The research was aimed to find means how to handle water at Phetchaburi diversion dam for coping with stream
pollution in Phetchaburi River through irrigation watershed management. There eight sampling points for
collecting water samples since the year of 2002 to 2013 for analyzing water quality in relation to release water
flow in consecutive velocity of 22.4, 100, and 377 m3/s in order to obtain the better diluted stream water.
Accordance with the same trends of water quality indicators, this study was taken in BOD and DO as the
representatives for determining the role of flow velocity in dilution capability. The results found that the BOD
were gradually decreased from Phetchaburi diversion dam all the way to agricultural zone and jumping up
during passing the city zone, and still jumping up in estuarine zone. Whenever the BOD decreases, the DO
values were also decreased because of bacterial organic digestion process occurring while it flows except very
high flow velocity. The flow velocity not more than 30 m3/s is recommended to release from Phetchaburi
diversion dam for eliminating stream pollution by dilution process.