Watersheds and Its Management !
Meaning of Watershed:
A watershed is defined as any spatial area from which rain or irrigation water is collected and drained through a common point. It is synonymous with a drainage basin or catchment area. Given the general topography of peninsular India and farm size distribution patterns, the joint property of a group of 5-10 farmers usually adds up to a landscape unit which suffices for creating a watershed.
In natural landscapes, such units generally co-exist in a serial continum. Each watershed unit, however, is a well-defined topographically delineated area with a distinct boundary. Further, a landscape watershed may constitute simple or complex agro-ecologies.
It may be made up of an upland ecology or a combination of upland, lowland ecologies or in some cases, a topo-sequential complex of ecologies representing upland, lowland and coastal ecosystem. The upland part of the watershed generally represents dry land rain fed agro ecology. The lowlands, mainly used for the production of paddy or other high water requiring crops.
Each watershed framework thus provides the necessary inter-ecological linkages and is a logical unit for the integration of the sustainable use of land and water. It helps augment soil and groundwater resources. It also enjoins the biophysical, social and economic inputs, which if optimally managed, lead to diversified high agricultural production, control of environmental degradation and provide a mechanism for the recharge of groundwater aquifers. Watersheds thus ensure water adequacy to crops and reduce the risks of yield loss due to water related stresses.
Objective of Watershed Management:
The overall objectives of all watershed management programmes are:
(i) To increase infiltration of rainwater into the soil,
(ii) To manage and utilize runoff for useful purpose or simply soil conservation measures, and
(iii) To control damage by run off.
Thus watershed management is an integration to technologies within the natural boundaries of drainage areas for optimum development of land, water and plant resources to meet the basic minimum needs of the people in that watershed in a sustained manner. Therefore, watershed management implies the scientific use of soil and water resources in the catchment. It entails protection against all form of land deterioration with crop production stabilization.
A watershed is the whole region (area) contributing to the supply of a river or lake.
A watershed is an area of land and water bounded by a drainage divide within which the surface run-off collects and flows out of the watershed through a single outlet into a larger river or lake (Brown, 1960).
Watersheds can be small or large. Small watersheds are drainage areas of a few thousands of hectares. Larger drainage areas are usually called river basins.
Watershed covers all land and water areas which contribute run- off and soil sediment to a common point which may be drain, channel, stream or river. The terms watershed, catchment, drainage basin and river basin are used synonymously. Water management provides clean and uniform water supply for beneficial use and to control damaging run-off. The same measures are used for controlling soil erosion by the water and watershed management.
Concepts of Watershed:
The river system is hierarchical. Rainwater on a ridge is divided in the direction of its slopes. The ridge acts as a ‘water divide’, meaning that the rainwater diverges from here in two or more directions following land slopes. Within the area surrounded by the ridge the surface flow of water coverage’s to feed the drainage system. The area within the ridges, i.e., the catchment area of the drainage system is defined as the watershed.
The rainfall, draining the upland follow slopes to converge into small gullies and streams which, in turn, meets large tributaries and finally, the main river course. Any of the constituents of a river a tributary, a stream or a gullyâ€”has a defined catchment. The catchment is identifiable on the group or on a topographical map as the watershed.
Each watershed has a network of streamsâ€”the ‘drainage system’, occupying an area surrounded by a continuous ridgelines. The rainfall in the watershed turns into surface and ground water flows to pass through its drainage and ground water system tending to exit the area through the lowest part of the trunk stream.
Watershed and Landform:
In a watershed, there are 3 distinct landforms:
1. The upland with or without a plateau top with slopes varying between 20â€” to 45 – and above.
2. The midland with wasteland and grazing lands in the slopes varying from 10â€” to 20- and
3. The terraces and agricultural fields in the low lands with slopes below 10â€” (Fig. 23.9)
All water flowing in the stream or in the ground water region will have the affinity to flow from the upland or plateau top through the midland region to finally converge in the low lands. Hence, the plateau- top, highland and midland slopes together make the recharge area for the ground water that can be tapped in the low-lying terraces and plains.
Upland is thus an area for water harvesting, midland an area for small dams and springs development and lowland for tapping ground water source. Hence proper compensatory measures may have to be devised for the occupants of the midland and upland regions.
Generally, upland areas have the potential for horticulture and livestock development. Midland often provides a good ambient for dry land agriculture while the lower valley area may strengthen their traditional agricultural practices through introduction of proper irrigation and land treatment e.g. terracing and leveling.
Watershed a Social Indicator:
Watershed is a geographic entity. Through centuries of social conflicts and adjustments a pattern has emerged by which different socio-economic groups is found to have occupied different locations of watershed.
1. The people living in the upland portion of the watershed, with stiffer land slopes, thinner soil cover and scanty natural vegetation are perpetually poor and deprived.
2. The people occupying the upper slopes and plateau regions of the watershed owing to their proximity to forests are likely to possess appreciable knowledge, skill, attitude and practice to sustain on forest based economy.
3. The people occupying the midland portion of the watershed adapt themselves to dry land farming, rainfed agriculture with single crop. They too resort to livestock farming or horticulture and acquire appreciable knowledge and skill to live under constrained natural ambience.
4. The people occupying the lower slops and valley regions with terraced or plain agricultural land having stronger socio- economic groups. They normally have easier access to water and irrigation (Fig. 23.8)
For creating small surface water based irrigation systems, upland is considered as the provider, midland the host while the lowland is the consumer. This is how the water resources is being shared presently.
Factors Affecting Operation of the Individual Watershed:
(i) Topography of the Land:
The run-off water from steep slope will reach main channel quicker than gentle slope. The watershed condition can be changed either by vegetation or structural erection.
(ii) Shape of watershed:
The shape of watershed has a bearing on the time of concentration of run-off.
(iii) Soil Type:
The soil texture determines the infiltration rate of rainfall.
(iv) Size of Watershed:
The size of watershed affects the amount of channel and storage. The size of watershed indicates the types of control needed.
The amount and pattern of rainfall are the most important factors in determining what happens in a specific watershed.
The type of vegetative cover plays an important role in watershed management. Vegetative cover controls infiltration, run-off and sediment products.
(vii) Land Use:
Land use affects the rates of water infiltration and run-off, types and nature of vegetation. Land can be used in such a way as to provide high degree of watershed protection.
Problem in Watershed Management:
The main problem in a watershed is the soil erosion by rainfall. The run-off water transport the sediments which may block the channel head, dam, reservoir and storage structures. The de-gradational processes in the catchment areas may result in an alarming rate of sedimentation in the reservoirs, structures and at the river mouth. Nizamsagar reservoir in Andhra Pradesh is an example of an alarming rate of sedimentation. It has lost 50 per cent of its capacity to provide irrigation to the command areas.
Objectives of Watershed Management:
The main objectives are:
(a) To control damage by run-off,
(b) To manage and utilize run-off for useful purpose or soil conservation purpose, and
(c) To increase infiltration of rain water.
Watershed management implies the scientific use of soil and water resources in the catchment area for optimum development of land, water and plant resources to meet the basic minimum needs of the people. Watershed management provides protection against land deterioration, loss to productivity and flood hazards.
Measures contributing to watershed protection can be classified in two broad categories:
1. The land use and treatment measures, which are effective in increasing the infiltration of rainfall and the water-holding capacity of the soil and of preventing soil erosion on watershed land. They include measures such as contouring, terracing, strip cropping, rotation, grassed waterways, pasture and woodland improvement. This category also includes watershed- stabilization measures such as control of gullies (by constructing structures or vegetation).
2. Management of water flow after it has left the fields and farm waterways and reached the small branches and creaks. These measures include flood water retarding structure like constructing diversion ditches, dams etc.
Land Use and Stream Flow:
Physical characteristics of the soil determine the stream flow, namely, infiltration, percolation and storage capacity. When infiltration and percolation rates and soil storage capacities are high, immediate run-off will be low. Under these conditions, the rainfall water will move into the soil. Woodland and grassland vegetation reduces the rate of surface run-off, thus reducing soil erosion and -floods.
Retention and Stream Flow:
By managing vegetation so that both the depth to which water is stored and the water-holding capacity of the soil particles are increased (thus, increasing total storage capacity), more rainfall can be stored. In addition, it may be possible to increase the total storage capacity of deep soils by encouraging or introducing deeper-rooted vegetation. Such vegetation would provide space for storage of precipitation (rainfall) as that less would go to stream flow.
On the other hand, evaporation is reduced where vegetation is present but the total loss by transpiration plus evaporation is commonly greater than the loss by evaporation alone from a bare area. Thus, storage opportunity is greater under vegetation than in bare areas.
Soil a Storage Reservoir:
Soils are not static. They are dynamic in the sense that they are continuously changing in structure, colour, depth, etc. They swell with the addition of water; they may shrink and crack as they dry. They creep downward on steep slopes.
The effects of land use and vegetation on the soil are reflected principally in changes in the structure, number, size, shape and stability of the aggregates. These changes in turn affect the size, shape, and distribution of the soil pores. These pores provide space for the storage of the water and serve also as avenues for water movement.
Watershed conservation involves the conservation, management and use of all soil and water and all things that depend on themâ€”tree, shrubs, grass, crops wildlife, fish, livestock and so on. It also includes proper construction and maintenance of dams- bridges, road, pond etc.
Components of Watershed:
A watershed is the land from which water flow into stream, river or other point of drainage. Essential watershed elements are geography of the watershed, area, vegetation, growing on the watershed, the land in cultivation, land use (includes farming, grazing, wildlife, industry or other urban use) and watershed land conditions of erosion and soil fertility.
The Watershed A Physical Unit:
A watershed is generally considered to be the geographic area of over land drainage that contributes waters to the flow of a particular stream at a chosen point. It is a ‘water-collecting’ and ‘water-handling’ unit.
Features of Watershed Plan:
The watershed plan is basically a water management plan. Although varying in scope, content, and degree of detail to fit an infinite variety of local conditions, it could be described as having the following salient features:
1. Watershed plan includes measures as:
(a) Major gully stabilization by structures and vegetation planning,
(b) Diversion ditches to collect run-off into controlled channels,
(c) Small flood water retarding structures,
(d) Erosion control and water management measures along highways, rail roads and other non-farm developments.
2. The plan should include a map showing physical land conditions based on the techniques of soil conservation surveys and the land capability classification to serve as a guide to the planning of land use adjustments and application of treatment measures on individual farms and ranches.
3. The map would be accompanied by a report describing the pertinent physical features of the area, the land and water problems, the opportunities for their correction.
The increases that could be expected in agricultural production as well as other benefits that would achieve by regulation of run-off, better distribution of available water, disposal of excess water and control of erosion. Moreover, it should set forth criteria for planning. The use and treatment of land on individual farms and reaches in such a way as to secure the maximum benefits in water management and sediment control.
Watershed planning is rather a tool to aid local people for water management, plus a framework for sound farm conservation planning on areas ranging in size from a few square miles up to several hundred square miles.
Watershed plans involve co-operation and major responsibility on the part of almost every landowner and operator within the area. In community watershed concept, the development of watershed concept rests with farm group and urban community leaders, soil conservation agency and other agencies.