Extended Abstract
Introduction
According to global data, flood is one of the most important natural disasters, which has the highest number of occurrences and a high volume of coverage. Therefore, it has extensive damage and consequences that cause destruction all over the world every year. Watershed management can be very important and effective in preventing floods and their damage. The analysis of watershed projects is one of the most fundamental measures that is carried out for the long- term planning in relation to executive plans and natural resources management. The loss of water and soil resources in watershed has been increasing in recent decades due to irrational exploitation of resources. This problem leads to a decrease in the useful life of dam reservoirs, a decrease in the production, and loss of the country’s capital, while intensifying the occurrence of floods and increasing sediment generation and deprecation of equipment and structures. The construction of watershed structures played an obvious and significant role in reducing erosion and consequently preventing these sediments from entering the reservoir, which is the supplier of the major part of the drinking water in Ilam city. In the current study, an assessment of the impacts of watershed management implementations of Gol-Gol, Ilam province watershed has been done using HEC-HMS.
Materials and method
Gol-Gol watershed is located in Ilam province, in Ilam and Malekshahi county as one of the sub-basins of Ilam dam. Gol-Gol watershed has geographic coordinates of 46°  to 46°  east longitude and 33°  to 46°  north latitude. Its area was estimated to be 24950 hectares. The maximum and minimum height of the Gol-Gol watershed is 2605 and 1076m above sea level, respectively. The average rainfall is 570.6 mm and the average temperature is 16.8 . In this study, first, all the data and related maps were collected and the HEC- HMS hydrological model was calibrated and validated using the observed rainfall and runoff data. In this stage, the data of 2004-2005 period were used for calibration and the data of 2006 were used for the validation. Also, the effectiveness of the hydrological model in simulating the flood hydrograph in the calibration and validation stages were evaluated using peak flows, average flows, Nash-Sutcliffe efficiency and correlation coefficient. Then, a daily event with a return period of 25 years and time steps of 10 minutes was selected to investigate the effects of watershed implementation, and their results were analyzed in this stage as two main scenarios (the presence of a watershed structure and the absence of a watershed structure).
Results and discussion
The calibration results showed that the maximum observed flow rate is 90 m³/s, while the simulated flow rate is 89.7 m³/s. The correlation coefficient and dispersion of the data used in the calibration stage were 80% and the NSE was 79%, which show the acceptable results of the simulation. Then, the results were validated using the optimized parameter of the model during the one- year period of 2006. The maximum observed flow rate was 67.7 m³/s and the simulated value was 67.1 m³/s. The efficiency index of the model was obtained with a correlation coefficient of 77% and a cliff settlement factor of 76%. Based on the results of the calibration and validation of the model and the low percentage of difference between the observed and simulated flow rate in the used events, it was determined that HEC- HMS model has the necessary efficiency to simulate the Gol-Gol watershed basin. In addition, the evaluation of the effect of watershed structures on the flow rate results showed that in the first scenario (the presence of a watershed structure), the simulated peak flow was calculated as 137 m³/s, but the calculations showed the value of 114.2 m³/s for the second scenario (the absence of a watershed structure).
Conclusion
This study was performed with the aim of evaluating the effect of watershed management measures on runoff in the Gol-Gol watershed, using the HEC-HMS hydrological model. In order to check the effect of the proposed devices, the flood behavior was simulated for the existing events. The criteria of peak discharge and flood volumes were determined for evaluation and their values were calculated for two situations before and after watershed measures. The results of the statistical comparison showed that these measures have a significant effect on reducing runoff and discharge in the study area. The calibration results also showed that the curve number parameter is one of the most sensitive and effective parameters on runoff. Since the numerical value of this parameter depends on the hydrological group of the soil of the region, land use, hydrological status of the region and previous soil moisture status, these factors can be controlled by management measures. The evaluation phase of watershed structures and their efficacy on the flow resulting from a storm with a return period of 25 years showed a decrease of 22.8 m³/s of he maximum flow, which can finally be concluded that the creation and construction of watershed structures is the best solution to prevent floods in this area, which confirms the effect of structures on reducing floods.