Geomorphologic Instantaneous Unit Hydrograph Based Hydrologic Response Models for Ungauged Hilly Watersheds in India

Hydrologic response of two fourth-order hilly watersheds of the Ramganga river basin in the central Himalayan region of India has been predicted in this study. Geomorphologic Instantaneous Unit Hydrograph (GIUH) was derived using two models: (i) Horton’s stream-order ratios based model (GIUH-I); and (ii) Nash’s two-parameter gamma distribution based conceptual model (GIUH-II). The travel times for the overland-flow and the stream-flow in Horton-Strahler stream ordering system of the watersheds were determined analytically and probabilistically for GIUH-I model; while a dynamic component (mean velocity of flow) was estimated for the GIUH-II model using two approaches: (a) as a function of effective rainfall intensity (GIUH-IIa); and (b) on the basis of time of concentration concept (GIUH-IIb). Based on eight single-peaked isolated storm events for each watershed, the statistical analysis and coefficient of efficiency showed better overall correlation between predicted and observed direct runoff hydrographs, particularly in terms of the magnitude and time of occurrence of peak runoff rate, by GIUH-IIb as compared to GIUH-I and -IIa models. Moreover, the GIUH-IIb has additional advantage (compared to GIUH-IIa) for being independent of the measured velocity of flow corresponding to the peak flow rate at the watershed outlet. Since, these models require no historical data of rainfall and runoff, they can be effectively used to predict direct runoff from ungauged hilly watersheds for water resources planning and management in the region.