Talat Farid Ahmed, Ashfaq Ahmed Sheikh, Hashim Nisar Hashmi and Muhammad Azeem Afzal
The hydrological investigations are fundamental to infrastructure development including roads, bridges and other social facilities. The rainfall-runoff analysis is central to all such hydrological analysis. Rainfall excess values are often used as input to various hydrological models for the designing of various hydraulic structures such as bridges, culverts, spillways and flood control works of all sort. This part has more uncertainties due to a complex nature of the watersheds and normally requires calibration of parameters involved in such relationships generally imperial in nature. Under the current study, an attempt has been made to evaluate an appropriate excess rainfall method by carrying out a comparative study on local catchments in Mangla watershed of Pakistan. Four methods namely 1) Initial and uniform loss rate 2) Exponential loss rate 3) SCS curve number, and 4) Holtan loss rate have been evaluated in the study, using HEC-1 computer package. A comparison has been made to see which method is the most effective method, applicable to the catchments for which parameters were optimized and also for other similar catchments. The study has been carried out on four different locations located in the same hydrological region. It has been concluded that Exponential method of rainfall excess is suitable for local conditions for catchments ranging up to 100 square miles. For larger areas, a catchment should be divided into sub-catchments for effective results. For a different catchment whether physically similar and located in a same hydrological region, a different set of optimized parameters is required.
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