives one centimeter of direct runoff from a rain storm of specific duration. This requirement calls for selection of storms of so small a duration which would generally produce an intense and nearly uniform effective rainfall and would produce a well defined single peak of hydrograph of short time base. This important assumption is called principle of linearity or proportionality or superposition.

In other words the hydrograph of direct runoff resulting from a given pattern of effective rainfall will remain invariable irrespective of its time of occurrence. This assumption is called principle of time invariance.

In practice, however, uniformly distributed effective rainfall rarely occurs on large areas. Also on large areas effective rainfall is very rarely uniform at all locations, within its specified duration. Obviously bigger the area of the drainage basin lesser will be the chances of fulfilling the assumptions enunciated above. The limiting size of the drainage basin is considered to be km 2. Beyond it the reliability of the unit hydrograph method diminishes.

When the area of the drainage basin exceeds a few thousand km 2. The catchment has to be divided into sub-basins and the unit hydrographs developed for each sub-basin. The flood discharge at the basin outlet can then be estimated by combining the sub- basin floods adopting flood routing procedure.

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The reason is that the storm rainfall gets mixed up with the snow pack and may produce delayed runoff differently under different conditions of snow pack. Obviously the principle of time invariance is really valid only when the time and condition of the drainage basin are specified. But it is not practicable to derive separate unit hydrograph for each possible time- intensity pattern.

This is so because due to variability in proportion of surface, subsurface and groundwater runoff components during smaller and larger storms of same duration, the maximum ordinate peak of the unit hydrograph derived from smaller storm is smaller than the one derived from larger storm.

Obviously the character and duration of recession limb which is a function of the peak flow will also be different.River discharge is the volume of water flowing through a river channel. This is the total volume of water flowing through a channel at any given point and is measured in cubic metres per second cumecs.

The discharge from a drainage basin depends on precipitation, evapotranspiration and storage factors. Hydrographs can be used to illustrate discharge. These can be used to show annual discharge patters of flow in relation to climate. Over the short term, a flood or storm hydrograph figure 1.

They cover a relatively short time period, usually hours or days rather than weeks or months. Storm hydrographs allow us to investigate the relationship between a rainfall event and discharge. The starting and finishing level show the base flow of a river. The base flow is the water that reaches the channel through slow throughflow and permeable rock below the water table.

As storm water enters the drainage basin the discharge rates increase. This is shown in the rising limb. The highest flow in the channel is known as the peak discharge. The fall in discharge back to base level is shown in the receding limb. The lag time is the delay between the maximum rainfall amount and the peak discharge.

The shape of a hydrograph varies in each river basin and each individual storm event. The hydrographs below show two contrasting environments. Rural areas with predominantly permeable rock increases infiltration and decreases surface runoff. This increases lag time. The peak discharge is also lower as it takes water longer to reach the river channel. Urbanisation is the main human impact on a storm hydrograph. As roads, pavements and buildings are constructed the surface becomes impermeable.

Laying drains leads to the rapid transportation of water to river channels which reduces the lag time. Large drainage basins catch more precipitation so have a higher peak discharge compared to smaller basins.

Smaller basins generally have shorter lag times because precipitation does not have as far to travel.

The shape of the drainage basin also affects runoff and discharge.A hydrograph is a graph showing discharge i. At the beginning, there is only base flow i. After the storm commences, the initial losses like interception and infiltration are met and then the surface flow begins.

The hydrograph gradually rises and reaches its peak value after a time tp called lag time or basin lag measured from the centroid of the hyetograph of net rain. Rising limb- The rising limb of hydro graph, also known as concentration curve, reflects a prolonged increase in discharge from a catchment area, typically in response to a rainfall event Recession or falling limb: The recession limb extends from the peak flow rate onward.

The end of stormflow aka quickflow or direct runoff and the return to groundwater-derived flow base flow is often taken as the point of inflection of the recession limb. The recession limb represents the withdrawal of water from the storage built up in the basin during the earlier phases of the hydrograph. Unit periodThe time duration of the unit storm i.

## Simple Hydrograph: Components and Parts (With Diagram)

Lag time tp The time from the centre of a unit storm to the peak discharge of the. Recession time Tr The duration of the direct surface runoff after the end of the excess or net rainfall, is called recession time in hydrograph analysis. Propositions of the Unit Hydrograph The following are the basic propositions of the unit hydrograph: i Same runoff duration.

For all unit storms of different intensities, the period of surface runoff i. For unit storms of different intensities, the ordinates of the hydrograph at any given time, are in the same proportion as the rainfall intensities iii Principle of superposition.

### Discharge & Hydrographs

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Discharge is normally measured at gauging stations that are situated at different points along the river. The discharge of a river changes over time depending on a few factors. The most influential factor is the weather. A storm hydrograph is a specific type of hydrograph that, surprise, surprise, shows precipitation and discharge during and after a storm. The main difference between a normal hydrograph and a storm hydrograph is that a storm hydrograph is over a much shorter period of time.

Below is a storm hydrograph for the fictional River Shui:. First is the lag time.

## Hydrograph Components

The lag time is the time difference between the peak precipitation and and the peak discharge. Conversely, a short lag time indicates that the precipitation is entering the river fairly quickly. The rising limb is the steep part of the discharge line that has a positive gradient, indicating that the discharge is increasing. The falling limb is the opposite showing that the discharge is falling.

The shape of a hydrograph is altered by a few different things. One factor is the shape of the drainage basin. Drainage basins come in a wide assortment of shapes. Roughly Circular shapes are common as are more elongated and narrow shapes. This is because all points in the drainage basin are again, roughly equidistant from the river so all the precipitation reaches the river at the same time. The size of the drainage basin obviously has an impact on the hydrograph.

Basins with steep slopes will have a high peak discharge and a short lag time because the water can travel faster downhill. Finally the drainage density of a basin will affect the lag time and the steepness of the falling limb. Basins with lots of streams and rivers a high drainage density will have a short lag time and a fairly steep falling limb because water will drain out of them quickly. If a river is surrounded by non-porous and impermeable rocks e. This is much faster than groundflow, interflow and throughflow so the lag time is reduced.

Unconsolidated soils allow water to infiltrate and so act as a store in a drainage basin. In addition, water travels slowly through soil via throughflow. This reduces the peak discharge while increasing the lag time of a river.

As a result, water travels quickly as overland flow, reducing the lag time of a river. The intensity of a storm will obviously impact the peak discharge of the river. Not immediately obvious is the type of storm or precipitation that takes place.

Flood hydrographs

A winter storm i. If the area surrounding the river has thick vegetation cover then lots of precipitation will be intercepted, greatly increasing the lag time.In this article we will discuss about:- 1.

Meaning of Runoff Hydrograph 2.

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Components of Hydrograph 3. Factors Affecting the Shape 4. Base Flow Separation. Sometimes, it is also known as storm hydrograph, flood hydrograph or simply hydrograph. It also shows the distribution of total runoff with respect to time at a certain point of measurement.

### Runoff Hydrograph: Meaning, Components and Factors | Geography

All hydrographs have three characteristics regions viz. These characteristics regions are shown in the schematic diagram of the hydrograph Fig. The hydrographs are mainly in two types, i. 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.

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.

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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. 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. 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.Thanks for sharing this informative information.

For Storm components all over information you may also refer Hydrograph and its components. It is an inverted U shaped diagram. Rising Limb : It is the ascending curved portion of the hydrograph. The rising limb rises slowly in the early stage of the flood but more rapidly toward the end portion. The shape of rising limb depends on duration and intensity distribution of rainfall. This is because in early stages the losses is more and water reaches to the stream faster. This is the part which is taken as matter of interest by hydrologists.

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Peak of hydrograph occurs when all parts of basins contribute at the outlet simultaneously at the maximum rate. Depending upon the rainfall-basin characterstics, the peak may be sharp, flat or may have several well defined peaks. It extends from the point of inflection at the end of the crest to the beginning of the natural groundwater flow. Labels: flood hydrographHydrographwater.

Unknown August 20, at PM. Floyd Pokharel November 4, at AM. Unknown March 27, at PM. Spark Mallikharjuna April 24, at AM. Unknown December 21, at PM. Older Post Home. Subscribe to: Post Comments Atom.To browse Academia. Skip to main content. Log In Sign Up. Fidelis O. The quantity of surface runoff is controlled by a complex variety of factors.

Included among these are precipitation intensity and duration, permeability of the ground surface, vegetation type and density, area of drainage basin, distribution of precipitation, stream-channel geometry, depth to water table, and topographic slope.

Also, in this term paper, runoff processes, runoff components, polluted runoff, runoff effects, mitigation and treatment of runoff and methods of estimating runoff were discussed. The rate of runoff flow depends on the ratio of rain intensity to the infiltration rate.

If the infiltration rate is relatively low, such as when a soil is crusted or compacted, and the intensity is high, then the runoff rate will also be high.

National Soil Erosion Research Laboratory. From AMS glossary, surface runoff is the water that reaches streams ranging from the large permanent streams to the tiny rills and rivulets that carry water only during rains by travelling over the surface of the soil. Thus, surface runoff takes place only over the relatively short distance to the nearest minor channel. According to the Department of Atmospheric Science, Runoff is the movement of land water to the oceans, chiefly in the form of rivers, lakes, and streams.

Runoff consists of precipitation that neither evaporates, transpires nor penetrates the surface to become groundwater. Even the smallest streams are connected to larger rivers that carry billions of gallons of water into oceans worldwide.

Excess runoff can lead to flooding, which occurs when there is too much precipitation. Science Daily defines surface runoff as the water, from rain, snowmelt, or other sources, that flows over the land surface, and is a major component of the water cycle Wiki encyclopedia defines surface runoff as the water flow that occurs when soil is infiltrated to full capacity and excess water from rain, melt water, or other sources flows over the land.

This is a major component of the water cycle. Robert E. Horton and Robert E. Horton Runoff that occurs on surfaces before reaching a channel is also called a nonpoint source.

If a nonpoint source contains man-made contaminants, the runoff is called nonpoint source pollution. A land area which produces runoff that drains to a common point is called a watershed.