عنوان مقاله [English]
The normal regimes of discharge and sediment load can be altered by the impact of natural variations and man-made disturbances. It is proved that temporal trends of discharge and sediment yield will occur due to anthropogenic impacts. Sediment transport via rivers is one of the important processes changing river morphology, bank stabilization, soil formation, and many other earth-related processes. Seasonality of water discharge controls the intermittently high sediment loads in rivers. Accurate estimation of sediment amount carried by the river flow is necessary for river management and to design of soil-water conservation projects. Also, according to the sediment variations based on changing river flows,assessment of spatio-temporal changes of runoff-sediment can be usefule in determining and cotrol of sediment source. The soil erosion phenomenon is the main cause of fertile soils losses from arable lands, also, the climatic conditions, hydrological, geological and excessive human pressure on the watersheds results carrying our more sediments in Iran.
In this study, the monthly changes of discharge and sediment yield were analysed in a 20-year period through 15 hydrometric stations in Azerbaijan Province, Iran. The monthly streamflow and sediment regime distributions over different months were investigated in a long-term observations over Azarbaijan Province. The linear sediment rating curve with a logharithmic transformation was used based on the discharge-sediment load relationship. The monthly streamflow and sediment load at the gauging stations were derived from the daily and random measured data, respectively. Then the wet-dry spells were determined using the Runs theory, and the occurrence pattern of wet and dry periods were recognised by using Power Laws Analysis techniqu. The results showed that the maximum discharge and sediment were observed in Spring season and especially in May, which can be related to occurrence of rainfall and snowmelt and high amount of river discharge. In other words, the highest and lowest amounts of sediment and discharge rates have been occurred in the spring and summer seasons. The high amounts of suspended sediment can be related with highe rainfalls in the spring season.
Results and Discussion:
According to the resuslts, it was found that the highest flow discharge was observed in Sarighamish station (434.1 cubic meters per secound), while the the maximum sediment rate has been occurred in Ghasemlou station having amount of 5174.9 tons per day. As well as, the Pole-Bahramlou station had the highest logarithm range of variation in monthly discharge and sediment load having 1.2 cubic meters per secound and 2.3 tons per day amounts. The slope of low sediment line had positive in the Plbe-Bahramlou, Sarighamish, Choplojeh, and Dizaj stations which indicates the frequency of occurrence in long periods of low sediment load in theses stations. The frequency and durations of wet and dry spells were drawn using double logarithmic plots. It was found that the dry periods waith low sediment yield were frequently occurred with respect to wet periods with high amount of sediment load. Also the wet spells with high sediment load had longer durations. Then, the frequency and durations of high and low sediment periods were represented on double-logarithmic plot to calculate the severity and duration of different periods of sediment transport. The results showed that the occurrence patterns of wet-dry spells and sediment transport rate were distinguished by using Power Laws Analysis technique.
The monthly changes in discharge and sediment load in the study area can be explained by rainfall seasonality, characteristics of the watershed, hydrologic response behavior and land use (cultivation activities in agricultural lands) which are the main factors controlling components of discharge and suspended sediment load. The results proved that the patterns of wet and dry spell durations can be determined using power law technique. The changes from high to low sediment loads indicates that the decreasing sediment yield play important roles in characterizing changing sediment load during the rainy season. The interpretations of double logarithmic plots of (frequency versus length period) can be used to compare the occurrence patterns of high and low amounts of sediment in the stations over different discharge regimes. Further applications of power law method are suggested to obtain a better understanding for the interpretation of high and low amounts of sediment load and erosion regions in different climatic conditions. As a concluding remarks, the high value of sediment transport were observed in April, May and June. Based on comparison of double-logarithmic plots, the sediment variations were consistent with runoff variations in Chapar-abad, Doroud, Naghadeh, Oshnavieh and Peighale, while the remaining stations in the study area had inconsistent and different ouucurence patterns.