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In this article we will discuss about:- 1. Definitions of Watershed 2. Features of Watershed 3. Significance 4. Watershed Drainage Divide 5. Classification 6. Deterioration 7. Watershed Delineation 8. Demarcation 9. Coding 10. Human Impacts 11. Analysis.
Contents:
- Definitions of Watershed
- Features of Watershed
- Significance of Watershed
- Watershed Drainage Divide
- Classification of Watershed
- Deterioration of Watershed
- Watershed Delineation
- Demarcation of Priority Watershed
- Coding of Watershed
- Human Impacts on Watershed
- Analysis of Watershed
1. Definitions of Watershed:
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The watershed is also nomenclature as drainage basin, catchment or drainage area. It refers to the area drained by a stream in such a way that all flow originating in that area is discharged through a single outlet. The topographic divide or watershed boundary line which covers the watershed, demarcate the area in which overland flow approaches towards the drainage system (stream) which finally becomes as surface runoff at the outlet. The ground water flow may not conform the surface drainage boundaries, because phreatic divide does not always coincide with the topographic divide.
There are various definitions of watershed, cited by different investigators/agencies; few of them are mentioned as under:
1. It is a divide, which separates one watershed from the other.
2. The watershed is the area, which is drained by a stream or a system of connecting streams in such a way that all stream flow originated in the area is drained through a single outlet.
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3. The drainage basin is the area which contributes runoff to the main stream and its tributaries.
4. It is a hydrological entity, in which hydrologic cycle gets complete.
5. As per oxford dictionary, the watershed is the separating ridge between two river systems.
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6. The watershed and drainage basin are synonymous terms indicating to an area surrounded by a ridge line that is drained through a single outlet. If it refers to the area that contributes the runoff to a river or stream, then it is called as catchment of the river.
7. Chow stated that the watershed is a divide separating one drainage basin from another. Drainage basin is referred as catchment in the British literature.
2. Features of Watershed:
The informations on watershed features regarding hydrological or morphological studies are very important in addition to the type of watershed falling in a particular area.
There are various important features of watershed concerns, are listed as under:
i. Size – The watershed size varies from micro to very large. Accordingly, they have different characteristics.
ii. Drainage Divide – Also called watershed boundary. It plays very significant role to divide the water flow from the area, i.e., whether the water is flowing towards or away from the area within the watershed.
iii. Topography – It basically refers to the terrain of the land within watershed boundary. Terrain affects the trend of flow, e.g., if the area is steep then flow of water is very quick which may result into occurrence of severe soil erosion. On the other hand, if terrain is flat, then there is very little possibility of soil erosion occurrence.
iv. Soil type – The soil is also considered as one of the very important features of watershed, because it affects the watershed behavior, significantly. For example – a belt of sandy, soil in watershed absorbs huge amount of rainwater from the rain event resulting into very low contribution for overland flow/runoff yield from there.
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On the other hand, the area under hard rocks/clay soils produces the runoff/overland flow with very high rate, because of very less infiltration loss. The soils also have significant effect on runoff, erosion and ground water recharge.
3. Significance of Watershed
:
The watersheds essentially comprise various features but in varying degree. Because of this reason, there is significant variations in their behavior. The behavior may be in terms of hydrological (rainfall-runoff-sediment yield), crop yield or in socio-economical aspects.
Consideration of these behaviours are very important, because watershed provides all means for nature’s creators. For example – a watershed provides drinking water as well as water for recreation, irrigation and industrial activities. Watersheds are also significant for plants and animals as they provide food, shelter and water to them.
4. Watershed Drainage Divide
:
The drainage divide is the boundary between two different land segments. The land segment may comprise very small size (micro/macro) area. The divide may be any kind of ridgeline.
In case of watershed study the drainage divides are of following three types:
i. Continental divides
ii. Major drainage divides; and
iii. Minor drainage divides.
In continental drainage divides the water flows on each side of these divides into different oceans. In major drainage divides the waters on each side of the boundaries do not meet via the same river or stream, but they reach to the same ocean. In case of minor drainage divides the water gets separate at the divide, but later rejoins.
5. Classification of Watershed
:
The watersheds are classified into a number of classes depending on the classification mode.
In general, the watershed classifications are based on the following parameters:
i. The size
ii. The drainage
iii. The shape; and
iv. The land use pattern.
1. Watershed Classification Based on Size:
These are presented in Table 27.1.
2. Watershed Classification Based on Shape:
Hydrologically, the shape of watershed is very important because it controls the time for the runoff to concentrate at the outlet, i.e., the time of concentration; and thus affecting the watershed’s hydrological behavior.
The most common shapes are given as under:
i. Fan shape; and
ii. Fern shape.
The fan shape watersheds are normally in circular shape; and fern shape watersheds are in elongated form.
3. Watershed Classification Based on Soil:
These are mentioned as below:
i. Red soil watershed; and
ii. Black soil watershed.
4. Watershed Classification Based on Slope of Land:
Under this classification, the watersheds are classified as follows:
i. Hilly watershed; and
ii. Flat land watersheds.
5. Watershed Classification Based on Climate:
These are as below:
i. Humid watershed; and
ii. Arid watersheds
6. Watershed Classification Based on Land Use Pattern:
These are as below:
i. Highland watersheds
ii. Tribal settlement watersheds; and
iii. Settled cultivation watersheds
Also, the categorization of watershed is done based on the size of stream/river, and the point of interception of the stream/river.
Small and Large Watersheds:
The following are the factors to distinguish the small and large watersheds:
1. Size:
i. The size of small watershed varies from few acres to 100 acres.
ii. In case of large watershed the size exceeds 100 acres.
2. Overland Flow:
i. It is predominant in small watersheds, as they have less network of drainage system.
ii. In large watersheds, the channel flow is dominant.
3. Channel Storage:
It is significant in large watershed, while insignificant in small watershed.
4. Land use:
It affects the overland flow, significant in small watersheds. In large watersheds since channel flow predominant, therefore, the land use in large watersheds is not effective.
5. Intense Rainfall:
In small watersheds intense rainfall causes rapid peak flow at outlet, whereas in large watersheds it is not so.
6. Roughness of Soil Surface and Channel:
The surface roughness of soil or land creates effect on overland flow, i.e., there would be less overland flow when surface roughness is more. On the contrast, its effect on channel flow is not much, but roughness of channel causes effect on channel flow.
Thus, considering above features distinguishing the small and large watersheds, a small watershed may be defined as a well bounded area which is so small that its sensitivity to intense rainfall is not suppressed by the contesting channel.
6. Deterioration of Watershed:
It is caused due to faulty use and improper management practices, followed.
However, the following points also lead to deteriorate the watershed, given as:
1. Adoption of faulty agricultural, forestry and pasture management practices.
2. Fire consequences.
3. Unscientific mining and quarrying.
4. Alignment and construction of roads in faulty way.
5. Extension of industrial activities.
6. People apathy etc.
The deterioration of watershed causes following bad effects:
(i) Reduction in biomass production
(ii) Poor returns from agriculture, grasslands, forests, fruit crops etc.
(iii) Siltation of reservoirs, lakes, channels etc.
(iv) Reduction in water quality and quantity, both
(v) Creation of poverty in the area.
7. Watershed Delineation:
The delineation of priority area can be performed to some extent by reconnaissance survey and study of topo-sheets. However, this technique is slow and also not provide very accurate result. Demarcation of priority areas can also be done in better way by using the areal photograph. Normally, the contour maps of 1:60,000 scale are most suitable, but the photographs of larger scale such as 1:15,000 can also be used for the purpose.
The demarcation of priority areas should be accomplished on watershed basis, because a comprehensive watershed management approach is essential for use of proper soil conservation measures. For demarcating priorities areas on watershed basis, the preparation of framework of watershed delineation over the entire watershed is essential.
Furthermore, it is also necessary that the size of watershed to be delineated should be from 10,000 to 20,000 ha, because for small watersheds the formulation of soil conservation plans and their execution over a reasonable period is practically possible and easy, also.
The steps for demarcation of small size watersheds are described as under:
1. Divide the entire watershed into different sub-watersheds considering important tributaries. The size of sub-watershed should be few lakh hectares. Use suitable scale for delineation. Normally, 1:1 million and 1:250,000 scales are followed.
2. Again, divide each sub-watershed into small, following distinct tributaries and streams passing through the respective sub-watersheds. The size of small watersheds should be in the range of 50,000 to 1,00,000 ha. Delineate these small watersheds using the scale 1:50,000. And superimpose the delineated sub-watersheds in step- (1) on the base map of the area.
3. Further, sub-divide each small watershed (as obtained in step 2) in the size ranging from 10,000 to 20,000 ha.
In the watershed if there are large number of small streams, that drain the runoff directly into the main stream, then demarcation of small size watershed is difficult. For such conditions the demarcation is carried out by combining all the streams into sub-watershed, small watershed etc. This should be started from the down-stream end and proceeded to upstream side.
8. Demarcation of Priority Watershed
:
It is carried out by determining the comparative severity of erosion and probable sediment yield from different watersheds.
The method is devised under following steps:
1. Determine the erosion intensity of different watersheds, called erosion intensity unit, and grade them in accordance to their increasing severity. Also, find out the probable sediment yield of respective watersheds and grade them by their order. For grading the least eroding units are assigned by the number 1 or 0.50, while more eroding units by higher weights such as 2, 3, 4…
2. Calculate the area of each erosion intensity unit and also determine their total area.
3. Multiply the area of each erosion intensity unit to its weight assigned. The obtained value is termed as weighted product. Compute the total weighted value by adding all together.
4. Compute, the erodibility index of erosion intensity unit by dividing the total weighted value with its total area, i.e.
Where,
IE = erodibility index of sub-watershed, (%)
Tw = total weighted value of sub-watershed
Ta = total area of sub-watershed.
5. Measure the distance between erosion intensity unit and the reservoir in which runoff is discharged; and assign the weight to each, as per given in Table 27.7. This weight is added to the erodibility index of each sub-watershed. The consideration of distance between erosion intensity unit and the reservoir is based on the fact that, the silt loads from nearer sub-watersheds tends to reach into the reservoir sooner as compared to those which are at farther distance.
6. After determining the total weights for each sub-watershed, arrange them into 3 to 5 suitable priority classes such as-
(i) Very high
(ii) High
(iii) Medium
(iv) Low
(v) Very low
For priority determination, the data required and their resources are cited in Table 27.8.
9. Coding of Watershed:
Coding of watershed is done to fix a kind of identity based on some specified features. It is carried out by several ways, viz., in digits, combination of letters and digits or by using the name of stream associated with the watershed. The coding of watershed in the river valley project is normally done in the combination of letters and digits.
The coding is carried out by using the following steps:
1. Find out the name of main river/stream draining the watershed. Let it is ‘Ram Ganga’.
2. Take the first letter of the name of main stream, which is ‘R’ for the Ram Ganga.
3. Determine the name of sub-catchment. Let, it is Naurar sub-catchment of Ram Ganga.
4. Take the first letter of the name of sub-catchment, as the second letter for coding of the watershed, which is obtained as ‘N’.
5. Combine both the letter, i.e. ‘RN’.
6. Use the digits for watersheds delineated within the sub-catchment, e.g. RN1, RN2, RN3 etc.
7. Lastly, code the sub-watershed by adding another digit after a hyphen, i.e. RN 1-1, RN 2-2 etc. The coding should be started from the downstream end and proceeded to upstream end. For this purpose, the tributaries serially from down-stream to up-stream are taken into consideration, in respect of their joining sides, i.e., left or right hand side of the main stream. The word ‘L’ for left hand side and ‘R’ for right hand side joining tributaries to the main stream, is also added in the code of watershed.
The coding method adopted by All India Soil & Land Use Survey is also given in Table 27.9.
10. Human Impacts on Watershed:
A well-managed watershed creates a kind of space for development of human need resources. It is general observation that most of the mega cities in the world are developed along the waterways. And all the waterways are linked with the watershed as a water generating source point.
The waterways are always under disturbance condition, because of different types of activities in action by the population. The cultivation practices in watershed always tend to generate sediment load to the linked waterways, as result there is development of siltation problem.
Ultimately, the siltation causes several in-conductive effects. The cutting of forest vegetations from the watershed causes to increase the runoff generating potential, which results into severe runoff from the watershed that creates flood occurrence at the down-stream end.
In irrigation command, the area falling under watershed boundary, if not properly taken care about drainage point of view, then there is possibility of development of water logging and salinity problems. This problem may affect the water quality and others, too. In other words, the watershed is likely to get deteriorated.
In general, the water pollution takes place in following two ways:
i. Point source; and
ii. Non-point source.
In which, the point source pollution refers to that which is confined to a specific point such as a disposal site or point of leaking pipe. And the non-point source pollution is that which takes place when the water containing pollutants is moved to some other places such as used for irrigation purposes etc., and there is developed pollution.
In this condition, the pollutants are deposited at greater distance from the source point. In addition, there are several other points under human based impacts on watershed, are considered.
11. Analysis of Watershed:
The quantitative land-form analysis is carried out for the developed watersheds, in which flowing water (overland flow or runoff) and mass movements over long periods of time are the promising parameters for development of surface features.
The watershed analysis is accomplished under following heads:
It includes the analysis of watershed’s geometry and its stream/channel system to evaluate the linear aspects of drainage network, aerial aspects of drainage basin and relief aspects of channel network and contributing ground slope, also. The analysis of watershed geometry including drainage network, considers the projected property of watershed on a horizontal plane is termed as ‘planimetric’, whereas relief aspect counts the vertical inequalities of drainage basin.