بررسی تأثیر اقدامات آبخیزداری بر ویژگی‌های سیل در آبخیزهای آب‌ماهی و چیکان-مورزیان استان فارس

Introduction and aim
Watershed management project evaluation is a fundamental aspect of future planning for implementation projects and natural resource management in many countries. Given the long history of watershed management projects in our country, it is imperative to assess the impacts of these projects and evaluate their effectiveness. Due to the lack of necessary equipment for measuring and recording changes in watersheds, the use of distributed hydrological models to simulate basin behavior before and after watershed management activities has become an effective tool for achieving these goals. Today, the use of hydrological models to simulate the impacts of management activities on decision-making processes has become crucial.
This study aims to quantitatively evaluate the effects of implemented watershed management activities on peak flow, time to peak, and flood volume in the Abmahi and Chikan-Morzian watersheds, which have homogeneous conditions. The HEC-HMS model was used to conduct this research. To determine the impact of structural and biological measures, concentration time, channel slope, and curve number were calculated through repeated field visits before and after the implementation of activities. The results of this study can be used for management planning of similar projects in other watersheds with similar climatic and physiographic conditions.
Materials and Methods
The Chikan and Murzian watershed, located 80 kilometers north of Sepidan city, is one of the sub-watersheds of the Kor River and the Doroodzan Dam. Geographically, this area is located within the range of 51°26'01.23" to 52°04'37.03" East longitude and 30°25'52.88" to 30°34'15.30" North latitude. The area of this watershed is 12,333 hectares, with a maximum elevation of 3,125 meters and a minimum elevation of 1,811 meters above sea level. The Ab-Mahi watershed, with an area of 158.7 square kilometers, is located 82 kilometers northwest of Shiraz city and within the geographical range of 52°01'46" to 52°14'31" East longitude and 30°25'02" to 30°36'57" North latitude. The minimum and maximum elevations are 1,769 and 3,070 meters above sea level, respectively.
This research employed the HEC-HMS model to simulate rainfall-runoff under two conditions: with and without watershed management interventions. For the scenario lacking these interventions, necessary data were sourced from detailed executive studies compiled by the Fars Province General Directorate of Natural Resources and Watershed Management. Conversely, the scenario incorporating watershed management assessed the hydrological impact of structural measures by analyzing their influence on basin slope and time of concentration. The effect of biological measures was evaluated by examining changes in the weighted average Curve Number (CN) of sub-basins before and after their implementation. Following this, key physical basin parameters, including sub-basin areas, weighted average CN, initial abstractions, and lag time, were calculated. Subsequently, physical calculations related to the watersheds, statistical data and information such as rainfall, and parameters required for the model, including the weighted average curve number of the sub-watersheds, initial abstraction, and lag time, were obtained. In order to convert rainfall to runoff, the Soil Conservation Service (SCS) method (now known as the Natural Resources Conservation Service (NRCS) Curve Number method) was used for both pre- and post-watershed management practices conditions. The Curve Number (CN) map of the watershed was prepared by integrating maps of vegetation cover, hydrologic soil groups, and land use, as well as through repeated field visits. The Kirpich method was used to estimate the time of concentration and lag time of the watershed.
Results and Discussion
The results of evaluating the impact of watershed management practices in the studied watershed reveal various aspects of their effects on the hydrological behavior of the basin. The results showed that the implementation of structural watershed management practices significantly altered the lag time between pre- and post-implementation conditions. The greatest difference in lag time was observed in sub-watershed F, and the smallest difference was observed in sub-watershed 9. This difference is attributed to factors such as slope, main channel length, soil permeability, and watershed area. The differences in peak time before and after the implementation of practices in sub-watersheds 11, 9, and F were 10, 10, and 60 minutes, respectively. Furthermore, in sub-watersheds 11, 9, and F, where the highest number of structures were implemented, the results clearly demonstrated the impact of the implemented structures. Among these sub-watersheds, sub-watershed F showed the highest difference in discharge before and after implementation, with a value of 27.7 m³/s, while sub-watershed 9 showed the lowest difference, with a value of 3.5 m³/s. The implementation of mechanical practices did not affect the amount of runoff discharge. The results of evaluating the impact of biological watershed management practices showed that these practices reduced the Curve Number (CN) in sub-watersheds 11, 9, and F by 10, 5, and 5 units, respectively, in pre- and post-implementation conditions. 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. Additionally, the implementation of biological practices significantly reduced runoff volume but did not affect the time to peak. Overall, the impact of biological practices on reducing runoff volume was greater than that of structural practices.
Conclusions and Recommendations
This study evaluated the impact of watershed management practices on flood characteristics, including peak discharge, runoff volume, and time to peak. For this purpose, in-stream hydraulic structures and biological practices were investigated. The results showed that watershed structures effectively reduce peak flood discharge and increase time to peak. Watershed structures did not have a significant effect on total runoff volume. Furthermore, the results of evaluating biological practices showed a significant impact of these practices on reducing runoff discharge and peak discharge. However, they did not affect time to peak. These findings emphasize the importance of integrating structural and biological practices in integrated watershed management. Integrating these two approaches can lead to maximum effectiveness in flood control, soil conservation, and water resources management. Therefore, it is recommended that similar projects be evaluated in other regions with similar climatic and physiographic conditions to utilize the results in decision-making and improve watershed management.