Assessment of the Impact of Watershed Management Measures on Flood Characteristics in the Ab-Mahi and Chikan-Morzian Watersheds, Fars Province

Introduction and Goal
Today, in countries, evaluating watershed management project is one of the most fundamental aspects of future planning for implementation projects and natural resource management. Therefore, given the long-term history of implementing watershed management projects in the country, it is necessary to evaluate and examine the effects of these projects. However, the lack of necessary equipment to measure and recording changes in watersheds has made the use of hydrological distributed models to simulate watershed behavior before and after watershed management activities an effective tool in achieving these goals. Today, use the capabilities of hydrological models in simulating the effectiveness of management activities plays a decisive role in the decision-making process. Therefore, this study was conducted using the HEC-HMS model to evaluate the effects of implemented watershed management activities on peak flow, time to peak, and flood volume in the Ab-Mahi and Chikan-Morzian watersheds. To determine the impact of structural and biological measures, concentration time, channel slope, and curve number were calculated before and after activities were implementation.
Materials and Methods
In this study, the effects of structural measures (gabion check dams, stone and cement structures, dry-stone terraces) and biological (almond cultivation, fencing, and control sample) watershed management measures on hydrological indicators (time to peak, peak discharge, and flood volume) in two watersheds: Ab-Mahi and Chikan-Morzyan, within the Doroodzan Dam watershed. Then, rainfall and runoff simulations were performed using the HEC HMS model in two scenarios with and without watershed management measures. In the scenario without watershed management measures, the necessary data were obtained from detailed operational surveys collected by the General Department of Natural Resources and Watershed Management of Fars Province. In the scenario with watershed management measures, the hydrological effects of structural measures were evaluated by their impact on watershed slope and concentration time, while the effects of biological measures were assessed through changes in the weighted mean Curve Number (CN) of sub-watersheds before and after implementation. Then, the physical indicators of the watershed (watershed area, weighted mean curve number (CN), initial losses, and delay time) were calculated. In order to convert rainfall into runoff, the SCS method was applied for both scenarios before and after the implementation of watershed management measures. The CN map was prepared based on the integration of soil hydrological and land use group data in the ARC GIS 10.8 software. Also, the concentration time was calculated using the Kirpich method.
Results and Discussion
The results of evaluating the impact of watershed management measures in the studied watershed reveal various aspects of their effects on the hydrological behavior of the watershed. The results of this study showed that the implementation of structural watershed management measures caused a significant change in the amount of delay time in conditions before and after the implementation of the operation. So that the largest change in delay time was related to sub-watershed F (28.32 min) and the smallest change was related to sub-watershed 9 (4 min). This amount of change was due to slope, main channel length, soil permeability, and watershed area, etc. The amount of the change in peak time before and after the implementation of practices in sub-watersheds 11, 9, and F were 10, 10, and 60 min, respectively. The largest number of structures was in these sub-watersheds, and the results showed the effects of implementing the structures. Moreover, the greatest and smallest changes in streamflow before and after the watershed management interventions corresponded to sub-watersheds F (27.7 m³/s) and 9 (3.5 m³/s), respectively. The implementation of structural measures had no significant effect on runoff volume. In this study, the assessment of biological watershed management measures demonstrated that these measures reduced the Curve Number (CN). The reduction in CN before and after the interventions in sub-watersheds 11, 9, and F was 10, 5, and 5, respectively. This reduction led to a decrease in peak discharge from 79.7, 5.4, and 197.5 m³/s to 75.8, 3.5, and 147.7 m³/s, respectively. Furthermore, the application of biological measures resulted in a considerable reduction in runoff volume. However, they had no impact on time to peak. Overall, the effects of biological interventions on reducing runoff volume were greater than those of structural measures.
Conclusions and Suggestions
In this study, the effects of watershed management measures on flood characteristics (peak discharge, outflow runoff amount, and time to peak) were evaluated, and waterway structures and biological measures were examined. The results indicated that watershed management structures were effective in reducing flood peak discharge and increasing the time to peak. However, their effect on the total runoff volume was not statistically significant. Furthermore, the assessment of biological practices demonstrated a significant impact on reducing both runoff volume and peak discharge, while no considerable effect was observed on the time to peak. These findings highlight the importance of integrating structural and biological measures within integrated watershed management. Combining these two approaches increased the effectiveness of measures taken in flood control, soil conservation, and water resource management. Therefore, it is suggested that similar projects be carried out in other regions with similar climatic and hydrological conditions so that the results obtained can be used in decision-making and improving watershed management.