Change in flood hydrograph response due to variable precipitation amount during different return periods in Namin watershed

Flood as a natural hazards occurs under the influence of many factors. Heavy rainfall in the upstream basin is one of the causes of floods. Rainfall and discharge simulation is one of the methods that can be used to understand the hydrological behavior of the watershed against different rainfall regimes, which ultimately leads to proper management of water resources and floods. Floods are one of the topics of interest to researchers and one of the most important topics studied in natural disasters around the world. Due to the fact that access to accurate information for flood study is limited in watersheds, hydrological models have been used to estimate flood characteristics and predict the trend of runoff changes, which in many cases has yielded good results. Namin city is one of the flood vulnerable areas in Ardabil province and is hydrologically an area with a rapid hydrological response, which has been selected for the present study in simulating the hourly flood hydrograph. The results can be used in flood analysis due to different rainfalls. The aim of this study was to study the changes in rainfall amount and the effect of its values in different return periods on hourly flood hydrographs in the Namin watershed in Fandoghlou forested area in Ardabil province.

In the present study, Landsat satellite images, ENVI 5.3 software and Google Earth were used to classify the land use map of the study area. Due to the fact that Landsat satellite images are provided to users in the form of geometric correction, several control points were selected from 4 images and their accuracy was evaluated, which was acceptable. For this purpose, in ENVI 5.3 software, green, blue and red bands were combined and then due to errors, the effects of diffusion and atmospheric absorption at the image level, radiometric, geometric and atmospheric corrections were performed on satellite images. The atmospheric correction was performed on the image by FLAASH method. Then, training sample was prepared to classify land use in ENVI5.3 environment from land use classes with the help of visual interpretation, Google Earth, auxiliary slope map, digital model of 12.5 m area. Training samples were taken from land use, forest use, barren land use, agricultural use (irrigated), pasture use, and residential use. Then land use classification was performed using images. For land use classification, the command of minimum distance from the mean and support vector machine was used, which among the methods used, the SVM method had the highest accuracy, and also to increase the classification accuracy of the area slope map using digital elevation model map. The thermal and multi-bands were combined into other bands of the images, and then a comprehensive output was prepared from all of these bands, and the classification was done using the SVM method due to its high accuracy. After land use classification, the accuracy of the classification was evaluated. For this purpose, experimental samples were taken from the land uses in the study area and the overall accuracy and kappa coefficients of the land use classifications were calculated. All classified images had a high accuracy of 98%. Then the design rainfall values in different return periods has been calculated using Cumfreq software and the hourly flood hydrograph is simulated by using Wildcat software. The Wildcat5 model is based on the SCS model, which is a rainfall-runoff simulation model. The required inputs to the model include the following. First, the rainfall characteristics for each year will be determined with different return periods (average rainfall and rainfall distribution). In the following, the parameters related to soil and surface cover of various watershed land uses to calculate excess rainfall (basin runoff) will be determined. For this purpose, curve number (CN) was assigned according to the type of coverage of each user. Then, time parameters were calculated to enter the flow time from the upstream to the watershed outlet and entered the model. The appropriate method for transforming rainfall to effective rainfall and also converting effective rainfall to unit hydrograph were selected based on the curve number method. In this section, factors such as the length of the main stream and the average slope of the watershed were calculated. Then, the area of each land use was determined and in the mentioned section, they were included in the model along with the curved number. Finally, the amount of watershed concentration time was calculated. According to the required inputs, first the maximum 24-hour rainfall during the existing statistical periods was extracted from Namin rain gauge station. According to the aim of this study, which is to investigate the hydrological response of the Namin watershed to changes in rainfall during the return period 2, 5, 15, 25, 50 and 100 using the Wildcat5 hydrological model to investigate the effects of changing rainfall during the return period in the flood hydrograph in the Namin watershed. Cumfreq software was used to estimate the amount of rainfall in different return periods.

According to the results, the rainfall changes in the study area showed that the rainfall values in the return period of 2 years and 100 years were 23.27 and 47.02 mm, respectively. Based on the results of hourly flow simulation, the amount of runoff in the 2-year period was 1.43 mm and the highest amount in the 100-year return period was 7.42 mm. Accordingly, the results showed that the the peak flow rate of the flood hydrograph will be equal to 100.07 cubic meters per second under the rainfall with 100-years return period. According to the results, the increase in discharge from the 2-year to 5-year return period is equal to 20.73 cubic meter per second, which has increased by 111.51 percent. While, with the increase of rainfall from the return period of 50 to 100 years, the amount of increase in flow will be equal to 15.38%.

The results of rainfall-runoff simulation at hourly time scale in the Namin watershed showed that the rate of change in discharge relatively will be more in the lower return periods. According to the simulations performed for maximum rainfall and runoff in Namin watershed in different return periods, it was observed that with increasing rainfall from 2 year return period to 100 years, runoff values and maximum flow intensities will increase. A noteworthy point is the amount of change with increasing return period. Overall, the results showed that the intensity of flow rate increase in the low return period will be more than the return period of more than 50 years. Based on the results of climate forecasts, the intensity of rainfall will increase due to climate change. Therefore, an increase in the intensity of flood events can be expected, which will show the importance of studying the effects of rainfall intensification. Accordingly, it is important to predict the flood-related effects of climate change in the study watershed.