Runoff simulation in the Rana ungauged watershed of Mahanadi river basin using regionalization and transposition techniques in HEC-HMS model

Accurate estimation of runoff in ungauged watersheds remains a major challenge, particularly in data-scarce regions such as India. HEC-HMS model used for simulating rainfall-runoff processes in dendritic watersheds across diverse geographic regions, from large river basins to smaller urban and natural watersheds. In this study, HEC-HMS was calibrated and validated for the gauged Brutang watershed using daily observed runoff data across four time periods (1997–2001, 2002–2006, 2007–2011, and 2012–2016) and subsequently applied regionalization and transposition techniques to simulate runoff in the ungauged Rana watershed of the Mahanadi river basin. Model performance varied across periods, with the best simulation during 2007–2011 (R 2 = 0.78, NSE = 0.73) and the weakest during 1997–2001 (R 2 = 0.73, NSE = 0.69). Calibration showed highest efficiency in 2007–2009 (R 2 = 0.86, NSE = 0.83, PBIAS = 6.1%). Validation results indicated strong agreement during 2010–2011 (R 2 = 0.86, NSE = 0.82), but lower reliability in 2000–2001. Improved validation performance may be attributed to relatively stable hydro-climatic conditions and smoother runoff response. Comparison of regionalized and transposed simulations in the Rana watershed yielded high reliability (R 2 = 0.91–0.93, NSE = 0.90–0.91, RMSE = 5.25–6.68 m 3 /s), demonstrating the suitability of both methods for ungauged watershed modelling. The comparison between regionalized and transposed runoff simulations showed strong methodological consistency between two transfer methods; however, due to the absence of observed runoff, these statistics should not be interpreted as evidence of direct predictive validation.