Hyetograph-hydrograph transformation model for small ungauged watersheds
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Date
2004-06
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G.B. Pant University of Agriculture and Technology, Pantnagar - 263145 (Uttarakhand)
Abstract
The well established techniques used for determination of runoff hydrograph require historical runoff data and other complex information for evaluating various parameters, which are generally not available in case of ungauged watersheds. Therefore, an attempt has been made to develop a model which is capable of transforming available rainfall hyetograph in to direct runoff hydrograph by using information about land use pattern and topographical features of the area. In the development of models, the storm runoff has been estimated by using SCS curve number method. The model formulation was attempted considering uniform and nonuniform rainfall distribution patterns. In case of uniform rainfall distribution, the model was formulated on the basis of one step rainfall input and accordingly named as One Step Rainfall Input Model (OSRIM). While in case of nonuniform rainfall distribution, the entire storm duration was divided into smaller time increments in a way that the rainfall intensity within an increment is almost constant and the model was formulated considering multiple step rainfall input and called as Multiple Step Rainfall Input Model (MSRIM). The developed models were applied for their verification using the observed data of a small hilly watershed known as “Jandoo-Nala watershed” comprising an area of 17.71 ha in Dehradun district of Uttaranchal State. In case of small ungauged watersheds, the developed methodology will be very useful in designing, planning and operation of various soil and water conservation structures, flood control works, water storage & conveyance structures and also in watershed management & planning. It was found that the value of initial abstraction ratio λ = 0.15 provided a better prediction of direct runoff volume using SCS curve number method for the study area. In case of One Step Rainfall Input Model (OSRIM), the coefficient of correlation between model predicted and observed values of the peak rate of runoff was found to be 73.0%. The values of peak rate of runoff predicted by using Multiple Step Rainfall Input Model (MSRIM) were found to be yielding a good correlation with the corresponding observed values as the coefficient of correlation was found to be 83.0%. A non-linear model of exponential form between peak runoff rate (qp, m3/s), rainfall depth (P, mm), maximum potential retention (S, mm) and curve number (CN) was developed having coefficient of determination (R2) as 96.61%. In case of OSRI model, the time to peak coincided with the time of concentration while in the NLR model because of its inherent weakness did not predict time to peak value. It was observed that there was a very good correlation (97.0%) and coefficient of efficiency (97.19%) between the observed and MSRI model predicted time to peak values. The qualitative analysis revealed that, in general, the predicted ordinates of hydrographs of the selected storm events were in reasonably close agreement with the corresponding ordinates of observed hydrographs.
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