عنوان مقاله [English]
Due to the importance of rivers in water supply, it is crucial to understand the flow condition and river regime. The river flows are affected by dam construction, climate change, flow diversion, backwater and urban development, which cause various problems, such as drought, flow reduction and environmental issues. Hydraulic structures have great impacts on the hydraulic conditions of rivers downstream. The reservoir dams as the most important hydraulic structures constructed on rivers, have considerable changes on flow regime conditions, therefore, drought and river flow drying area are major impacts of dams on downstream rivers. One of the common types of droughts is the hydrological drought, which is followed by meteorological drought with a delayed time. The most commonly definition of hydrological drought is based on a truncation level. The hydrological drought occurs when the river flow falls below a defined threshold level. The Yamchi and Sabalan dams are two main important dams in Ardabil Province due to the water supply for drinking water and irrigated agricultural lands.
Due to the consequences of short and long-term socio-economic and environmental effects of droughts as a natural disaster, it is necessary to study the frequency, severity, and spatial extent of drought spells. Accordingly, the drought characterization plays an important role in macroeconomic planning and decision-making to achieve sustainable development goals. The purpose of this study is to evaluate the effect of the Yamchi and Sabalan dams on the flow regime and hydrological drought occurrence of the downstream rivers, areas.
The used data in this study were the daily discharges, which were obtained from four hydrometric stations located upstream and downstream of the dams under the study. In this study, the seasonal threshold of was chosen and drought characteristics of the upstream and downstream stations of each of the dams were evaluated and then the drought severity at different durations was calculated using the Runs theory. Then, the Easy Fit software was employed to calculate the drought intensity at different return periods. The drought severity amounts were plotted over different return periods and the Severity-Duration-Frequency curves that were drawn for the upland and downstream of the constructed dam reservoirs. The percent change in discharge at the upstream and downstream stations of the studied dams were calculated to quantify the degree of influence.
According to the results, the highest drought occurrence in both studied dams were observed at 1-month duration and at Pole-almas and Dost-beiglou river gauge stations. It can be concluded that the severity of the observed hydrological droughts increases under the influence of both Yamchi and Sabalan dams as the return period increases. Also, the most severe drought spells occurred is related to the Sabalan dam and its upstream station (Arbabkandi). According to SDF curves, the drought severity increases in higher drought durations in the upstream and downstream stations of the Yamchi dam. According to the results, the percentage of changes in the downstream hydrometery station of the Sabalan dam is positive in almost all water years, which confirms the positive effect of Sabalan Dam on hydrological drought reduction and flow regulation. The highest percentage of changes in both of these dams is for to June, which indicates the difference in discharge values in upstream and downstream of the dams. In other words, this indicates that both of the studied dams, especially the Sabalan Dam, have increased the monthly base flow rate over the downstream reaches.
It can be concluded that the construction of the Yamchi Dam caused a decrease in the flow amount in the downstream area and increased the occurrence of hydrological drought events. While, the Sabalan Dam regulated the flow regime of the Garesou river in the dam downstream and reduced the severity of hydrological drought spells and modifies the river flow regime for all months of the year.