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The watersheds are natural hydrological entities that cover a specific aerial expanse of land surface from which the rainfall runoff flows to a defined drain, channel, stream or river at any particular point. The terms region, basin, catchment, watershed etc are widely used to denote hydrological units. Even though these terms have similar meanings in popular sense, technically they are different. Size of a watershed is governed by the size of the stream occupied by it.  Size of the watershed is of practical importance in development programmes.  For example, size of irrigation cum hydel project has its watershed size several thousands of square kilometers but for a farm pond the size may be few hectares only.  In deserts and flat terrains with little incipient drainage, it may be difficult to delineate small sized watersheds whereas in undulating and hilly terrains smaller sized watersheds could be easily delineated.  Hence the aerial extent of watersheds vary widely in the various attempts made earlier for demarcation of watersheds.


A systematic delineation of river basins was first attempted in 1949 by Central Water and Power Corporation (CWPC) under the able leadership of  Dr.A.N.Khosla , in which entire country has been distinctly delineated into 6 Water Resources Regions (Watershed Atlas of India, AISLUS, 1990).  The 6 Water Resources Regions has been further divided into 66 major river catchments. These delineations have been extensively used in the planning and development of surface water resources of the country.

Subsequently it was realized that there is a need for systematic delineation of the rivers systems at a national level on a convenient working scale.  In this context, Ministry of Agriculture, Government of India had assigned the work of preparation and codification of River Systems of India to Mansinghal consultants.  In 1970, the Mansinghal Consultants suggested / published the methodology based on the area concept. The suggested hydrological units are : Water resource region, river basins, river sub-basin, watershed, sub watershed, mini-watershed and micro watershed. The recommended size of the watershed is from 250-750 Sq. Km.

Increasing thrust in  water sector demanded finer details of hydrological units for implementation of various watershed management schemes . One of the major landmarks in this direction was by the All India Soil and Land Use Survey (AISLUS) Organization of the Dept. of Agriculture and Cooperation by publishing the National level watershed atlas on 1: 1 million scale using the base map from Irrigation Atlas of India in the year 1990. In this Atlas, the entire river systems of the country have been divided into 6 Water Resources Region, which has been further, divided into 35 basins and 112 catchments.  These catchments have been further divided into 500 sub-catchments and 3237 watersheds.  The atlas consists of 17 sheets on 1:1 million scales along with a Compendium of watersheds giving details of other related information such as area within the basin, sharing states and stream names etc.  This atlas is being extensively used for various purposes.

The attempts made so far for delineation of hydrological boundaries up to watershed level has some limitations. The atlas giving details of watershed  is available on 1 million scale. However, for effective implementation of water resources development scheme there is growing requirement of a larger scale map. Further, these maps are available in hard copy and require digitization for integration in GIS platform. The terminologies and codification system adopted for watershed is also cumbersome.


In the last two decades, watershed management has gained the top most priority in water resources sector. Implementation of any water management measure requires   a suitable hydrological unit. A properly delineated watershed forms a convenient hydrological unit for computation of water balance parameters and thus implementation of water management schemes.  The watershed approach has become a pre requisite for any developmental programme, because land and water resources have maximum interaction and synergic effect, when developed on watershed basis.  Watershed approach is, therefore, increasingly applied in various development programmes like command area development, soil and water conservation, flood control, soil erosion control, river valley projects, land reclamation, people and resource dynamics etc.  It is also equally important for various hydro-power and irrigation projects, assessment of ground water resource, pollution and artificial recharge studies. For proper planning and execution of any development programme on watershed basis, it is essential to have watersheds in the form of maps along with relevant attributes.
Increasing application of GIS techniques in water sector requires digital maps and  geodatabase.

Under the Hydrology Project Phase – I, CGWB had the mandate to prepare the GIS data set on 1:250, 000 scale for various thematic layers relevant to groundwater for integration in the dedicated software and consequently in the Hydrological Information System (HIS) developed in the project. Watershed was one of the layers envisaged for integration in the software to facilitate the Ground water assessment on watershed basis.  CGWB was identified by Ministry of Defence as one of the nodal agency for creation of digital data under the Hydrology Project. Keeping in view the growing requirements of watershed delineation on 1:250,000 scale and its utilities in various departments for implementing diverse developmental activities an urgent need was felt to delineate the watershed boundary on  1: 250,000 and bring the data in GIS platform which can be utilized by different departments/agencies for various purpose. The present Watershed Atlas is the outcome of the effort  made by CGWB in this direction.