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Delineation of Watersheds:
The watershed is a hydrological unit bounded by natural ridges. It allows the runoff due to rainfall to drain in a well-defined drainage pattern of streams within the watershed boundary. All India Soil and Land Use Survey Organization of Government of India has prepared the WATERSHED ATLAS OF INDIA delineating the country in five stages viz. Region, Basin, Catchment, Sub Catchment and Watershed.
The Six water resources regions as suggested by Dr. A.N. Khosla in 1949 have been adopted by numbering tem in clockwise direction. They are:
Name of Regions
1 Indus River Region
Each water resource region has been delineated into BASINS, which are constituted by a single major river by group of small rivers or a major tributary of the rivers like Ganges, Indus etc.
Following interstate river basins covers Tamil Nadu State
Delineation of Watershed into Smaller watershed in the District Atlas:
The entire nation has been delineated into Watershed upto five levels in the 1:1 million scale. Watershed Atlas of INDIA. These watersheds can be sub divided into smaller watersheds if large-scale maps are used. The District Watershed Atlas are prepared based on the 1:50000 scale Survey of India Topographic Sheets and the taluk maps of Tamil Nadu Govt. Which are in 1:1 mile or 1:50000 scale.
While using this large scale maps the delineation can be to identify the smaller watersheds. In critical areas the contours are not available to effectively delineate the watersheds the Aerial Photographs are used to identify the ridge and the delineate the small watersheds.
The smaller watersheds are codified and given specific names to know their levels. A well-defined relationship can be created between the stream order, the watershed level and area of the watershed. The watersheds are being delineated based on the stream or river drainage pattern. A stream is formed due to drain the water from place and forms the course for the water. After drop, sheet and gully erosion the stream takes a definite path. This stream is known as first order stream. When two first order stream joins they form a bigger stream to take runoff from both streams. This is known as the second order stream. When two second order streams join they form the next higher order i.e. . Third order stream. However when a first order stream joins with the second order stream the resultant stream is stream is only second order stream. It is observed that the watersheds can be further delineated into sub watershed , mini watershed and micro watershed. It may however be seen the rivers in plains will have watersheds, sub watersheds, mini watersheds and micro watersheds. The streams in hilly and undulating region will have many further smaller watersheds as the drainage pattern is dense and the rivers will have upto 10th order stream if it is originating from hilly regions.
The lower order micro watersheds are codified as micro watershed grade 1, micro watershed grade 2, micro watershed grade 3 etc. in a larger river like Cauvery one can observe micro watersheds upto grade 5.
An illustration of delineation of watersheds into tiny micro watersheds is detailed in the table below.
While delineating the watersheds in to micro watersheds the terms meso, nano, peso watershed etc., are avoided, as they may not be under stood well at the field level. The word micro watershed and itís various grades are adopted the word lMIC O WATERSHED is being used in the field level planning. The word MACRO watershed is also avoided in this ATLAS as it creates confusion between MICRO and MACRO watersheds for those new to the watershed concept.
The table above will give an idea about the micro watershed relationship with area of watershed and the stream order. This will avoid the concept of calling an area of 500 ha i.e. 5-eq. Km as a micro watershed without seriously considering the drainage pattern. The micro watershed cannot be linked directly with as area without studying the stream morphology.