This work aims to investigate the impacts of hydrologic drought on agricultural water resources under climate change scenarios. The study area is Tseng-Wen Reservoir basin in southern Taiwan, which receives temporally uneven precipitation. It is thus a basin prone to suffer from drought during dry season. General circulation models (GCMs) are the main tool to tackle climate change issues through the help of prescribed scenarios. This work used several approaches, including spatial downscaling, temporal downscaling and hydrologic model, to solve the coarse-resolution problem of GCMs and then to analyze the effect of climate change in the study area. The following are important findings: (1) According to future climate projections, droughts may become more frequent (hereafter referred to as scenario droughts), but their duration and magnitude may become more diverse than those of the baseline droughts. (2) The times of start and end of scenario droughts may occur earlier than those of baseline droughts. (3) Scenario low flow during the dry period tends to decrease in January and February, but to increase in March and April. (4) Moderate adjustment of irrigation period to adapt to climate change is suggested.