Watershed Management: Introduction, Objectives, Factors, Selection and Estimation!
Introduction to Watershed Management:
A watershed is an area from which runoff, resulting from precipitation, flows past a single point into a large stream, a river, lake or an ocean. The terms watershed, catchment area or drainage basin are used in the same sense. A watershed may be only a few hectares as in case of small ponds or hundreds of square kilometres as in case of rivers.
All watersheds can be divided into smaller sub-watersheds. As each watershed or sub-watershed is an independent hydrological unit, any modification of the land use in the watershed or sub-watershed will reflect on the water as well as sediment yield of the watershed.
Objectives of Watershed Management:
Watershed management or protection implies the proper use of all land and water resources of a watershed for optimum production with minimum hazard to natural resources.
The different objectives of a watershed management programme are –
(1) To control damaging runoff,
(2) To manage and utilize runoff for useful purposes,
(3) To control erosion and effect reduction in the sediment production,
(4) To moderate floods in the downstream areas,
(5) To enhance groundwater storage wherever applicable, and
(6) Appropriate use of the land resources in the watershed and thus developing forest and fodder resources as well as protection of the environment.
Adoption of conservation farming practices to improve agriculture, controlled grazing to keep the pastures productive, water management for irrigation and drainage and all other types of erosion control measures could be considered as parts of the watershed management programmes.
The problems of each watershed are different and programme for their management could also be different laying emphasis on the problems of the particular watershed.
Factor Affecting Watershed Management:
The important factors which affect the watershed behaviour and which need to be studied for developing a management programme are –
(1) Size and shape of the watershed,
(2) Topography,
(3) Soils and their characteristics,
(4) Precipitation,
(5) Land use, and
(6) Vegetative cover.
The kind and amount of data necessary for planning watershed improvement projects will depend in part upon the nature and purpose of the project.
The data necessary for planning can be grouped under the following broad categories –
(1) Hydrological information,
(2) Soil and land use data, and
(3) Economic and social data.
Hydrological information includes data about precipitation, climatological parameters required for determining crop water requirements, stream flow data and data about sediment flow.
Soil and land use data includes existing land use information, soil data, topographic and geologic maps and types of vegetation.
Economic and social data are needed to work out the cost benefit ratios of the project.
Selection of Priority Areas for Watershed Management Programme:
In a watershed management programme, particularly in case of large watersheds it may not be possible to treat the entire area of the watershed with land treatment measures. In such cases, the watershed should be divided into sub-watersheds and sub-watersheds which need treatment should be identified.
The methods used for determining the priority of the sub-watersheds for treatment from soil erosion and sediment yield point of view are –
(1) Reconnaissance surveys,
(2) Soil and land use surveys,
(3) Interpretation of aerial photographs, and
(4) Sediment observations.
A reconnaissance survey of the entire watershed gives an idea of the relative erosion status of the sub-watersheds, but this is approximate information and is to be used when other methods are not available.
Detailed soil and land use surveys include the erosion information of the sub-watersheds. A careful interpretation of these reports could provide information on the relative erosion status of the various sub-watersheds.
The main difficulty with soil survey is that it requires much time and personnel. Scientific interpretation of aerial photographs provide qualitative information regarding land, including location, general texture of soil, general evaluation of sand, gravel and boulder deposits, drainage characteristics, spread of vegetation etc.
From the aerial photographs information on the erosion conditions of the areas can be gathered and areas, which are comparatively under severe erosion can be determined.
Actual measurement of the silt load contributed by each of the sub-watersheds will give a clear picture of the extent of erosion in the sub-watersheds. Observations in respect of silt loads recorded over a period of three to five years will indicate the sub-watersheds which are contributing higher silt loads. The main difficulty with this procedure is that the data has to be collected over a period of years so that reliable conclusions from the data can be drawn.
Estimation of Sediment Yield of Watershed:
The universal soil loss equation essentially gives the soil loss of individual plots. The gross erosion in a watershed consists of erosion in individual plots, gully and stream bank erosions. Sediment produced from sheet and rill erosion may get deposited before it moves to the outlet of the watershed.
The concept of sediment delivery ratio (Dr) is used to estimate the sediment yield of a watershed. The sediment delivery ratio is defined as a fraction (or percentage) of gross erosion that is transported to the point of measurement and is expressed as –
The SDR depends on range of local factors including the size of the watershed, its characteristics like land use, slope, erosion conditions etc. Its values are therefore applicable for the local conditions for which these values are determined and have to be used with caution.
The US Soil Conservation Service (1971) has developed a general SDR values versus drainage area relationship from data of earlier studies. The relationship shows that the SDR varies approximately inversely as the 0.2 power of the drainage area.
Values of SDR given in Table 28.1 may be used as approximations. A higher value of SDR is used when the eroding soil is very high in silt or clay and a lower value is used if the eroding soil is coarse textured.
Regression equations, wherein the sediment yield is related to several factors like watershed size, land cover, slope, soil properties, etc. are also used for estimating the sediment yield.
Conceptual models for estimating sediment yields from watersheds have also been developed and some of these approaches are given in Singh (1989).
Watershed Work Plans:
These are project proposals for watershed management. The work plans describe the watershed and its problems and set forth the general sequence in which works of improvement are to be installed, the estimated cost, economic justification and the responsibilities of those participating in the project for installing, operating and maintaining those measures needed for the protection and improvement of the watershed.
As the problems of each watershed are different, their work plans will also be different. As an example, a watershed work plan may have the following contents.
I. Description of the Watershed:
The description of the watershed should be given in sufficient detail so as to present a clear picture of the conditions in the watershed as they relate to the problems in the watershed and the proposed works of improvement.
The different watershed characteristics needed are as follows:
1. Location:
Name of the river basin, tributary, physiographic region, latitude, longitude and principal communication lines.
2. Size and Shape:
Size (in sq. km. or hectares), shape (long and narrow, fan shaped), length width ratio.
3. Climate:
Precipitation, its annual, seasonal and monthly distribution, forms of precipitation. Rainfall, storm pattern, rainfall intensity, duration and its areal distribution. Other climatic factors include air temperature (mean and extremes) soil temperature, evaporation, relative humidity, wind velocities and direction, and solar radiation.
4. Geology:
Nature of parent rocks, fractures, faults, extent of outcrops, weathering and groundwater recharge areas.
5. Slopes:
Proportion of the watershed in different slope groups, length of slope and mean slopes.
6. Surface Drainage:
Detailed information about the principal streams and their tributaries.
The information should include:
(i) Nature of Flow – Perennial, intermittent, spring-fed, ephemeral or seasonal.
(ii) Drainage Net – Order of the streams, stream density and stream lengths.
(iii) Morphologic Characteristics of Streams – Stream channel profile and location of lakes, ponds and swamps.
7. Soils:
Major soil groups of the watershed and their hydrologic groupings. Physical and chemical properties as required.
8. Physiography:
Elevation of different watershed areas, information about mountain ranges etc.
9. Water Uses and Needs:
Source of surface and groundwater, use for domestic, irrigation, power generation, recreation, adequacy, future needs, etc.
10. Land Use and Cover Conditions:
Existing land use and cover conditions including forest lands, range lands, cultivated lands, waste lands, habitations and miscellaneous uses. Forest types and area under each classification (like stock forest, degraded forest, scrub etc.
Hydrologic: conditions legal status (reserved, demarcated—protected, un-demarcated— protected, unclassified, private, community etc.), present management, areas under regeneration, felling and logging practices, closures, rights and forest fires.
Extent of range lands, their major classification (hayland, grazing land etc.) closures, grazing incidence, grazing practices (migrating, settled etc.), cattle population (number, types), important grass species and their distribution and hydrological conditions.
Extent of agricultural lands, land capability classification, area tinder each class and subclass, area under irrigation, major crops, rainfed area along with crops grown, orchards and their extent etc.
11. Economic Data:
General economic conditions of the people, important professions and their dependence on watershed resources, markets and marketing practices, return from forests including fire wood, and minor forest products, return from range land, grazing value, returns from cultivated land including irrigated and rainfed etc.
II. Watershed Problems:
In this section problems resulting from flood water, erosion and sediment damage and those associated with conservation development utilization and disposal of water are to be discussed.
1. Flood water Damage:
The problem described by outlining-
(i) The amount and value of land improvements and other property exposed to the flood hazard,
(ii) The frequency and seasonal occurrence of floods,
(iii) The significance of small frequent flood or large infrequent floods in the total problems,
(iv) Land use limitation due to flooding, and
(v) Any other pertinent aspect of the problems.
2. Sediment Damage:
The effects of sedimentation of water supplies, reservoirs, channels, drainage, irrigation development and agricultural areas are assessed.
3. Erosion Damage:
The extent of sheet, gully and channel erosion, the sources of sediment causing downstream damage, the effect of erosion on agricultural production and on the general economy of the watershed are outlined.
4. Problems relating to Water Management:
The needs for irrigation, drainage, agricultural and non-agricultural water supply and other water areas management needs are discussed.
5. Special Problems:
Problems unique to the watershed like land slips, landslides, torrents, highway erosion, mines etc. are mentioned.
6. Socio-economic Problems.
III. Proposed Management Programme:
These consist of measures for agricultural lands, forest lands, grazing areas, irrigated areas and protective measures for other areas.
Different factors to be described under each item are as follows:
1. Agricultural Lands:
Recommendations should be given for each land capability class.
(i) Agronomic Practices:
Crops proposed, rotations, cultural operations, introduction of improved seeds, use of manures and fertilizers, green manuring, use of improved implements, plant protection measures, expected costs and yields.
(ii) Engineering Measures in Agricultural Lands (Class Wise):
Proposed measures, justification, specifications of individual measures with plans and designs, calculations and cost estimates.
2. Conservation Irrigation:
Selection and design of suitable method of irrigation for each part of land, duration of irrigation, conveyance system, location and specifications, land leveling and land smoothing, plan for disposal of excess water and agronomic measures.
Type of surface drainage systems, their specifications, location of outlets and their design, design of intercepting and relief drains, specifications of subsurface drainage systems (if adopted), maintenance needs and cost estimates.
3. Protection against Floods:
Channel improvements, flood retarding structures, their location, designs and cost estimates.
4. Forest Land:
Management practices for existing forests, degraded forests, shrubs and farm forestry. Engineering measures to be adopted on forest and range lands.
5. Grazing Lands:
(i) Pastures – Species selected method of raising, mechanical practices like contour furrowing and water spreading, management practices, yields and costs.
(ii) Grazing Lands – Improvement practices recommended; grazing systems to be prescribed.
6. Road Construction:
Layout of roads, protection of roadsides from erosion.
7. Special Problems:
Control measures recommended for special problems like landslides, stream bank erosion, gullies and ravines.
IV. Effect of Works of Improvement:
Under this topic the likely benefits viz., increased agricultural production, flood control, reduction of sediment loads, increased forest produce etc., are outlined.
V. Comparison of Benefits and Costs:
The discussion should show the ratio of the average annual benefit to average annual cost of the project. Only primary benefits are to be used for benefit and cost comparison. A tabular form of estimating benefits as shown in Table 28.3 is useful.
Models for Watershed Behaviour:
Computer- based models have been developed for studying watershed behaviour. These include models for predicting rainfall-runoff relations with changing land uses in a watershed, estimating erosion and sediment yields and estimating movement of solutes influencing water quality. Listing of some available watershed models is given in Haan et al. (1982).
In each of the models, input data have to be provided. Using specific algorithms, the model provides an output. Computer software packages are available for many such models and the use of such software packages has to be done after a proper understanding of the procedures adopted in developing the software.