Little is done to connect hydrological modelling with stakeholder participation. This study incorporates agricultural development and climatic changes within the Water Evaluation and Planning hydrological model. This is done with a participatory approach involving four scenario workshops, 400 household surveys and two focus group discussions in the period of 2010-2012 for the ungauged Huai Sai Bat sub-basin as a case study in the Mekong region. The modelling results indicate future increased streamflow during the wet (monsoon) season in response to shifts in the regional climate. Modelled land-use and management changes brought about large unmet water demands, primarily in the dry season.