Definition of runoffPortions of runoffRunoff processSurface runoffFactors affecting runoffRunoff cycleConditions of runoff cycleSummary of Rainfall-Runoff process

Definition of Runoff

Runoff can be defined as the portion of the precipitation that makes it’s way towards rivers or oceans etc, as surface or subsurface flow.

Portion which is not absorbed by the deep strata.

Runoff occurs only when the rate of precipitation exceeds the rate at which water may infiltrate into the soil.

Portions of Runoff

1. Surface runoff

2. Groundwater flow

3. Direct precipitation over the river stream.

Runoff Process

When rainfall occurs: A part of rainfall/precipitation is intercepted by vegetation.

Some part is stored in depressions on the ground surface known as depression storage (Sd), which later infilterates or evaporates.

Some part of rainfall is absorbed by the soil, the amount of which depends upon the soil moisture condition at the time of percolation.

Now if the rain continues further; The water starts infiltrating/percolating to the water table and if the rate of

rainfall or the rate at which the water is reaching the ground exceeds the infiltration rate (f), resulting the surface detention (D).

This water flow overland and joins the rivers, lakes, streams, oceans, etc, and is known as surface runoff.

One can say runoff as surface runoff.

1) The surface runoff is important for maximum flow.

2) Where as the ground water flow is important for minimum flow.

3) Direct Precipitation over the river or stream is negligible.

So for peak flow we are generally concerned with surface runoff and therefore we can say runoff as surface runoff.

Factors Affecting Runoff

1. Precipitation characteristics,

2. Shape and size of the catchment,

3. Topography,

4. Geological characteristics,

5. Meteorological characteristics,

6. Character of the catchment surface,

7. Storage characteristics.

1. Precipitation characteristics

It is the most important factor for runoff

Runoff depends on the type of the storm and it’s duration, which causes precipitation.

Runoff depends on the intensity of rainfall.

More the rainfall, more will be runoff.

If the rainfall intensity is very less and it rains as light showers then much of the water will be lost in infiltration & evaporation resulting less runoff.

If precipitation is in case of snow then less runoff.

2. Shape and size of the catchment

Runoff depends upon size, shape and location of the catchment.

Generally more rainfall on smaller area resulting in greater runoff.

Less runoff in larger catchments because due to uniform rainfall over the entire area, thus only few tributaries of the stream feed water to a main stream during a particular storm.

Types of catchment

A. Fan shaped catchment

B. Fern leaf catchment

A. Fan shaped catchment

All the tributaries are approximately of the same size.

Gives greater runoff because the peak flood from the tributaries is likely to reach the main stream approximately at the same time.

B. Fern leaf catchment

The tributaries are generally of different lengths and meet the main stream at the regular intervals.

In such a narrow catchments the peak flood intensity is reduced since discharges are likely to be distributed over a long period of time.

3. Topography

Runoff depends on surface smoothness and slope.

If slope is steep, flow will be quick and less evaporation and absorption, resulting greater runoff.

If the catchment is in mountainous area and on the windward side of the mountain, then more rain fall resulting more runoff.

4. Geological characteristics

It is one of the important factor.

It includes the type of surface soil, subsoil, type of rock and their permeability characteristics.

If soil and subsoil is porous, seepage will be more resulting reduction of the peak flood.

If the surface is rocky, then absorption will be nil resulting more runoff.

If rocks have fissures, are porous in nature, have lava tunnels water will be lost resulting less runoff.

5. Meteorological characteristics

Runoff may also be affected by temperature, wind and humidity.

If temperature is low and ground is saturated then runoff will be greater.

If temperature is high and greater wind velocity give rise to greater evaporation loss and resulting in less runoff.

6. Character of the Catchment surface

Runoff depends upon the surface conditions like drained, undrained, natural or cultivated.

If the surface has no natural drainage then absorption loss will be more.

If more area of a catchment is cultivated resulting less runoff.

Vegetal cover reduces the runoff in smaller storms.

No vegetal cover reduction in bigger storm.

7. Storage characteristics

The artificial storage such as dams, weirs, etc and natural storage such as lakes, ponds, etc tend to reduce the peak flow.

They also give rise to greater evaporation losses.

Runoff Cycle

It is a part of hydrological cycle

The part between the precipitation from the atmosphere over land areas and it’s subsequent discharge through streams channels.

Conditions of Runoff Cycle

1. End of dry period

2. Shortly after beginning of rainfall

3. Near the end of isolated heavy rainfall

4. After the end of rainfall

1. End of dry period

At the end of dry period and just beginning of heavy rainfall all the surface and channel storage gets depleted (dried) expect from lakes, reservoirs and ponds resulted from the previous rains.

The only source of stream flow is the ground water flow entering the river channel.

2. Shortly after beginning of rainfall

Shortly after beginning of rainfall and before interception, depression storage have been satisfied.

The stream flow intercepted by vegetation and buildings can’t contribute to runoff.

This intercepted water is eventually returned to the atmosphere through evaporation.

At this stage, a part of precipitation falls directly on the stream which gives an immediate increment to stream flow.

3. Near the end of isolated heavy rainfall

After many hours of heavy rainfall virtually all depression storage and interception requirements gets filled up.

The soil moisture deficiency is also satisfied to a considerable extent.

Infiltration rate is near the minimum.

Similarly the flow into the filled depression is essentially balanced by over land flow and infiltration.

Thus at this stage over land flow mainly contribute to stream flow.

Sub surface flow also contributes to stream flow.

4. After the end of rainfall

When rain and overland flow ceases (stops), the stream flow consists of only base flow and channel storage.

Evaporation takes place quite from soil moisture.

Transpiration also takes place from vegetative cover.

Water from depression storages also continues to infiltrate.

Also the gravity water still not drained up to the water table continues its downward journey to join water table.

Summary of Rainfall-Runoff Process

When a rain starts falling, it is first of all intercepted by buildings, trees, and other objects, which prevent it from reaching the ground. This quantity is known as rainfall interception.

Since this quantity is generally very small, it is not of much importance for intense rains; but many a times, large portions of lighter rains are disposed of in this manner.

The difference between the total rainfall and that which is intercepted is called ground rainfall.

when the rainfall rate exceeds the interception rate, water starts reaching the ground and infiltration into the sub soil starts. The maximum rate at which the soil in a given condition can absorb water is known as its infiltration capacity.

The excess rainwater gets collected into the innumerable small and large depressions existing in the basin, filling them to their overflow levels. This quantity is known as depression storage.

All this storage is either evaporated or used by vegetation, or later infiltrates into the soil. None of it appears as surface runoff.

If after the depression storage is filled, the rain intensity (p) continues to exceed the infiltration capacity of the soil (f), the difference appears as rainfall excess, which initially accumulates on the ground as surface detention (D), and then flows as overland flow on the basin surface before entering a stream channel.

The water that reaches the stream channel of a basin in this manner is called surface runoff (SRO) or direct runoff (DRO).

The surface runoff can, therefore, occur only from those storms, which can contribute to excess rainfall, and are simply not dissipated in fulfilling the interception, depression storage, and infiltration needs of the basin. Hence,

Excess Rainfall = Rainfall – Interception - Depression storage – Infiltration

The sum total of interception and depression storage for a basin is usually called the initial loss or basin loss or initial basin recharge.

Excess rainfall can therefore be represented as:

Excess rainfall = Rainfall – Initial Basin loss – Infiltration

The sum total of initial basin loss and infiltration, is called potential infiltration.

Since for intense rains, the initial loss is very small as compared to excess rain, it is usually ignored in hydrological analysis of such rainfalls; or is considered to be included in the infiltration itself.

The excess rainfall is, thus, represented as:

Excess Rainfall = Rainfall – Potential Infiltration

The rain that falls in the beginning of a storm before the depression storage is completely filled is called the initial rain,

and

the rain that falls near the end of the storm at a rate less than the infiltration capacity is called the residual rain.

The intervening period is the net supply interval. The infiltration occurring after the net supply interval is called the residual infiltration.

Runoff and surface runoff are two different terms and should not be confused.

Runoff or Discharge or the Stream flow includes all the water flowing in the stream channel at any given section.

While

Surface Runoff or Direct Runoff includes only the water that reaches the stream channel without first percolating down to the water table.

Yield of a drainage basin is same as runoff, with the only difference that it is expressed over long periods (M.m3/year),

while

Runoff is expressed for short periods (m3/sec or m3/hr).

0.0000

100.0000

200.0000

300.0000

400.0000

500.0000

600.0000

700.0000

Baseflow

Surface Response

Runoff =Surface runoff + Ground water inflow (i.e. Base flow).

0.0000

100.0000

200.0000

300.0000

400.0000

500.0000

600.0000

700.0000

0.0000 0.5000 1.0000 1.5000 2.0000 2.5000 3.0000 3.5000 4.0000

Total Hydrograph

Surface Response

Baseflow

Continuous process represented with discrete time steps