GIS-based Spatial Runoff Coefficient Mapping in Mohammadabad Katul watershed, Golestan Province

Extended Abstract
Introduction
Determining the runoff coefficient in a distributed way can be used to identify the runoff producing areas. The runoff coefficient represents the ratio of runoff to total precipitation in different areas, where the previous soil moisture is not taken into account. The runoff coefficient without considering the effect of soil moisture is called the potential runoff coefficient, which is determined based on different parameters in hydrological studies. The changes in the watershed runoff coefficient depend on the topographic characteristics and especially the slope. Assessing the spatial changes of runoff coefficient at the watershed scale is very important for understanding the hydrological cycle under natural and disturbed condition. The location of homogeneous and similar units in terms of hydrological behavior in the watershed is determined and identified by determining the spatial map of runoff production. Meanwhile, determining the watershed response is important in the production of flood runoff volume. It should be noted that the similar units based on hydrological response are usually defined based on runoff production using field measurements. In this regard, the spatial data mapping provides the possibility of preparing a map of the runoff coefficient in a short time and will increase the accuracy of the work. Anthropogenic intervention in the natural water cycle through the destruction of vegetation in watershed areas, land use change, development of impervious surfaces lead to increasing the possibility of flooding in various areas. Various factors affect the occurrence of floods, which can be mentioned the intensity of rainfall, the slope of the land permeability, relief, characteristics of vegetation and different soil conditions. The runoff coefficient is one of the important parameters for estimating the peak flood of hydrological models and identifying important areas of sediment production and pollutants in runoff producing areas. Several factors have been used in determining homogeneous hydrological units with similar runoff coefficient, e.g., rainfall distribution, soil moisture, bedrock depth, evaporation, geology, land use, soil and slope.
Methodology
The purpose of this research is to surface runoff potential mapping using combined table and the soil conservation service curve number (SCS-CN) method in the Mohammadabad Katul watershed in Golestan province. The area of study area watershed is 404 square kilometers and a main river length is 30.5 kilometers. The minimum and maximum elevation of the study watershed is 455 and 3671 meters above sea level, respectively. The average annual precipitation of the study watershed is 530 mm, and the average annual temperature is 16.5 centigrade degrees. Also, the average slope of the study area is 41.6%. The land use map of the study area was obtained from the General Office of Natural Resources and Watershed Management of Golestan province and the land use types were modified during the field surveys, then the land use map was digitized in the GIS environment. The soil map of the study watershed was also prepared based on previous studies and then digitized using GIS and the soil map has been prepared. The digital elevation model of the study area with a cell size of 30x30 meters has been obtained from topographic map with a scale of 1:25000. The slope map of the watershed was prepared from the DEM and then classified into four slope classes according to runoff coefficient table. Based on this, the necessary information to determine the runoff coefficient including the land use map, slope and soil texture was prepared. Then, the values of the potential runoff coefficient were determined using the combined table in the study watershed. Also, by incorporating the required layers, the curve number map of the study area has been prepared. The maximum 24-hour precipitation data of the nearest rain gauge station (Fazel abad) has been analyzed. After statistical analysis, the best probability distribution function fitted to the data has been selected for further analysis. Then, the runoff height and the value of the runoff coefficient were determined in 5, 10, 25, 50 years, return periods using the SCS-CN method. In the next step, the results of the combined table method and the SCS method were compared in estimating the amount and spatial distribution of the potential runoff coefficient.
Result and Discussion
The results of this study showed that the runoff coefficient was 39% according to the combined table method. The maximum coefficient of potential runoff in the study watershed was estimated to be 0.55, corresponding to the steep and agricultural land use of the study areas. Also, the minimum runoff potential value coefficient was 0.13. Based on the results, the combined distributed method incorporating land use, soil texture, and slope layers has the ability to determine the spatial changes of the runoff coefficient. In the following, the fitted frequency distributions were evaluated based on the goodness of fit criteria, and the Combined Laplace frequency distribution was chosen as the best frequency distribution to calculate the rainfall values in different return periods. The value of the runoff coefficient was 0.29 in 50-year return periods.
Conclusion
In the present study, the runoff coefficient map was prepared using the combined table method and SCS-CN method. In this regard, the GIS layers of slope, soil, land use and combined table were used to prepare the potential runoff coefficient map of the watershed. It seems that the calculated runoff coefficient based on the integration of the influencing maps takes into account the main and effective factors on runoff production, and provide an accurate spatial runoff coefficient map. As a concluding remark, it can be said that the combined and SCS-CN methods have provided similar results, but the runoff coefficient numbers provided by the combined method are higher than the SCS method. Considering the effect of several factors, including rainfall characteristics, relief, land permeability, vegetation characteristics and physiography on the flooding of a region, it is necessary to develop a method that can be used to map the runoff coefficient to the flooding map based on the affecting available factors. The integration of different layers in GIS and the use of a combined method is a useful tool for determining the runoff coefficient in ungauged watersheds, which allows the use of the runoff potential map in reducing the possible effects of floods.