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The urbanization effects on water quantity in drainage system of Bangkok Metropolitan
Area can be determined by the comparison of annual water account in Hydrological cycle of
selected polder system, covering 700 Km2 on the eastern side of Bangkok. By
collecting the historic records of social data and hydrological data in year 1991 and
1992, the mathematical equations of urban water balance were developed to estimate such
effects.
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The results showed that urbanization caused some changes in water balance system. Water
quantity of the outer system or natural routes, such as precipitation,
evapo-transpilation, infiltration and surface runoff were 1072, 131, 293, 648 mm. In the
year 1991 and 1037, 121, 273, 643 mm. in the year 1992 respectively. While the water
quantity of the Inner System or water utilization routes, such as quantity of water
supply, ground water and sewage in drainage system were 1065, 239, 1953 mm. in the year
1991 and 1099, 238, 1980 mm. in the year 1992 respectively. It is noticed that increasing
of sewage water in drainage system were due to 2 major causes :
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1) Increasing of surface runoff (because of rapidly urbanization) causes impervious area
ratio range 0.80 to 0.82 and runoff ratio range 0.729 to 0.734, while infiltration rate
decreases from 27.33 % to 26.33 %.
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2) The water supply quantity is increasing due to increasing of population, The second
cause play more important role because about 66 % of annual amount of water in drainage
system comes from the water supply utilization.
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