There is a need to assess the areal depth of the probable maximum precipitation (PMP) over specified catchment areas for the safe construction of dam spillways. The large number of dams in India, coupled with their risk of failure, makes this need imperative both for the maximum use of water resources and for public safety. On the basis of rainfall data for the heaviest storms that occurred in different parts of India during the period 1880–1983, improved estimates of one-, two-, and three-day point PMP for India have been made. In this paper the distribution of areal PMP over specified catchment sizes is provided for the first time. The areal reduction factors (ARF) were based on envelope curves of major storms to give the ARF for areas of 10–20,000 km2. These factors were found to vary from 1.0 to 0.41, though there was no real difference between durations of rainfall. These values of ARF were then multiplied by values of one- to three-day PMP. The resulting maps allow a broad description of the spatial distribution of areal PMP and also provide a rapid and consistent estimate of the probable maximum flood (PMF) from the PMP.

For 500 km2 the areal PMP varies from 40 to 120 cm for one-day duration; from 70 to 200 cm for two-day duration; and from 75 to 270 cm for three-day duration. The pattern of PMP is consistent with the geography and available moisture.