Estimation of Areal Precipitation from point measurements

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Estimation of Areal Precipitation from point measurements

  • Most often interested in quantifying rainfall over an entire watershed. Has to be inferred from some sort of weighted average of available point measurements P(xi)

  • Several methods to determine weights. All require

Arithmetic Average

  • Arithmetic average:

  • Note that all weights equivalent

  • Method OK if gages distributed uniformly over watershed and rainfall does not vary much in space.

Theissen Method

Isohyetal Method

  • most accurate method if have a sufficiently dense gage network to construct an accurate isohyetal map. Can account for systematic trends, i.e., orography, distance from coast.

Hydrologic Frequency Analysis

Return Period

  • Return period (T) of an event is the average time (recurrence interval) between events greater than or equal to a particular magnitude.

  • For example, 25 year return period storm occurs on average once every 25 years and has a probability of 1/25 of occurring in any one year.

To estimate return period from flood/drought/rainfall records

  • Select annual maximum/minimum of a particular duration from historical record to form annual maximum/minimum series.

  • Rank annual maximum/minimum from largest to smallest (or smallest to largest if interested in drought)

Calculating Probabilities of Occurence


  • For example, 10 year return period storm has prob. of occurrence 0.1 in any 1 year. How probable once in 10 years?

    • T = average recurrence interval for event =10 years
    • Probability of occurrence in any one year = 1/T=0.1
    • Probability = 1 - (1-1/10)10 = ______ at least once in ten years
  • What is the probability that a 20 year return period storm occurs at least once in 10 years?

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