The Monthly Changes of Surface Runoff Intensity and Concentration in the Gorgan-Roud River, Golestan Province

Extended Abstract
Considering the relative stability of the physical characteristics of a watershed, the variability of the precipitation over space and time, and the direct relationship between rainfall and runoff, the variations of runoff can be expected and analyzed to understand the nature of variability. Determining changes in the amount of runoff caused by rainfall and detecting the time of rivers' floods can provide a prediction of floods' occurrence and, consequently, reduce their damages. The increasing importance of water resources management in recent years, erosion, and sediment highlights the need for understanding the rivers' behavior and regimes.  Regarding the changes in the river flow rate, estimating temporal and spatial variations of runoff changes can be effective in determining and controlling the dependent processes of soil erosion in a watershed and river bank, droughts, floods, and water quality and utilization. The analysis of the river flow variability, its duration and influencing factors, is necessary for an optimal river management/operation as the main sources of water uses.
Methodology
The monthly and annual runoff volumes of different stations were calculated based on the monthly discharge data in different years during the study period. Then, the variability indices were used to study the seasonal variations in the runoff volume at each hydrometric station. Next, using Annual Distribution of Regulating Coefficient and Concentration Rate indices, the seasonal variation in runoff volume of twenty river gauge stations located in Golestan Province were evaluated in 38 years. The values of Annual Distribution of Regulating Coefficient indicated the uniformity/ non-uniformity of changes in runoff volume at the studied river gauge stations. In addition, the annual variation of runoff volume was plotted in triple diagram models based on average runoff volume and time variables. The Kriging method was also used to draw the triple diagram models using two independent variables in a surfer environment. The Annual Distribution of Regulating Coefficient and Concentration Rate indices were considered as dependent variables. The variability of the implemented indices were analyzed over a time period of 38 years.
Results and Discussion
According to discharge data in different years, the monthly and annual runoff volumes of the stations were calculated during the study period. Based on the monthly spatial distribution, the results showed that the maximum amount of runoff volume of the stations were observed in March. The highest amount of surface runoff amounts occurred in Aghghala, Ghazagli, and Basirabad which respectively had an average annual runoff of 33.9, 33.5, and 32.6 million cubic meters. The highest uniformity in runoff occurrence was related to Nodehkhandoz, Tamar, Galikesh, and Gholitappeh stations, respectively with an annual Distribution of Regulating Coefficient of 0.19, 0.21, 0.23, and 0.24. The lowest Rate of runoff concentration was at Nodehkhandoz and Tamar stations respectively with 0.26% and 0.25%. The results also indicated a direct and significant relationship (R² = 0.60) between Annual Distribution of Regulating Coefficient and Concentration Rate (p < .05). Ramian station had the highest Concentration Rate with a value of 0.62%. The highest significant decreasing and increasing trends, in Mann-Kendall test, were observed at Shirabad and Nodehkhandoz stations
Conclusions
According to the findings, there was a correlation between the annual distribution of regulating coefficient and the concentration rate. The higher values of the Annual Distribution of Regulating Coefficient and the Concentration Rate of runoff volume can be attributed to physiographic properties of watershed such as its slope, vegetation, and soil permeability. In other words, the process of changes in the runoff volume at these stations can indicate the temporal and spatial variations of precipitation, human protection measures such as dam construction in the basin, or the amount of permeability during the statistical period. In conclusion, with the non-uniform distribution of runoff volume in different months of the year, it can be expected that variations between the minimum and maximum values of runoff volume will also be high. Indeed, the higher the uniformity of the monthly distribution of runoff volume, the lower the variations between the minimum and maximum changes in the runoff volume. Variations in the amount of monthly runoff in the studied area can be related to the characteristics of the area, the hydrological response, and land use (agricultural land plowing season), as one of the main factors controlling runoff.