Objective: In response to severe droughts and climate change impacts in the Chaharmahal and Bakhtiari province, numerous watershed management structures, particularly earth dams, have been constructed to revive groundwater and surface water resources. However, the cumulative hydrological impacts of these structures on downstream flow regimes, especially in snow-dominated mountainous basins, remain a subject of debate. This study aims to evaluate the hydrological effects of existing earth dams on surface runoff and downstream flow in the Sudjan-Qaleh Shahrokh sub-basin (located upstream of the Zayandeh Rud Dam). Furthermore, it seeks to assess the capability and reliability of the HEC-HMS model in simulating these structural interventions and snowmelt-driven hydrological processes in data-scarce mountainous environments.
Method: The study was conducted in the Sudjan watershed (975 km²), characterized by a cold, semi-arid climate and a snow-dominated hydrological regime. Daily precipitation and discharge data from 10 hydrometric and synoptic stations over an 8-year period (2017–2025) were utilized. Spatial data, including DEM, land use, and geology, were processed in a GIS environment to extract the Curve Number (CN) map, with CN values ranging from 45.5 to 66 across nine sub-basins. The HEC-HMS model (v4.11) was employed using the SCS-CN loss method, SCS unit hydrograph, Muskingum routing, and linear baseflow modules. The model was calibrated (2017–2021) and validated (2022–2025) using the Nelder-Mead optimization algorithm. To evaluate the dams' impacts, three major earth dams with a total storage capacity of 509,220 m³ were simulated for the 2017–2018 hydrological year. Two scenarios were compared: a "with dams" scenario (incorporating stage-area-volume curves and assuming three full fillings per year) and a "without dams" baseline scenario.
Results: The HEC-HMS model demonstrated satisfactory performance, particularly at the monthly scale. During calibration, the daily Kling-Gupta Efficiency (KGE) was 0.64 and the monthly KGE was 0.84, with a Percent Bias (PBIAS) of approximately -22%. The negative PBIAS indicates a slight systematic overestimation, primarily attributed to the lack of direct Snow Water Equivalent (SWE) data in the model. Hydrological simulations revealed that the presence of the earth dams resulted in a negligible reduction in the annual outflow volume—only 0.19% (approximately 406 × 10³ m³). Notably, the main annual peak discharge remained entirely unchanged at 56.8 m³/s (recorded on May 10, 2018). However, a systematic increase in summer baseflow was observed during the recession limb (June to August), ranging from 0.1 to 0.6 m³/s. Reservoir analysis showed that Reservoir 1 dominated the retention process, accounting for 79.7% of the total volume reduction. The minimal impact on flood peaks is fundamentally due to the extremely low ratio of the dams' total storage capacity (509,220 m³) to the basin's annual runoff volume (~210 million m³), which constitutes merely 0.24%. Consequently, the structures act primarily as baseflow regulators rather than flood peak attenuators.
Conclusions: The existing earth dams in the Sudjan sub-basin do not exert significant control over extreme flood peaks generated by spring snowmelt. However, they play a crucial, albeit localized, role in enhancing summer baseflow, thereby supporting downstream agricultural demands without causing a substantial deficit in the total annual water transfer to downstream basins. The HEC-HMS model proved to be an efficient tool for evaluating cumulative watershed interventions, despite inherent limitations in snowmelt modeling. From a water resources management perspective, these findings highlight that small-scale earth dams in snow-dominated regions cannot replace comprehensive flood risk management strategies. Future watershed management plans must consider increasing storage capacities or integrating real-time forecasting systems. Furthermore, rigorous pre-construction hydrological modeling is essential to balance dry-season water supply with flood risk mitigation in vulnerable mountainous catchments.