Background and Objectives: The escalating global population and the shift toward consumerist lifestyles have intensified the demand for water and food resources, leading to overexploitation of natural landscapes. This has resulted in reduced soil permeability, degradation of vegetation cover, increased flooding, intensified erosion, downstream sedimentation, recurring droughts, and associated socioeconomic challenges. To address these issues, watershed management operations have been implemented across various regions of Iran to enhance runoff infiltration, reduce peak flood discharge, and mitigate sedimentation behind dams. However, due to the relatively recent implementation of such measures, comprehensive quantitative and qualitative evaluations remain limited. This study aims to assess the impact of mechanical watershed management operations on runoff storage and flood reduction in the Karizak watershed, located in Khalilabad County, Razavi Khorasan Province, Iran. The primary objective is to analyze the effects of constructed earthen and gabion structures on water infiltration and peak flood attenuation, while providing insights for optimizing future watershed management strategies.
Materials and Methods: The study area, the Karizak watershed, spans 5,695.1 hectares in the central district of Khalilabad County, characterized by a cold arid climate and an aridic-thermic moisture and temperature regime. Topographic data, slope, and elevation classes were derived using a digital elevation model (DEM). Field surveys documented the characteristics of nine earthen check dams and four gabion structures. Infiltration rates were measured using double-ring infiltrometers in sediment-laden and control (sediment-free) areas within the reservoirs of selected earthen check dams (numbers 4, 5, and an older dam from the 1970s) across three zones (southeast, south, and southwest). Evaporation and infiltration volumes were estimated for 10-day intervals from the time of dam construction until the end of the 2018-2019 water year, based on reservoir water surface mapping. Rainfall and land-use data were analyzed to evaluate the watershed’s response to precipitation before and after the implementation of watershed management measures.
Results: The results revealed that infiltration rates in sediment-laden areas were significantly lower than in control areas due to the filling of soil pores with fine sediments. In the earthen check dams, approximately 10 million cubic meters of water (92% of the flood volume) infiltrated, while 0.9 million cubic meters (8% of the flood volume) were lost to evaporation. This high infiltration rate, relative to evaporation, is likely attributed to the timing of rainfall (April to mid-June) when temperatures are lower. The mechanical structures effectively reduced peak flood discharge, mitigating flood-related damage and enhancing groundwater recharge through qanats. However, issues with the siting and height of the structures relative to upstream runoff volumes were identified. Economically, the cost-to-benefit ratio was 10.6 for mechanical measures, 4.4 for biological measures, and 4.9 for the entire watershed, indicating the economic viability of the implemented watershed management operations.

Conclusion: Watershed management operations in the Karizak watershed have significantly enhanced water infiltration and reduced peak flood discharge, thereby improving water and soil resource management. The structures facilitated the storage of approximately 11 million cubic meters of water, primarily contributing to qanat recharge and mitigating drought impacts. Nonetheless, optimizing the design and placement of structures is critical to further improving efficiency. This study underscores the importance of continuous evaluation of watershed management operations and the development of robust methodologies for performance analysis. Future studies should employ advanced simulation models to determine the optimal number and placement of structures to maximize the effectiveness of watershed management initiatives.