2-Estimation of Areal Probable Maximum Precipitation (PMP) over Saurashtra and Kutch by Grid Point Transposition

Abstract

A generalized physical approach of estimating areal probable maximum precipitation (PMP) by physical method for the Saurashtra and Kutch region has been developed. The highest average areal raindepths of different size areas and durations from the major rainstorms were considered which occurred over the region during 1891-2002. The analysis is carried out to determine the greatest rainfall amounts experienced by various size areas and durations as per hydrometeorological practices. For each rainstorm, the area considered for the study is the area enclosed by peripheral isohyet of the rainstorm. It was found that the region experienced about 9 severe rainstorms during the 112-year period 1891-2002. These rainstorms were analyzed by Depth-Area-Duration (DAD) method. The transposition limits of major rainstorms have been identified. The DAD raindepths were then moisture maximized at its original location of occurrence and then transposed at different grid points. After applying various corrections, PMP values at different grid points were estimated for an area of 500, 1000, 5000 and 10,000 km2. With the help of these PMP estimates at different grid points, generalized PMP maps for areas 500-10,000 km2 have been prepared. These values of PMP for different size areas and durations will be useful for estimating design storm raindepths of PMP magnitudes for any sub-catchments in the Saurashtra and Kutch region falling in range of 500 to 10,000 km2 areas. The PMP estimates obtained by the generalised method are well comparable with values obtained by the physical method with limited transposition for 1-day and 2-day durations. However, for 3-day duration, limited transposition method was found to give higher estimates. The main advantage of this method is that it gives more satisfactory and consistent results than estimates made on project wise basis.