Design flood projections for ungauged catchments using updated unit hydrographs under climate change scenarios

A new variant of GcIUH (Geomorphoclimatic Instantaneous Unit Hydrograph) called equivalent GcIUH (EGcIUH) is proposed for deriving flood hydrographs from ungauged catchments (lacking streamflow data) under climate change scenarios. The EGcIUH overcomes shortcomings of GcIUH arising from uncertainty in (i) the choice of threshold area for stream network demarcation from a digital elevation model, and (ii) the estimation of the characteristic flow velocity. The first uncertainty is alleviated by considering the self-similarity hypothesis of stream networks, whereas the second uncertainty is addressed by specifying equations for estimating the flow velocity corresponding to different precipitation intensity ranges. The effectiveness of EGcIUH in simulating flood hydrographs is demonstrated by considering both finer (sub-daily) and coarser (daily) resolution rainfall data through case studies considering isolated precipitation events from catchments in the United States and India. Furthermore, projected changes in design flood hydrographs are determined for the Indian catchments through analysis considering their EGcIUHs and design storm hyetographs corresponding to the prevailing conditions and four future CMIP6 climate change scenarios. EGcIUH appears promising, and probability density functions of two-parameter Gamma, Nakagami-m, and Weibull distributions (in that order) are the most suitable for representing its shape.