Estimating the Runoff Coefficient by Combining Arc CN-Runoff, SCS-CN and ICAR Empirical Relationship (Case Study: Selseleh Study Area - Lorestan Province)

Introduction: Determining the value of the runoff coefficient is one of the biggest problems and the main source of uncertainty in many water resources projects. In most researches related to water resources, the runoff coefficient values are taken from the tables of values depending on the conditions of the studied area. The main concern in this method is the selection of values that are subjectively chosen from a vague method and reflect a personal judgment of useful data, hence an appropriate method for determining the runoff coefficient should be selected.
Materials and methods: In order to carry out this research, data including digital elevation model information, land use classes, soil texture, and meteorological and hydrological statistics (rainfall and runoff) related to the research area for a 20-year statistical period (2001-2021) were used. Using the obtained land use map, the land use map of the area was classified into 9 land uses. Also, to prepare a map of soil hydrological groups, the soil texture map was used at a depth of 200 cm in the study area of ​​Selseleh, and according to the type of soil texture, the soil hydrological group was extracted for each area. Finally, the soil hydrological group was divided into three categories (A-B-C). The map of the land use layer and soil hydrological groups was entered into the Arc CN - Runoff tool environment, and finally the Intersect operation was applied on two layers and the land use layer - hydrological group (Land soil) was prepared, and this new layer is only for display. The surface is covered by two layers and the map of runoff curve number, soil surface maintenance, volume, height and runoff coefficient is prepared based on this layer. Finally, the value of the runoff coefficient was estimated in three conditions of dry, medium and wet moisture conditions and a comparison was made.
Results: The results of the research showed that in the study area, the runoff coefficient (CR) in three conditions of dry, medium and wet moisture conditions is equal to 0.26, 0.53 and 0.77, respectively. Therefore, the dry humid state has decreased by 68% compared to the average, and the more humid state has increased by 37% compared to the average. Correlation analysis between the runoff coefficient and characteristics of the study area showed that in the study area, the runoff coefficient series is influenced by 6 physiographic characteristics of the study area: area, slope, waterway length and Gravel's coefficient, maximum height and density of waterways. The value of the runoff curve number (CN) in dry, medium and wet conditions for the whole area was estimated as 65, 81 and 92, respectively. The amount of soil surface retention (S) in dry, medium and wet moisture conditions for the whole range was estimated as 138.74, 59.60 and 23.42 mm, respectively. The amount of runoff height (Q) in dry, medium and wet moisture conditions for the whole area was estimated as 27.78, 55.73 and 79.31 mm, respectively. The amount of runoff volume (V) in dry, medium and wet moisture conditions for the whole area was estimated as 3710.64, 7164.03 and 10070.46 thousand cubic meters, respectively.
Conclusion: These conditions require the implementation of basic measures to increase vegetation cover, including digging holes and leveling along with plans to increase vegetation cover, generally in the form of planting and sowing pasture plants in the area, and rainfall in the area can provide sufficient moisture for their success. Establishing a rainwater collection system can also be effective due to the low permeability of the soil in the region, and it can be used to increase vegetation and other uses.