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Tuesday, February 1, 2011

Rainfall Measurement

The instrument for measuring rainfall is called a rain gauge. A rain gauge measures the volume of water that falls onto a horizontal surface delineated by the rain gauge rim. The volume is converted into a rainfall depth through division by the rain gauge surface area. The design of a rain gauge is
not as simple as it may seem at first glance as there are many errors and inaccuracies that need to be minimised or eliminated.
rain gauge
There is a considerable scientific literature studying the accuracy and errors involved in measuring rainfall. It needs to be borne in mind that a rain gauge represents a very small point measurement (or sample) from a much larger area that is covered by the rainfall. Any errors in measurement will be amplified hugely because the rain gauge collection area represents such a small sample size. Because of this amplification it is extremely important that the design of a rain gauge negates any errors and inaccuracies.

The four main sources of error in measuring rainfall that need consideration in designing a method for the accurate measurement of rainfall are:
1 Losses due to evaporation
2 Losses due to wetting of the gauge
3 Over-measurement due to splash from the surrounding area
4 Under-measurement due to turbulence around the gauge.

Source : Tim Davie. 2002. Fundamental of Hydrology.New York : Routledge 270 Madison Avenue.
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