• The amount of rain collect by a rain gauge in 24 hours is know daily rainfall and the amount collect in one year is know annual rainfall.
  • This annual rainfall at a given station should be record over a number of years say 35 to 40 years or so.
  • In India this rainfall cycle period is take about 35 years.
  • When we talk of the rainfall of a given place we generally refer to the average annual rainfall of that place.
  • Thus when we speak of rainfall figures of a particular place, it means that this figure has been averaged over a long period of about 35 years.
  • This is know as normal rainfall.
  • But in any given year the rain may not be equal to this amount.
  • It may be less than this average value or may exceed it.
  • The ratio of the actual rainfall in a given particular year at a given place, to the normal rainfall of that place is know as index of wetness.

\[Index of wetness= \frac{Actual rainfall in a given years at a given place }{Normal rainfall of that place}\]

  • Index of wetness thus gives some idea of the wetness of the year in relation to normal rainfall.
  • 50 per cent index of wetness means a rainfall deficiency of 50 per cent.
  • A deficiency of about 30–45 per cent is know as large deficiency, 45– 60 per cent as serious deficiency and more than 60 per cent as disastrous deficiency.
  • The year in which the rainfall is less than the average annual rainfall, is call a bad or a sub-normal year and if it is more than the average it is known as a good year.
  • If the rainfall in a particular year is approximately equal to the annual average value, then it is know as a normal year or an average year.



Average annual rainfall & index of wetness