Digital Watershed Atlas of India

 Watershed:-

Watershed is a natural hydrologic entity governed by the terrain topography from where runoff is drained to a point. The term watershed is a general phenomenon thus its size and area depends on the scale of the base map used for delineation and codification.

Recognizing the importance of management of soil and water resources in the country following the natural system, the department of Agriculture and Cooperation developed the delineation and codification system and “Watershed Atlas of India” has been published (Anon, 1990). The  atlas has been devised based on the drainage map on 1:1 million scale following stream hierarchy where the whole country has been divided into six River Resource Region, 35 Basin, 112 Catchments, 550 Sub catchments and 3257 Watersheds. The codification has been made in a simplistic manner following alphanumeric system.

A watershed can be symbolized as 1A2B3 where “1” stands for River Resource Region, “A” designates the Basin in that river resource region, “2” indicates the Catchment within the basin, “B” indicates Sub catchment and “3” stands for the watershed number in the sequence of stream hierarchy.

Methodology:

Delineation and Codification:-

India is drained by many rivers and their tributaries where flood and droughts are frequent visitors. To combat these situations and to sustain agricultural production, the country’s land mass needs to be divided into smaller hydrological units that led to conceptualization of “Delineation and Codification of Watersheds in India”.  The state of art of the delineation and codification system is that it is an open system and the process can be extended to micro level using the drainage map of larger scale .The Watershed Atlas meets up the requirement of planning at National and State level. One of the unique characters of the atlas is that it provides the basic framework of watersheds of the country and all the land based developmental programme of the country can be recognized with national code of watershed.

             The delineation has done in seven stages starting with Water Resource Regions and their subsequent division and subdivisions into Basins, Catchments, Sub-catchments, Watershed, Sub watershed and Micrwatersheds in decreasing size of the delineated hydrologic unit.

Each of the sub-catchment then divided into watersheds following the lower order streams, a group of tributaries or the left and right bank of higher order stream moving downstream upwards using the same base.

When the left and right banks of a higher order stream are being delineated as separate watershed it would be advisable to cut across the stream at the upper end at a suitable confluence point to avoid the ambiguity in locating the ridge. The number of watersheds in a sub-catchment is restricted up to 9. The size of individual subwatersheds is generally restricted around 5,000 to 9,000 hectares, which is considered a viable working area for implemental programmes. The division of watershed is restricted to 22 alphabets (avoiding a, e, i, l and o). The code of sub-watershed will be 2A1C1b.

1.          Water Resource Regions (WPR): The six WRRs suggested by Dr. A. N. Khosla in 1949 have been adopted as such with slight modifications in their numbering which has been done in a clockwise manner, starting with Indus drainage as numbers like 1, 2, 3 etc.

r  Indus drainage – 1

r  Ganges drainage – 2

r  Brahamputra drainage – 3

r  All drainage flowing into the Bay of Bengal except those at 2 & 3 – 4

r  All drainage flowing into the Arabian Sea except Indus drainage – 5

r  Western Rajasthan mostly ephemeral drainage – 6

2.          Basins: Each WRR has been divided into basins which constitute individual big rivers like Krishna, Narmada, Chambal etc. or a combination of smaller ones which are contiguous to each other, such as basin between Cauvery and Krishna have been divided into lower and upper basins or left bank and right bank basins as in the case of Brahamputra. Basins are assigned letters as A, B, C………..Z.

3.          Catchments: Each basin has been divided into a number of catchments, which pertain to main tributaries or a group of contiguous tributaries or individual streams. Catchments are represented by numerals suffixed to basin code as 1, 2, 3…….9.

4.          Subcatchments: The catchments are further divided into a number of sub-catchments, which are mainly smaller tributaries and streams. Sub-catchments are indicated by suffixing alphabets to catchment code as A, B, C………Z.

5.          Watersheds: Each subcatchments has been divided into a number of watershed which are the smallest sized hydrologic units in the macro level category on the base of 1:1 million scale in the Watershed Atlas of India, published by SLUSI. Watersheds are designated by suffixing Arabic numbers to a code of subcatchment. For example a watershed code would be like 4G4D3, 2B2A3 etc. The watersheds codes are restricts to numerical 1 to 9 only.

6.          Subwatersheds: Each watershed is further divided into sub-watersheds on 1:50000 scale (SOI topographical map) in which main tributaries and streams are taken up for delineation of sub-watersheds.

Subwatersheds designated by small English alphabets as a, b, c………z which is suffixed to watershed code. The complete code of a subwatershed is appeared as 2A2E8a, b, c, etc. The small alphabet ‘e’ ‘i’ ‘l’ and ‘o’ are discarded in view of the cartographic consideration and to avoid its ambiguity with the sequence of code. Hence, the total numbers of codes for subwatersheds is restricted to 22.

7.          Micro Level Delineation: Planning to phase out the watershed management at catchment level and to formulate action programme needs, micro level delineation. The delineation of watershed boundary at micro level could easily be attained by superimposing the watershed boundary from Watershed Atlas on to a drainage map of 1:50000 scale.

The delineation and codification would follow the similar system based on stream hierarchy and codification from downstream upward that allows to get a microwatershed of 500 to 1,500 ha size viable enough for implementation of soil and water conservation programmes. The beauty of such delineation and codification is that it could be recognized with a national code with seven digits and symbolized as 1A2B3a1 where “a” stands for sub-watershed and “1” denotes the microwatershed. Thus 1A2B3a1 stands for a national code of a microwatershed belongs to Water Resource Region “1”, Basin “1A”, Catchment “1A2”, Subcatchment “1A2B”, Watershed “1A2B3”, Subwatershed “1A2B3a” and Microwatershed “1A2B3a1”.

Table: Average size and size ranges for each Hydrological Units

Various steps involved in delineation of watersheds at micro level are illustrated and is given below:-

Digital Watershed Atlas of India:-

The Watershed Atlas of India published by Soil and Land Use Survey of India  has now been brought under digital environment using GIS and RDBMS by the organization as step towards development of Spatial Database not only to serve the country in a sustained manner but to disseminate the information on line to the users through information technology  

The digital watershed atlas is customized in such a way that it can be given to the users both River Resource Region and Basin wise or State and Catchment basis. The beauty of the atlas is that the name of the river will blink on the monitor when the cursor is placed on any part of the atlas. Subsequently, detailed description of the watershed highlighting the area and the district and State it falls and its linkages with main drainage system will follow. The digital atlas is going to be the backbone of many future information systems and applications.