Digital elevation models (DEMs) are extensively used in hydrological modelling and deriving the geomorphological properties of catchments. Recently, hydrologists have shown interest in researching the effects of DEMs from different sources on simulated outputs. As part of these efforts, this study aimed at evaluating the effects of DEM and algorithm selection on geomorphologic instantaneous unit hydrographs (GIUH)-Nash model based direct surface runoff predictions from ungauged Debarwa river catchment in Eritrea. Four open-source DEMs and two quantum geographic information system (QGIS) algorithms (GRASS and SAGA) were applied and corresponding outputs were evaluated using five observed events. The two algorithms resulted in drainage networks of similar stream orders but different geomorphologic characteristics such as stream ratios. The subjective and objective goodness of fit results indicated that the performance of the model based on SAGA was unsatisfactory whereas that of GRASS algorithm-based GIUH-Nash model was acceptable for all the DEM-scenarios irrespective of their sources and resolutions. The study concluded that DEM in the calculation of flow hydrographs for the conditions of the Debarwa catchment selection has little impact on the GIUH-Nash model based direct surface runoff predictions and can be used indiscriminately. But, great care should be taken while selecting stream network generating algorithms, especially for catchments whose outlets are located near the confluence of two major rivers.