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The largest watershed in Thailand is the Chao Phraya River Basin which covers approximately 35% of Thailand’s land mass. A watershed is all of the land area that drains into one common location such as the Chao Phraya River, a pond, wetland, lake, estuary, and eventually, the oceans.
Everybody lives in a watershed, and everyone shares the water within that watershed. Therefore everything in a watershed is in some way inter-connected which is why it is so important to think about how our actions may impact upon others.
A watershed can be quite large or extremely small. An example of a small watershed would be the soi near your home. The gutters along the sides of your house or apartment are even a kind of 'micro watershed'. Watersheds are separated from each other by areas of higher elevation called ridgelines or divides.
The Chao Phraya watershed constitutes several sub-basin watersheds, which contain the rivers Ping, Wang, Yom, and Nan to the north of the country. In addition to these, it also contains many smaller riverine watersheds such as the Tha Chin, Pasak and Lopburi Rivers.
Illustration showing the drainage area of the Chao Phraya watershed (Source: http://www.maproom.psu.edu/dcw/)
Over 90 % of the area in the watershed is either used for agriculture or covered with forest. Agricultural land is generally concentrated in the southern sub-basins and the majority of forest cover occurs in the north.
In recent years there has been steady encroachment of people into forest areas which has led to conversion to agricultural land, whereas land close to urbanised areas has been converted for residential or industrial purposes. The need to protect the upper catchment of the Chao Phraya basin from degradation and soil erosion has been identified as a priority by government.
More than 11.5 million people live within the Chao Phraya sub-basin in and around Bangkok. This sub-basin is located in a low-lying plain that has an elevation range from 0 to 20 metres above sea level. Several factors contribute to widespread flooding in this area, typically during the rainy reason.
Topography of the basin – the gradient of the Chao Phraya from its upper reaches to the lower areas of the Chao Phraya sub-basin causes water upstream to flow rapidly and slow down at lower elevations.
Flood protection systems upstream – due to urbanisation and industrialisation, and for protection of agricultural land to minimise crop damage
Land subsidence, caused by pumping groundwater in areas of soft clay, lowers the height of river walls and causes drainage problems
Global warming – as global warming continues, sea levels will rise further and rainfall will tend to fluctuate to a greater extent
Change in land use in the Chao Phraya has led to an increase in flooding. (Source: http://article.wn.com)
Changes in land use, urbanisation and more complex irrigation systems mean that heavy rains during the monsoon season can no longer be controlled naturally. Soil retention or lake and reservoir storage which were once sufficient are now no longer, and water levels frequently rise above embankment protection during the rainy season leading to widespread flooding. Hence a need to manage these conditions is paramount.
The Monkey Cheek, or 'kaem ling', Project is a method of flood prevention supported by His Majesty King Bhumibol Adulyadej in which large areas of low lying land and many inter-connected canals are used as retention ponds storing flood waters. In 1995 he said, 'Is water beneficial or harmful? The truth is that water, or anything in this wide world is, by nature, both good and bad. If we make good use of it, it will be beneficial. If we misuse it, it will be harmful.'
Water gates developed to support flood prevention. (Source: http://www.thailandtatler.com/tag/king/)
Water gates are used to release water into the Gulf of Thailand by gravity at low tide and closed during high tide to prevent water entering the river. Pumping stations simultaneously pump water into the Gulf of Thailand. This monkey cheek scheme ensures that the floodwater does not over flow the river banks.
In September 2011 the monkey cheek project was implemented to prevent flooding in the west side of Bangkok, an area that covers 76 km2. Twenty-five fully operational water stations were used to contain six million cubic metres of water, releasing water slowing so that it did not flood homes or communities.
Along the Chao Phraya River, dykes and artificial embankments have been constructed in both rural and urban areas. Dykes along the Chao Phraya River are mostly concrete retaining walls or raised elevations of road.