Probable Maximum Precipitation Estimation Using Two Different Approaches of Hershfield's Method over Qareh-Su Basin, Golestan province, Iran

The probable maximum precipitation (PMP) and probable maximum flood (PMF) are among the important measures in designing hydraulic structures. One of the most suitable statistical techniques for estimating PMP is Hershfield method. In this study the maximum 24 hours precipitation was evaluated using 12-53 years of data from 6 rain gauge stations and 1 synoptic station located in Qareh-su basin. For this goal two statistical methods of Hershfield (in two different approaches of standard and revised) were used. For the first approach, frequency factor and the PMP24 were found to vary in the range of 16.98 to 18 and 335.08 to 524.93 mm, respectively. The corresponding values for the second approach was 2.06 to 4.01 and 88.35 to 153.41 mm, respectively. Also the amount of PMP24 with 50 and 100 year return periods using Gumbel distribution were respectively calculated as 579.2 and 613.59 mm in the first approach and 263.69 and 281.7 mm in the second approach. As a criterion independent to the climate conditions, the ratio of PMP24 to the maximum of 24 hours precipitation was used for compare PMP in stations. The results indicated that the second approach was more stable for the Qareh-su basin. The Precipitation Gradientwas then investigated for drawing the isohyets maps which presented an insignificant. Therefore the Inverse Distance Weighted method was used to draw the iso-PMP maps and to determine the Depth-Area-Duration curves. Using these curves, conversion of point PMP24 to areal PMP24 over Qareh-su basin is Possible.