The estimation of flood hydrographs has a high level of importance in ungauged watersheds.
Methods currently being widely used for the estimation of flood hydrographs utilize historical rainfallrunoff
data. For ungauged watersheds, the unit hydrograph may be derived using the geomorphologic
instantaneous unit hydrograph (GIUH) and geomorphoclimatic instantaneous unit hydrograph (GcIUH)
approaches. This research is aimed at comparing the accuracy of GcIUH-based Clark (GcIUH-Clark) and
GIUH-based Nash (GIUH-Nash) models. For this purpose, the Clark-IUH model option of the HEC-HMS
package and the Nash-IUH model were employed with and without the use of historical rainfall-runoff
data, respectively, to determine shape, peak discharge, and time to peak of direct surface run-off (DSRO)
hydrographs. The case study in this study was the Kasilian watershed, located in Mazandaran Province
of Iran, with an area of 67.5 km2. The results obtained from these models were compared with observed
DSRO hydrographs based on 3 performance criteria, namely EFF, PEP, and PETP.
The results clearly revealed the accuracy and applicability of these 2 models (the GcIUH-Clark model
and the GIUH-Nash model) for derivation of DSRO hydrographs