Estimating Height runoff by using curve number method and Arc- CN Runoff tool in Afjeh Catchment

  In Iran, there is a general risk of runoff and flood, and since this country has a dry to semi-arid climate, its predominant rainfall is not evenly distributed in terms of time and place. Routine runoff at the earth's surface can lead to risks such as groundwater abatement, social issues such as population migration, erosion and loss of soil fertility, sedimentation in reservoirs and water quality in rivers. Also, damage to the agricultural sector, subsidence, the destruction of residential buildings and the reconciliation of the urban, rural and nomadic order are all examples of controversy about the risks that runoff and flooding are due to. The severity of these hazards in the Afje watershed is due to the geographical location, the specific climate, geology and pond factors, and a large volume of flooding every year causes the destruction of residential areas, agricultural lands and many financial and financial losses.                             In this research, data collection was carried out through library and field resources. The main tools of this research were topographic map of 1: 25000 which was used by GIS software for the separation of layers and determining the boundaries of the basin; Land use maps and user data and hydrologic groups of the study area were also used to estimate runoff height using curve number method and Arc-CN Runoff instrument. Then Arc-CN Runoff tool was used to prepare layers and curve number map (CN). Finally, the runoff height of the studied basin was prepared in the GIS software. Soil Hydrology Group: According to the studies, in terms of breadth and extent, the Hydrologic Groups C has the largest area with relatively high runoff potential. By matching the map of Hydrologic Soil Groups and the mineralogical units of the basin it can be stated that the green mass tuff and thick conglomerates are in the hydrologic group D, and the thick layers of green tuff, marl and sandstone in the hydrologic group C and the rocky layer of limestone in The hydrological group B is located. Land use: The hydrological status of the land and types of uses in the Afje watershed basin are as follows. Most of the catchment area is covered by medium-sized meadows, which comprises 53.77% of the basin area and is located in the hydrological group C with relatively high potential of runoff production. The rock outcrops are in the next rank, which is in the hydrological group D. Runoff curve number (CN): Land use maps and soil hydrologic groups were combined and extracted for each curve number range (CN) and CN map was prepared. The Afjeh basin has a curtain number of 66 to 100. The highest value of the curve number is 100, which is related to the outflow of the Afjeh watershed basin, which is practically inert, so all precipitation becomes runoff. The Afjeh basin has a curtain number of 66 to 100. The highest value of the curve number is 100, which is related to the outflow of the Afjeh watershed basin, which is practically Impervious, so all precipitation becomes a runoff. In fact, the curve number 100 in stone is 5.59 square kilometers from the area of the Afjeh watershed. But gardens and Agricultural land have the lowest CN (curve numbers 66 to 77) in the Afjeh basin, and include 4.53 square kilometers of basin area. Therefore, the lower parts have a lower CN than the upstream of basin.                The runoff height in the four seasons was calculated based on the daily precipitation values occurring at mentioned times. Based on the average daily rainfall of spring with a value of 10.79 mm and runoff classification, in a small part of the gardens in catchment area due to high permeability of the soil, 2.54 mm of precipitation has become runoff, although it is due to stone due to The Impervious of the surface, 10.66 mm of precipitation, turned into runoff. According to the studies carried out and according to the potential maps of runoff production in the Afje watershed, in the garden with the value of the curve number 66 and daily precipitation (for example, in the winter with a rainfall of 4.04 mm), the runoff height is 0.76 mm And the peak of discharge of 0.47 cubic meters per second And has the lowest runoff potential. In winter, the upstream sediments of the basin with curve number 100, runoff height of 3.81 mm and peak of discharge of 2.65 cubic meters per second and almost all rainfall becomes runoff. Therefore, the shortage of water resources, the presence of dry and semi-arid climates in the country and the achievement of sustainable development leads to the optimal use of water resources.   keywords: hazard, Arc- CN Runoff, GIS, high runoff, catchment Afjeh