Development of geomorphological instantaneous unit hydrograph for Myntdu Leska basin - A research report by the National Institute of Hydrology

The study attempts to develop a computerized model for Geomorphological Instantaneous Unit Hydrograph (GIUH) for estimation of flood hydrographs resulting from intermittent storms of varying intensity. The model applied to the rainfall-runoff data of the Myntdu-Leska basin of Meghalaya infers that the channel network and geomorphological features are closely related to the retention and discharge characteristics of the basin.

The theory of GIUH assumes that rainfall that occurs over a basin is assumed to be composed of infinite number of non-interacting drops of uniform size. After spending some time in one state in the channel or overland region, the drop makes transitions to another state to reach the basin outlet. Assuming one parameter, exponential time distribution of one drop chosen at random from the basin defines the IUH of the basin.

Unit hydrograph approach is one of the hydrologic methods developed for the purpose of estimating and forecasting floods. The method is simple and reasonably accurate. An instantaneous unit hydrograph for a basin is derived using rainfall and corresponding runoff data and avoids the development of a large number of unit hydrographs of different durations. 

For ungauged watersheds a methodology, which does not require large amount of observed rainfall-runoff data, needs to be adopted. Since intermittent storms are very common in nature, a large number of different duration unit hydrographs are required to estimate the discharge from the basin for a varying storm pattern.

A general purpose software is developed for the model and is capable of directly estimating the DRH from the net effective hydrograph using the geomorphologic parameters of a basin of order less than or equal to 17. The rainfall-runoff data of Myntdu-Leska basin of Meghalaya were used in the study for calibration and validation of the model. The average hydrographs of two storm events of this basin were used to estimate the DRH. The comparison of estimated to actual hydrograph indicates that the response of the GIUH model gives a higher and delayed peak discharge, while the total runoff from the basin matches.

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