From upstream development to downstream water shortage: tracing hydrological changes in the Karkheh River Basin

Abstract The Karkheh River Basin in Iran grapples with water scarcity and ecological degradation due to uncoordinated upstream developments and ineffective water management. Despite the implementation of large-scale water infrastructure projects, including the Karkheh Dam, declining river flow has led to the dam’s operational failure and wasted national investment. This study aims to the underlying causes, and dynamics of these hydrological changes and their environmental consequences. Using the Mann-Kendall (MK) and Pettitt test, we analyzed spatial-temporal trends and detected the year of abrupt change (YAC) in precipitation and river flow across the basin. We further assessed changes in flow regime using the River Regime Impact Index (RI) and estimated the degradation of the downstream Hawizeh-Azim Marsh through a soil moisture estimation approach. The findings reveal no significant trends in precipitation across most stations in the basin, while river flow decreased dramatically after the late 1990s. RI values (0.63–0.11) reveal substantial alteration in flow regimes, primarily due to reduced flow magnitude rather than timing. These changes have significantly contributed to the inflow deficit at the Karkheh Dam and the shrinkage of the Hawizeh-Azim Marsh. Our findings underscore the urgent need for integrated and adaptive water resource management strategies in the Karkheh Basin to mitigate further environmental and socioeconomic harm.