Geomorphologic instantaneous unit hydrograph (GIUH) can be used as a transfer function forrnmodeling the transformation of excess rainfall into surface runoff, in which excess rainfall is arnproduction function to the hydrologic system. These models can be used to predict the temporalrnvariation of the surface runoff at the outlet of ungauged catchment, which is useful in thernhydrologic engineering applications. In this study the two types of geomorphologic approachesrnwere studied, GIUH and GIUH based Nash IUH models, for unit hydrograph derivation from thernobserved data.rnThe geomorphological characteristics of a catchment were related with the shape and scalernparameters of the Nash IUH to derive the complete shape of the GIUH based Nash IUH model;rnand GIUH was developed from geomorphological characteristics of the catchments andrnprobability density function of travel time of rainfall excess to the catchment outlet. These twornmodels were developed in seven catchments in Abay basin (Fettam, Hoha, Neshi, Uke, Andassa,rnAzuari and Jedebe) with range of areas 161 to 573km2.rnIn this study, velocity was set as a calibration parameter to calibrate the simulated unitrnhydrograph with the observed unit hydrograph in both models. A regionalized streamflowrnvelocity equation was developed from the simulated data with R2 equal to 0.89 and 0.96 forrnGIUH and GIUH based Nash IUH model respectively. Then, these equations were applied onrnchemoga catchment, which was considered as ungauged catchment, to observe the reliability ofrnthe models.rnThe performances of the calibrated models were evaluated using Nash-Sutcliffe efficiencyrn(NSE), Error (ERR), percentage error in peak discharge (PEP) and percentage error in time tornpeak (PETP). From model evaluation results, GIUH model efficiency (EFF) varies from 81.95 torn97.9 percent while GIUH based Nash IUH model efficiency varies from 75.68 to 97.51 percent.rnGIUH model is efficient in peak discharge estimation (R2=0.988) than GIUH based Nash IUHrnmodel (R2=0.884). In general, the result of both GIUH and GIUH based Nash IUH models arernalmost comparable and their results are acceptable but the GIUH model is superior.rniirnMoreover, a simple GIUH based regional triangular unit hydrograph equations were developedrnand compared with the SCS triangular unit hydrograph. The R2 value in peak dischargernestimation using SCS method is 0.48 while using GIUH method is 0.95. This implies that thernderived GIUH based regional triangular unit hydrograph equation is quite different from SCSrntriangular unit hydrograph and GIUH has better efficiency.rnFinally, this paper concludes that, the regionalized GIUH based equation for peak discharge,rntime to peak and base time of the triangular unit hydrograph can be used, within the errorsrnspecified in this research paper, for any ungauged catchment found in neighbor of the study arearnhaving similar character and varying in areas from 161 to 573km2. Further work may in thernfuture be required to explain any non linear relationship of rainfall runoff and homogeneity ofrnthe catchments.rnKEY WORDS: Geomorphologic Instantaneous Unit Hydrograph (GIUH), Nash IUH Model,rnILWIS (Integrated Land and Water Information System), Rainfall-Runoff modeling, UngaugedrnCatchment.