1.   Purpose and objectives

     The techniques are those used to approximate "the greatest depth of precipitation for a given duration meteorologically possible over a given basin at a particular time of year, with no allowance made for long-term climatic trends."

2.   Description

     The procedures are based on a meteorological approach, consisting of moisture maximization and transposition of observed storms. Storm  transposition involves adjustments for elevation, moisture-inflow barriers,  and distance from the moisture source. A 2-dimensional orographic air flow  model with limited application to the computation of orographic precipitation is described. Computations are based on a storage equation of water vapour and the continuity equation for air mass. Air flow is assumed to be laminar. Computations are made for single or multiple layers of air passing over the barrier.

     Alternative computations of orographic precipitation using subjective valuation of certain meteorological data and analysis are explained. A statistical PMP technique is included, based on general frequency equation in which the maximum rainfall is given as the mean rainfall value plus the standard deviation times a statistical factor.  

     Methods are also given for determining seasonal variation, and chronological and areal distribution of PMP. Techniques are suggested for computing PMP in regions where orographic influences are important. Tables of precipitable water in a pseudo-adiabatic atmospheric are included, as are world record and near record rainfalls. Examples from actual studies are given in the manual; numerous others are available in the references cited.

3.   Input

     This is a manual procedure. The quantity and types of data needed are  highly variable, depending on the area to be studied. Most commonly needed are historical records of precipitation and temperature, as well as meteorological parameters which serve as indicators of, or which affect 
moisture amounts in the air (dew point, relative humidity, pressure, wind speed and direction). In addition, observed storm data such as isohyetal analysis, mass curves, and storm location data are needed as well as topographic, and geographic information.

4.   Output

     Estimates of PMP for points or areas for a given duration.

5.   Operational requirements and restrictions

     a) A person with knowledge and skills in meteorology and statistics
        and some understanding of hydrology can learn the procedures.

     b) With the above skills, at least 3 months of training is necessary.

     c) Limitations: Procedures are applicable in the middle latitudes for
        basin sizes up to about 50,000 km2 and usually for durations from
        1 hour to three days.

6.   Form of presentation

     Report in English: Operational Hydrology Report No. 1, Manual for Estimation of Probable Maximum Precipitation, 269 pp.  WMO No. 332 (1986)

7.   Operational experience

     The technique has been used worldwide in temperate climates, and in a limited number of cases in tropical regions.

8.   Originator and technical support

     Hydrometeorological Design Studies Center.

9.   Availability

     From the HOMS National Reference Centre for USA, or HOMS Office, WMO Secretariat.

10.  Conditions on use

     None.