The Modified Rational Method was developed by H R Wallingford, and uses four hydrological constants to determine rainfall intensity. These are:

• SAAR - the Standard Average Annual Rainfall, (in mm/yr);
• M5-60 - The rainfall intensity sixty minutes into a five year storm;
• r - the ratio between the M5-60 and the M5-2day storm intensity;
• Soil - the rainfall acceptance potential of the soil

Each constant has fixed values for a specific location, and these are obtained from hydrological maps. MasterDRAIN includes images of these maps, in National Grid squares, by kind permission of H R Wallingford. The program also contains a database of over 4000 UK locations with the constants already extracted from the maps. This method of calculating rainfall is an essential part of the program. A fuller explanation of this method can be obtained from Vol. 1 of the 'Wallingford Procedure'.

This method limits designs to the UK, but to increase the functionality of the program, pre-calculated rainfall intensities can be loaded in for other locations, for example, in Europe or the USA.