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Reservoirs also known as Wewa (in local language) has been the heart of the national agriculture system in Sri Lanka. Unfortunately these large and small scale reservoirs had been destroyed as a result of political and personal agendas of humans. This has vitally effected the agricultural water system directly causing a reduction of national food supply. Which ultimately troubles the national economy.
To overcome this situation Reservoirs have to be preserved. Traditional process of protecting the reservoirs (Wewa) is very tedious process mainly for the following reasons.
Traditional Precise surveying takes time.
Process needed the coordination of many institutions such as Govi samithi (Farmers' Society), Department of Agrarian service, Survey Department are some of them.
High cost
Huge amount of labor, etc.
The process we propose may overcome some or all shortcomings to identify and to extract the FSL of reservoirs rapidly which enables to preserve the reservoirs by preventing unauthorized human activities.
Methodology
Study Area - Part of Puttlam District (80%)
Acquiring satellite images
To extract the area of Full Supply Level(FSL) of Reservoirs, Satellite image of Spilling period of most reservoirs has been taken.
Type of Satellite Image
For this purpose, among the freely available optical satellite images Sentinel 2 Satellite image was used due its higher spatial resolution.
Process of downloading the Satellite image
The Copernicus Open Access Hub (previously known as Sentinels Scientific Data Hub) provides complete, free and open access to Sentinel-1, Sentinel-2, ...)
Acquiring Satellite image data :2019/12/27
Extracting Water Bodies
Normalized Difference Water Index
The NDWI results from the following equation: Index = (NIR - MIR)/ (NIR + MIR) using Sentinel-2 Band 8 (NIR) and Band 12 (MIR). The NDWI is a vegetation index sensitive to the water content of vegetation and is complementary to the NDVI. High NDWI values show a high water content of the vegetation. (Gao, B.C., Remote Sensing of the Environment, p.257(1996))
Processing Software
QGIS project A Free and Open Source Geographic Information System ·
Processing
Bands 8 and 12 of Acquiring Satellite image of Sentiel 2
Deriving the NDWI from Raster calculate in Qis
In NDWI image High NDWI values show a high water content which resulted in pixel color nearing to white.
Digitizing the Required Reservoir here we have considered Wadigamangawa Reservoir (Wewa) Anamaduwa Divisional Secretariat, Puttalam District.
Result
Digitizing the NDWI image to extract following details in reservoirs
Reservoir’s Full Supply Level (FSL), Perimeter Points Coordinates
Wadigamangawa Reservoir
Digitized as polygon and Turning points as nodes
Labeled vertex and respective coordinate to stake out the FSL
For Coordinates check the file
Conclusion
Although in this matter wadigamangawa reservoir has reached its full level other reservoirs’ water level should be verified with local data before using the above methodology.
Spatial resolution of this data set is 20m which means it covers land area of 20m*20m which is 400m2or otherwise 15.8perch. Higher spatial resolution data may lead toaddress this matter with a better accuracy but higher resolution satellite images are not freely available.
Theoretically for water bodies has to provide a higher NDWI values throughout its region due to large vegetation cover on surface water of water bodies such as aquatic plants, in some part of water bodies provide a lower NDWI values resulting darker pixels.
For the initial stage to identify the water area manual digitizing process has been followed, a robust autonomous system could be developed in next stages.
Note: This method has only considered the FSL of reservoirs, neither adjacent lands nor the reservations of reservoirs.
by M.F.M. Mafaz -0774222389
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