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37 Rainfall Trend Analysis: Return Period

Part 4. Hydrometeorology

  1. Robin T Clarke

Published Online: 15 APR 2006

DOI: 10.1002/0470848944.hsa040

Encyclopedia of Hydrological Sciences

Encyclopedia of Hydrological Sciences

How to Cite

Clarke, R. T. 2006. Rainfall Trend Analysis: Return Period. Encyclopedia of Hydrological Sciences. 4:37.

Author Information

  1. Instituto de Pesquisas Hidráulicas, Porto Alegre RS, Brazil

Publication History

  1. Published Online: 15 APR 2006


This article describes the concept of return period, with particular reference to the analysis of rainfall records. Methods are described for calculating return levels, corresponding to different return probabilities, when rainfall is accumulated over different time intervals. In the case of intense, short-duration rainfalls, threshold models are illustrated. The article points out that the concept of return period is most easily interpreted where stationarity in the rainfall regime can be assumed, so that it is important to establish whether rainfall records show evidence of trends over time. Procedures are discussed for testing whether time trends exist in annual and monthly rainfall, in the occurrence and magnitude of daily rainfalls, and in annual maximum rainfall intensities over different durations. Where a trend in rainfall regime is apparent, an alternative to the concept of return period is suggested, on the basis of the probability of occurrence of an extreme event of specified magnitude, during a period extending into the future during which the observed trend can be assumed to continue.


  • return period;
  • return levels;
  • rainfall;
  • trend;
  • EV distributions