Introduction
In general, the role and importance of watershed operations in improving the condition of pastures, controlling erosion, preventing sedimentation and finally protecting water and soil is not hidden from anyone. The technical and principled implementation of watershed projects causes storage and infiltration of rainfall, increase of vegetation cover, stabilization of erosion and sedimentation, increase of the useful life of dams, adjustment of the slope of waterways and reduction of the speed and power of water destruction. Based on the studies conducted, due to the diversity of ecological conditions, it is not possible to reach a specific pattern or provide specific instructions with the same application in all watershed areas. For this reason, in addition to knowing the area and its issues, it is inevitable to prioritize watershed management operations according to the existing potentials. Examining the results of the conducted research shows that the implementation of watershed operations in watersheds has played an important role in controlling surface runoff. In a study, they evaluated watershed projects on the flood level of the Ramyan area using the HEC-HMS model, and the results showed that the flood has decreased in the period of different returns; In such a way that the greatest decrease in peak discharge and flow volume occurred during the return periods of 10 and 20 years, respectively, and with the increase of the return period, the effect of flood control measures on floods decreases. In this research, in order to evaluate the role of watershed projects in reducing peak flood discharge, HEC-HMS rainfall-runoff model was used, and the amount of discharge was calculated before the implementation of watershed management activities and after the implementation of watershed management activities, and the effect of these projects on the peak flood discharge was determined.
Materials and Methods
Hossein Abad Jiraft watershed is located between 57' 34'' to 57' 43'' east longitude and 28' 19' 13 to 28' 26' 34' north latitude. The area of the watershed is 11258.68 hectares. Also, the weighted average slope of the watershed is 17.83%. More than 60% of the area's land use is pasture, which is the dominant species. In this study, the HEC-HMS model version 3.1.0 was used to simulate rainfall-runoff. Also, the data of 20 climatology stations with a common statistical distance of 30 years (1986-2016) were used.The methodology must be clearly stated and described in sufficient detail or with sufficient references. In order to calculate the amount of losses, the American Soil Conservation Society (SCS) method was used in the HEC-HMS model. The amount of runoff (Q) depends on the rainfall (P) and the water retention volume inside the soil. In all floods, runoff (Q) is always less than or equal to precipitation (P). Similarly, after the occurrence of runoff, the actual amount of water retention (Fa) is less than or equal to the maximum retention capacity (S). As can be seen, CN is the most important parameter to be estimated. CN for average humidity conditions (AMII) is obtained from the relevant table. CN is estimated from the combination of 2 factors of hydrological group and land use at any point of the basin, and CN of the entire basin is calculated from weighted averaging. In this study, to prepare the curve number (CN) map of the region, first, the map of soil hydrological groups in four groups A, B, C and D was prepared by combining the slope map and the map of hydrological units.
Results and Discussion
According to the calculation method described in the previous section, CN was calculated in the sub-basins. The results show that the highest value of CN is estimated in sub-areas H12-1-1-2 with CN: 78.3 and sub-area H11int with CN: 76.9. According to the high value of the curve number in these two sub-areas, flash floods are expected to occur in these two the area is very large and generally the average area curve number is above 70. After completing the input data to the HEC-HMS runoff precipitation model, the model was run for different return periods (RUN) and the calculated peak discharge was extracted. A total of 67472 cubic meters of structures have been built in the entire Hossein Abad basin, including 3 stone and mortar structures with an area of 1139 cubic meters and 10 earthen dams with an area of 66339 cubic meters. The 13 created structures have a total reservoir volume of 222598.1 cubic meters. Due to the effects of watershed management activities, the concentration time of the basin changes and generally increases. This change can be estimated through two factors: the slope of the main waterway and the change in the condition of the cover (CN). The above information was calculated in the modified Kerpich relation of the concentration time and the delay time of each sub-basin of Hossein Abad, after watershed operation. Here, the changes include concentration time, CN and equating some factors with the effective level. By applying the effects corresponding to watershed operations in the runoff precipitation model, the model was run for different return periods (RUN) and the flood peak discharge after watershed operations was calculated.
Conclusion
Today, in order to implement watershed projects, a lot of money is spent in this regard, but perhaps less attention is paid to the effectiveness of these projects. In fact, without monitoring and evaluating these projects, it may not be possible to convince the policy-making managers to allocate credits and the people to voluntarily participate in the implementation and support of watershed projects. Watershed activities are considered as the most important flood risk reduction projects In this research, considering that the implementation of the Hosseinabad-Jiroft watershed project is very costly and is one of the important flood-prone areas of the region, the HEC-HMS rainfall-runoff hydrological model is used to simulate the peak flood discharge before and after the implementation of watershed activities in the Hosseinabad-Jiroft watershed to evaluate the effectiveness of this The projects were used to reduce the peak flow of floods. The results of this research show that watershed management activities in Hosseinabad-Jiroft region play an essential role in reducing the peak flood discharge. This reduction is more than 95% in some sub-areas, which indicates the impact of watershed projects and their effectiveness on the peak flood discharge. Therefore, it can be concluded that the HEC-HMS model is a suitable model for predicting runoff events. Due to the good performance of the HEC-HMS model, this model can be used to estimate floods in basins without statistics with appropriate calibration and obtain acceptable results. The HEC-HMS hydrological model has a good capability in predicting the real time of flood and therefore it was used in this research.
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