This study was conducted to evaluate the impact of future climate change and irrigation expansion on the available surface water resources of the Gilgel Abay catchment, which is located within the Abay River Basin in northwestern Ethiopia. Monthly time step climate data after downscaled and bias corrected under two SSP4.5 and SSP8.5 scenarios was used to predicted future streamflow in addition to irrigation expansion for the baseline period of (1988–2018) and for the future with two-time windows, the medium-term (2023–2060) and long-term (2061-2100) periods using WEAP model. The WEAP model was used to simulate water balance and streamflow evaluation after calibration (1982–2002), validation (2003–2012) and sensitivity analysis of the model with soil moisture methods. The model has been very good performance, with r, R 2 , NSE, PBIAS, and RMSE values of 0.9152, 0.871, 0.9997, 0.0113 and 0.0139, respectively, to predict the streamflow in the catchment. The average annual streamflow availability for the baseline, medium-term, and long-term periods was 4.365 BCM, 4.372 BCM, and 4.383 BCM. This surface water resource of the catchment reduced to 3.508 BCM and 3.368 BCM for the medium term and to 3.475BCM and 3.144 BCM for the long term under SSP4.5 and SSP8.5 emission scenarios respectively. Similarly, the water resource availability reduced to 2.83 BCM to 2.80 BCM under irrigation expansion scenarios for the medium- and long-term considerations. Under the combined effects, surface water availability in the catchment will reduce to 0.3556BCM and 0.3435BCM for medium-term and to 0.3481BCM and 0.3364BCM for the long-term periods under SSP4.5 and SSP8.5 emission scenarios respectively. The outcomes of this scenario based study revealed decreased streamflow under climate change and irrigation expansion will have an adverse impact on the water resource management in the catchment. Therefore, the upcoming water resources development and management have proper monitoring of the resources and implement relevant mitigation measures for an efficient and sustainable use of the resources. Keywords: Gilgel Abay catchment, climate change scenarios, irrigation expansion and surface water resources.