The role of rainfall simulation in determining the type of relationship between runoff production and soil chemical properties (Case study: Kuhe Gach watershed of Izeh city)

Introduction
Examining the details between water and soil relationships can be useful and effective in better understanding these relationships that univariate regression can play an important role in this important matter and cause be a useful analysis of these relationships and ultimately the design of appropriate structures in water projects. The purpose of this study is to determine the type of relationship between runoff production and soil chemical properties using rainfall simulator, one of the advantages of which is to investigate the partial relationships that occur between runoff and soil chemical properties in different land uses in a shorter time, with lower cost and higher accuracy. Knowing the general information about a watershed is useful, but what leads to a better understanding of hydrological relationships in a watershed is knowing the details of hydrological relationships that have been discussed in detail in this research and can be useful and effective in the proper design of different watershed management structures in the watershed by increasing the understanding of the relationships between runoff production and chemical properties of soil, and also from wasting time experts and also prevented the waste of capital. In this research, it has been tried to state that sometimes a large amount of information obtained by different researchers in general may be not useful and also cause confusion for researchers, and paying attention to the details in the relationship between water and soil, even if it is small, has much more useful and better results for both researchers and the implementation department.
 
Materials and Methods
In this study, in order to determine the type of relationship between runoff production and soil chemical properties in different land uses of Gachsaran Formation deposits, a part of Kuhe Gach watershed of the Izeh city with an area of ​​1202 hectares was selected. 
The area has geographical coordinates of 49°45′27′ to 49°47′9′′ E and 31°50′ 27′ to 31°53′32′′ north. In this study, the relationship between runoff and soil chemical properties in different land uses of Gachsaran Formation was determined using univariate regression. Then, sampling of runoff production in 6 points with 3 replicates and in different rainfall intensities of 0.75, 1 and 1.25 mm/minute in three uses of the range, residential area and agricultural lands with the help of the rain simulator was done. In addition, the same number of runoff sampling was performed on soil chemical properties such as organic matter, soil salinity, soil acidity and carbonate calcium. SPSS and EXCEL software were used for statistical analysis. The rain simulator is known as the most important device for simulating different components of the hydrological cycle, especially rain and runoff resulting from it, in the first stages of formation and occurrence of soil erosion process. However, in recent years, the necessity of measuring runoff in order to evaluate it using rainfall simulation and in short-term times has been considered in order to manage watersheds.
 
Results and Discussion
Soil chemical properties have a significant effect on runoff changes in different land uses, which were investigated in this study more detailed without interaction of chemical and physical properties of soil. Also, the complex role of soil chemical properties in relation to land use and runoff generation was shown. Although the displacement of rainfall simulators and the amount of water consumed in watersheds is very difficult, this study showed that the Kamphorst rain simulator can provide reliable and valuable data due to its easier displacement and homogeneous environment that its plot creates. Of course, one of its limitations is the height of the installation of the rain simulator, which is better to be placed at a higher height that is somewhat close to the terminal velocity of the raindrops. Therefore, it is recommended that instead of transporting soil from watersheds to fixed rain simulator laboratories, it is recommended to use simulators such as Kamphorst that can be easily transferred to watersheds.
 
Conclusion
The results showed that in total, in Gachsaran Formation and in range, agricultural and residential land uses and in all three intensities of 0.75, 1 and 1.25 mm/min, soil organic matter in six cases had a negative relationship and in three cases showed positive relationship with runoff production. Soil salinity showed a negative relationship in five cases and a positive relationship in four cases, soil acidity showed a negative relationship in six cases and a positive relationship in three cases, and soil carbonate calcium had a negative relationship in eight cases and in one case showed a positive relationship with runoff production.