Derivation of Intensity -Duration-Frequency -Area(IDFA) Curves for Mashhad City

A knowledge of rainfall distribution over a finite area is needed for small bydraulic structures design. Hydrologists have always been concerned about Depth- Area- Duration (DAD) for a specific storm in a watershed. An intense storm occurred on June 6, 1992 which claimed some lives in parts of Mashhad, Khorasan Province, Iran. Data of rainfall depth at 29 raingauge stations around the city and its suburbs were gathered. There also exist three automatic raingauges at Mashhad airport, the College of Agriculture, and Khorasan Water Authority and one at Toroq Dam in Mashhad suburb. A survey of hyetographs for June 6, 1992 of these stations revealed that the airport raingauge did not work well while dimensionless mass curves for other raingauges were quite similar. Two rain centers at the College of Agriculture (with 50 mm rainfall) and Wheat Silage (with 53 mm rainfall) were detected. The DAD relationships for this storm were derived. Intensity- Duration- Frequency (IDF) curves for the airport station were derived and were compared with those of the College of Agriculture and Khorasan Water Authority. In the absence of an abnormal topography, the Theissen method was utilized for deriving a regional IDF for Mashhad City. Intensity- Duration- Frequency- Area (IDFA) curves, a new concept in hydrometeorology, has been initiated for the first time for Mashhad. Combination of regional IDF with DAD of the most intense storm in Mashhad was a key to represent such a new concept.