The development of the Tehran metropolis necessitates alignment with spatial planning policies and consideration of resource carrying capacity, particularly hydrological constraints. This study investigates the impacts of climate change on the spatial distribution of precipitation in Sub-basin 4133 (Tehran and its surroundings) and delineates its implications for Tehran's urban policy-making. Due to the lack of observational data in high-altitude areas, TerraClimate reanalysis data were first validated ($R^2 > 0.85$). Subsequently, precipitation patterns over the past 70 and 20 years were analyzed, and future changes under four Shared Socio-economic Pathways (SSPs) were projected using the median of five CMIP6 models via the Change Factor downscaling method. Findings indicate a "spatial contraction of high-precipitation zones" over the last 20 years, leading to a reduction in the basin's hydrological inflow. Foresight results suggest that even under the sustainable scenario (SSP1-2.6), a decline in precipitation is inevitable, while the pessimistic scenario (SSP5-8.5) predicts a structural collapse of water resources in the northern highlands (dropping below 190 mm). This trend, potentially exacerbated by Tehran’s Urban Heat Island (UHI), shifts the precipitation regime toward destructive flash floods, reducing soil permeability, severing groundwater recharge, and intensifying land subsidence in Tehran’s southern districts. The results emphasize the urgent need for urban regeneration programs to adopt the "Sponge City" model and revise spatial planning documents based on the basin's new ecological potency.