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In recent years, geomorphological instantaneous unit hydrograph (GIUH) approach to estimate the runoff from a watershed due to different storm patterns has gained wide acceptability for watersheds with scanty data. In this study, a user-friendly event based computerized GIUH model, GIUH_CAL, was developed. GIUH_CAL model incorporates an infiltration component. Depending on the availability of soil and infiltration data, the model offers a choice among three methods of infiltration calculation, viz., Richards’ equation, phi-index method and Philip two term model. The graphical user interface (GUI) of the model is based on mouse-driven approach with pop-up windows, pull-down menus, sub-menus, toolbar, statusbar and button controls including comprehensive context-sensitive help file. GIUH_CAL model displays generated instantaneous unit hydrograph (IUH), unit hydrograph (UH) and direct runoff hydrograph (DRH) of the watershed graphically and processes statistically. Modelling efficiency (ME), coefficient of residual mass (CRM), root mean square error (RMSE) and coefficient of determination (r(2)) are used as the performance criteria. The model was tested for two distinctly different watersheds, i.e., a 4th order Guptamani watershed in tropical sub-humid region and a 3rd order Upper Chhokeranala watershed in semi-arid region of India. The results show that model performs satisfactorily for both test watersheds and performance is much better with Richards’ equation option of infiltration calculation as compared to phi-index method and Philip two term model. Thus, GIUH_CAL model can be adopted as a standard tool for modelling rainfall-runoff transformation process in scantily gauged watersheds. (c) 2007 Elsevier Ltd. All rights reserved.