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XXI I
Gage height (G.H.) is the water surface elevation above some arbitrary gage datum to which, in
most cases, referred the Mean Sea Level (M.S.L.) Ko Lak datum, otherwise the arbitrary assumed datum
(A.D.) being used.
Gauging station is a particular site on a stream, canal, lake, or reservoir where systematic
observations of gage height or discharge are obtained. The term "Staff gauging station" is applied only those
gauging stations where only a gage height record is observed at an installed staff gage. If a water stage
recorder is also installed, such a gauging station is referred as recording gauging station. In case of
discharge measurements being practiced, it is referred as stream flow calibration or rating station.
Million cubic meter (mcm.) is the volume of water expressed in millions of cubic meter.
One million cubic meter is the quantity of water required to cover 1 square kilometer to a depth of 1
meter and is equivalent to 811 acre-feet, or 408.6 cfs.-day, or 264.2 million gallons.
Runoff in mcm. shows the volume of approximately effective runoff from rainfall, passing a given point
for a given period of time.
Annual runoff is the volume of approximately effective runoff from rainfall accumulated during the
water year.
Stage-discharge relation is the relation between gage height and the volume of water per unit of time,
flowing in a channel.
Rating tables giving the discharge for any gage height are prepared from stage-discharge relation
curves or rating curves.
Water year is the 12-month period, April 1 through March 31, related to annual cycle of river flow
during which the river begins to rise, reaching its flood stage and subsiding toward its cyclic minimum
The water year is designated by the calendar year in which it begins and which includes 3 of 12
months. Thus, the year began April 1, 1969 is called the “1969 water year”.
Order of listing of gauging data
In this format, gauging data are listed according to their code running number order. It has been
found more convenient in locating the required data when the list is arranged in such manner. The station
code name contains the letters standing for river name and number in running order according to the age of
the station installed in that river. The station code, for example, for the Chao Phraya River at Wat Tha Hat
installed in 1905 is C. 1, which is the first stream flow rating station ever built in this river.