This study highlights the spatial and temporal impacts of climate change on rice–wheat cropping systems, focusing on irrigation water requirement (IWR) in the Bagmati River Basin of Nepal. The outputs from a general circulation model (HadCM3) for two selected scenarios (A2 and B2) of IPCC and for three time periods (2020s, 2050s, and 2080s) have been downscaled and compared to a baseline climatology. CROPWAT 8.0 model is used to estimate the water requirements. IWRs show different trends in different physiographic regions and different growth stages of rice and wheat. A decreasing trend of IWRs in the Mid Hills and the High Hills indicates that farmer-based small irrigation schemes are sufficient to meet the requirements. However, in the Terai region, where there is an increasing trend in IWRs, the deficit volume of water needs to be supplied from potential large-scale irrigation schemes.