The word “watershed” introduced in 1920 was used for the “water parting boundaries”. A watershed, also called a drainage basin or catchment area, is defined as natural hydrologic unit which covers specific aerial expanse and area of land surface from which the runoff flows to defined drain, channel, stream or river at any particular point. Watersheds are considered more efficient and appropriate unit for assessment of resources and subsequent planning for sustainable development of resources because land and water resources have optimum interaction and synergetic effect. The watershed approach is therefore increasingly being employed in various development programmes in India like soil and water conservation, command area development, erosion control, forest management, desert development programme, river valley projects, flood prone area management, reclamation of wastelands etc.
From the hydrological point of view, the different phases of hydrological cycle in a watershed are dependent on the various natural features and human activities. Watershed is not simply the hydrological unit but also sociopolitical-ecological entity which plays crucial role in determining food, social, and economical security and provides life support services to rural people.
Source: ArcGis help.
Watershed Management
Watershed is an ideal unit to interact among various natural resources, man, animals, ecosystem etc. as they all make a unique geo-hydrological entity. Therefore management of watershed becomes an important unit for management of resources and ecosystem.
A watershed is an area of land and water bounded by a drainage divide within which the surface runoff collects and flows out of the watershed through a single outlet into a lager river or lake. Watershed technology is used in Rain fed areas.
Watershed management implies an effective conservation of soil and water resources for sustainable production of food, fodder, timber and other agriculture and forest resources. It involves management of land surface, vegetation, surface water resources so as to conserve the soil and water for immediate and long term benefits to the farmers, community and society as a whole.
Drainage system is divided in many categories depending upon the size, zone of runoff, etc starting from region, basin, catchment, sub-catchment, watershed etc. Catchment area is the water collecting area or run off zone “All the areas from which water flows out into a river or water pool”.
Watershed characteristics
In general terms, in more compact watershed, the runoff hydrograph is expected to be sharper with a greater peak and shorter duration. For a watershed that is partly long and narrow and partly compact the runoff hydrograph is expected to be a complex composite of the above mentioned hydrographs. Morphometric analysis of watersheds is necessary to understand complex inter-relation of linear, aerial, and relief aspects of terrain in any watershed. For this purpose database of various parameters of drainage (coding, ordering, flow direction etc.) and land surface (slope, elevation, extent etc.) are calculated. Analysis of morphometric parameters are calculated and tabulated (stream order, stream length, drainage density, stream frequency, bifurcation ratio, shape index, circulatory ratio etc.) to hypothesize the approach for water resource development planning.
Following Morphometric parameters are important for watershed management:
A. Linear Aspects
2. Stream length
3. Mean stream length
4. Stream length ratio
5. Bifurcation ratio
B. Relief Aspects
C. Aerial Aspects
1. Drainage density
3. Stream frequency
5. Circulatory ratio
6. Elongation ratio
A. Linear Aspects
1. Stream Order: Stream order is as measure of the degree of stream branching of streams within a Watershed. First order stream are defined as those channels that have no tributaries. In this case, the flow is depending entirely on surface overland flow to them. The junction of two first order stream forms a second order channel. When a low order stream segment joins to the height order stream segment, then the order of the stream remained as it is higher order. Second order channel receives flow from the two first order channel that that form it and from overland flow from the ground surface and might receive flow from another first order channel that flow directly in to it. Third order channel is formed by the junction of two second order channels. It receives flow not only from the two second order channels that form it, but also direct overland flow and possibly from first order channels that flow directly in to it and possibly from other second order stream that might join it. In general, an nth order stream is a tributary formed by two or more streams of order (n-1) and streams of lower order. Numerical ordering begins with the tributaries at the streams headwaters being assigned the value one.
Fig: 3 .Stream Ordering
2. Stream length and mean stream length: Generally total length of stream segments in first order streams found maximum and decreases as the stream order increases. Stream length is a combined function of length of slope, rock structure underneath and rainfall intensity. The smallest streams measured in first order are located on the crest while longest measured is located on the valley floors. Generally higher the order longer the length of streams is noticed in nature. Mean stream length is division of total stream length in a order and number of stream segments of that order. Mean stream length of any order is greater than that of the lower order and less than that of its next higher order. The variation in this factor is probably due to high variation in the topography, slope characteristics, variation in rock type and structural variations.
3. Stream length Ratio: Ratio of the mean stream length of the one order to the next lower order is stream length ratio. In general, this factor increases from lower to higher order. The decreasing trend of mean stream length ratio shows that the terrain is undulating and area has many streams and generating complex stream configurations of all orders. Higher order streams are low in compared to lower stream order therefore this factor in higher order is high. This pattern is found in areas where runoff is high which create erosion hazards.
4. Bifurcation Ratio (Rb): It is defined as the ratio of the number of streams of any order to the number of streams of the next highest order. Values of Rb typically range from the theoretical minimum of 2 to around 6. The division ratio is calculated as
The division ratio of a watershed is the average of the bifurcation ratios of each stream order
Fig: 4. Bifurcations (division).
B. Relief Aspects
1. Relief Ratio: It is defined as the elevation difference between the reference points located in the drainage basin. Maximum relief is the elevation difference between the highest point and the lowest points. Maximum basin relief is the elevation difference between the basin outlet and the highest point located on the perimeter of the basin.
Rh is the relief ratio
H is the relief (m)
HL is the horizontal distance (m)
C. Aerial Aspects
1. Drainage Density: Drainage density is indicative of closeness of spacing of drainage channels. It is factor of total length of streams of all orders per unit drainage area in a watershed. Density factor is associated / influenced by climate, type of rocks, relative relief, infiltration capacity of soil / rocks, vegetation cover, surface roughness, land use, run off intensity index etc. It denotes how densely a watershed / basin are channelized. High values of drainage density are indicative of stable drainage system and low altitude but undulating topography.
2. Stream frequency: Stream frequency is the total number of stream segments of all orders per unit area. The pattern of stream frequency is generally similar to the drainage density in the watersheds because numbers of drainage lines are proportionately to the length of drainage lines.
3. Drainage Texture: Drainage texture is relative spacing of drainage lines. It is the total number of stream segments of all orders per perimeter length of that watershed. Infiltration capacity is important factor influencing drainage texture which broadly depends on drainage density and stream frequency.
4. Form Factor: Form factor is indicative of shape of the watersheds. High value of form factor show circular shape and low values show elongated shape of watersheds. It is computed as the ratio of basin area to the square of basin length.
5. Circulatory Ratio (RC): It is ratio of the area of basin to the area of the circle having the same circumference as the perimeter of the basin. This factor is influenced by length and frequency of streams, geological structures, geomorphic features, land use / land cover, slope, relief, rainfall patterns etc. High circulatory ratio is indicative of circular shape of watersheds. This parameter is helpful in assessment of flood hazards. Higher the circulatory ratio higher the flood hazards at a peak time at the outlet point because in circular shaped watersheds, water drains from almost all direction and meet near the outlet, as a result more water is accumulated through all the streams.
6. Elongation Ratio: It is the ratio between the diameters of the circle of the same area as the drainage basin and the maximum length of the basin. A circular basin is more efficient in the discharge of runoff than an elongated basin. Higher elongation ratios are indicative of low altitude undulating terrain where as low elongation ratios show hilly terrain.
7. Overland Flow : It is the length of water over the ground before it gets into a definite drainage. This factor relates inversely to the average slope of the channel. It is a component of total run-off in a drainage basin. When the rainfall intensity exceeds soil infiltration capacity, the excess water flows over the land surface as overland flow. Length of overland flow is calculated as one half of the reciprocal of the drainage density.
Slope of watershed: It determines the flood magnitude and speed of flowing water. Naturally, the steeper the slope of a watershed, the greater the amount of runoff. Soil erosion by water also increases as the slope length increases due to the greater accumulation of runoff. The average slope of the watershed can be determined from the topographic map of the watershed by using the following formula.
S= percent watershed slope (average)
M = total length of contours within the watershed (m)
N = contour interval (m)
A = size of watershed
For very small watershed, the average slope can be taken as the ratio of difference in elevation between the watershed outlet and the most distant ridge (delta H) and approximate average length of the watershed (L).
Time of concentration (Tc): The time taken by the runoff to reach the outlet from the farthest point in watershed.
Tc= 0.0195 L 0.77 Sg-0.385
Tc is Time of concentration in minute,
L is maximum length of flow m,
Sg is the watershed gradient or the difference in elevation between the outlet and the most remote point in the watershed divided by the length, L
Drainage patterns
This refers to the arrangement of streams in a drainage basin, which often reflects structural and/ or lithological control of underlying rocks.
Drainage patterns tell much about the substance of which the land surface is made, with little practice, interpretation and identification of geologic structures and rock types in an area can be made from analysis of stream patterns on satellite images, air photos or topographic maps.
Major drainage pattern are Dendritic patterns, Trellis pattern and Radial patterns.
Dendrite drainage pattern
Develops in area where the type of rocks remain the same all over the basin and where no geological processes, like folding or faulting have created structures that would control the development of river system. Weak rock structure usually form dendrite drainage pattern. It is characterized by the fact that tributaries flow in the same direction as the main stream, joining at an acute angle.
Trellis drainages pattern
It develops in area where softer and harder rocks alternates with one another or where folding and faulting results in the formation of structures that control the development of river system
Fig: 5. Dendritic and Trellis Drain System
Radial drainage system
It is made up of a pattern of stream flowing outward, down the slopes of a dome or cone-shaped up land.
Fig: 6. Radial Drainage System
Importance of Watershed Management
Runoff from rainwater or snowmelt can contribute significant amounts of pollution into the lake or river. Watershed management is necessary to improve production of food, fodder, fuel, conservation of soil and water and also help to control pollution of the water and other natural resources in the watershed. It is achieved by identifying the different kinds of pollution present in the watershed and how those pollutants are distributed and transported, and preparation of plans for control of pollution through technical interventions.
All activities that occur within a watershed will somehow affect that watershed’s natural resources and water quality. New land development, runoff from already-developed areas, agricultural activities, and household activities such as gardening/lawn care, sanitation system, water diversion all can affect the quality of the resources within a watershed. Watershed management planning comprehensively identifies those activities that affect the health of the watershed and makes recommendations to properly address them so that adverse impacts from pollution are reduced.
Watershed management is also important because the planning process results in a partnership among all stake holders, policy makers and policy executors in the watershed. That partnership is essential to the successful management of the land and water resources in the watershed since all partners have a stake in the health of the watershed. It is also an efficient way to prioritize the implementation of watershed management plans in times when resources may be limited.
Because watershed boundaries do not coincide with political boundaries, the actions of adjacent political units upstream can have much impact on the downstream areas. Impacts from upstream sources can sometimes undermine the efforts of downstream municipalities to control pollution. Comprehensive planning for the resources within the entire watershed, with participation and commitment from all municipalities in the watershed, is critical to protecting the health of the watershed’s resources.
Types of Watersheds
Watershed is classified depending upon the size, drainage, shape and land use pattern.
a. Macro watershed: 1000 -10,000 ha
b. Micro watershed: 100 -1000 ha
c. Mini watershed: 10 -100 ha
d. Mille watershed: 1 -10 ha
Objectives of Watershed Management
a. Improve production of food, fodder, fuel.
b. Control of Pollution
c. Minimize over exploitation of natural resources
d. Water storage, flood control, checking sedimentation.
e. Wild life preservation
f. Erosion control and prevention of soil
g. Soil and water conservation
h. Employment generation through industrial development
i. Recharging of ground water to provide regular water supply for consumption and industry as well as irrigation.
j. Recreational facility.
Steps in watershed management planning
Delineate and map the watershed’s boundaries and the smaller drainage basins within the watershed.
Inventory and map of natural resources in the watershed
Inventory and map the natural and manmade drainage systems in the watershed. Inventory and map land use and land cover.
Inventory and map soils.
Identify areas of erosion, including stream banks and construction sites
Identify the quality of water resources in the watershed as a baseline
Inventory and map pollution sources, both point sources (such as industrial discharge pipes) and nonpoint sources (such as municipal storm water systems, failing septic systems, illicit discharges).
Much of these information may already be compiled and available through the government departments such as Soil conservation and Watershed Development, Forests, Agriculture, Irrigation, municipal offices, planning and zoning of urban areas, inland wetlands, and public works etc.. Additional information specific to the watershed can be gathered from the ground studies of the general conditions of the receiving waters and the adjacent watershed areas.
Watershed management involves determination of alternative land treatment measures for, which information about problems of land, soil, water and vegetation in the watershed is essential. In order to have a practical solution to above problem it is necessary to go through four phases for a full scale watershed management.
a. Recognition phase
b. Restoration phase
c. Protection phase
d. Improvement phase
(i) Recognition Phase
It involves following steps
(a) Recognition of the problem
(b) Analysis of the cause of the problem and its effect.
(C) Development of alternative solutions of problem.
(ii) Restoration Phase
It includes two main steps.
(a) Selection of best solution to problems identified
(b) Application of the solution to the problems of the land
(iii) Protection Phase
This phase takes care of the general health of the watershed and ensures normal functioning. The protection is against all factors which may cause determined in watershed condition.
(iv)Improvement Phase
This phase deals with overall improvement in the watershed and all land is covered. Attention is paid to agriculture and forest management and production, forage production and pasture management, socio economic conditions to achieve the objectives of watershed management.
Watershed Management Programmes
Government of India and state governments have many live programmes being implemented in the country on watershed management with the objective of soil and water conservation, optimum utilization of natural resources, improvement of food, vegetables, fruit, fodder, fuel etc, mitigation of effects of draught, restoration of ecological balance.
(i) Drought Prone Area Programme (DPAP)
(ii) Desert Development Programme (DDP)
(iii) National Watershed Development Programme for Rain fed Agriculture (NWDPRA) (iv)Control of Shifting Cultivation
(v) World Bank Assisted Integrated Watershed Development Project
(vi) Integrated Watershed Management Programme (IWMP)
(vii) Neeranchal National Watershed Project
Watershed Management Practices: In India generally following practices are adopted for watershed management programmes
A. In Terms of Purpose
(i) To increase infiltration
(ii) To increase water holding capacity
(iii) To prevent soil erosion
B. Method and Accomplishment:
(i) Vegetative measures/Agronomical measures:
(a) Strip cropping
(b) Pasture cropping
(c) Grass land farming
(ii) Engineering measures/Structural practices
(a) Contour bunding
(c) Construction of earthen embankment
(d) Construction of check dams
(e) Construction of farm ponds
(f) Construction of diversion
(g) Gully controlling structure
(i) Establishment of permanent grass and vegetation
(j) Providing vegetative and stone barriers
| you can view video on Applications of RS/GIS in Watershed Management |  |
Remote sensing, GIS and GPS Copyright © by komal. All Rights Reserved.
