Objective: This study evaluates the hydrological impacts of earth dams on surface runoff in the Sudjan Qaleh Shahrokh sub basin and assesses the HEC HMS model's overall reliability.

Method: Utilizing daily precipitation and discharge data from 2017 to 2025, the HEC HMS model was carefully calibrated and validated. Three earth dams with a combined storage capacity of 509220 cubic meters were simulated for the 2017 to 2018 hydrological year under both with and without dam scenarios, assuming three complete annual reservoir fillings.

Results: The simulations indicated that the dams reduced the annual outflow volume by a negligible 0.19 percent, equivalent to approximately 406000 cubic meters. Crucially, the primary peak discharge at the basin outlet remained completely unchanged at 56.8 cubic meters per second. Conversely, a systematic increase in summer baseflow was observed, rising between 0.1 and 0.6 cubic meters per second.

Conclusions: Due to their low storage to runoff ratio, these earth dams do not significantly attenuate extreme flood peaks. Instead, they function primarily as baseflow regulators. By augmenting summer flows, they provide crucial support for downstream agricultural demands without causing substantial annual water deficits. The HEC HMS model proved highly effective for evaluating such watershed interventions.