Introduction
In order to estimate the environmental water rights of rivers in the sustainable management of water resources and the preservation of ecosystems in watersheds, various methods are examined, taking into account hydrological, ecological, and hydraulic conditions, and flexibility and compatibility with the region. Methods for determining environmental flows are classified into four groups: hydrological, hydraulic, habitat simulation, and comprehensive. The evaluations conducted indicate that the assessment of environmental needs in important rivers is a priority and therefore it is necessary to estimate them in accordance with the conditions of the region. In this regard, this important issue has been carried out in the present study using hydrological methods as a case study in the Tajan River using statistical data collected during the water years 1370-1395.
Methodology
The Tajan River basin, with an area of about 4015.88 square kilometers and geographical coordinates of 52 degrees and 56 minutes to 54 degrees and 59 minutes east longitude and 35 degrees and 56 minutes to 36 degrees and 49 minutes north latitude, is located in Zone 39 of the UTM geographical system in Mazandaran Province. The perimeter of the basin is close to 414 kilometers due to its large extent and elongation. The Shahid Rajaee Reservoir Dam is a double-arch concrete type that was built on the Tajan River about 40 kilometers south of Sari city at the location of Sulayman-Tangeh with a geographical longitude of 53 degrees and 13 minutes and a geographical latitude of 36 degrees and 14 minutes. The distinction and innovation aspect of this research compared to research on a similar subject is based on the fact that, at first, one of the rivers in the north of the country, which is located in the vicinity of the main and important dam in the study area, was studied. Secondly, the assessment of the effects of dam construction is very extensive and in addition to evaluating the hydrological characteristics on the river flow conditions, the situation resulting from its impact on other parameters is also carried out. The selection of a station (hydrometry) to analyze the flow conditions in the Tajan River depends on the location of the dam in the region. For this purpose, the statistics of the stations obtained from the Regional Water Company of Mazandaran Province are evaluated with respect to the dispersion of the stations relative to the dam.
Results and Discussion
This study was conducted to investigate the environmental characteristics and impacts of Shahid Rajaee Dam on the conditions of the study area. In addition, using hydraulic and hydrological methods, the amount of environmental flow of this river in the upstream and downstream stations was estimated and its impact on the characteristics of the river was presented. The chemical quality of Tajan River water for agricultural purposes was determined as C2S1 based on the minimum and maximum changes in the average values of two factors: sodium absorption coefficient and electrical conductivity according to the Wilcox classification. In terms of drinking, the quality of river water in all study stations was acceptable in terms of total hardness and concentration of dissolved substances in the upstream and middle areas. Biologically, in all studied stations, the river water is unsuitable for drinking purposes and requires purification and chlorination. Monthly flow rate changes at two stations, Soleiman-Tangeh and Rig-Cheshmeh, as upstream and downstream stations of the dam, were investigated during the 26-year statistical period from 1991 to 2016. The results for Soleiman-Tangeh station indicate flow control in the periods after the construction of the dam from the beginning of the water year to Farvardin. After that, due to the release of flow during the cultivation seasons, the peak flow value increases significantly compared to before the construction of the dam. In the hydrological method, the flow continuity curve transfer method, Tennant and Tasman, and in the hydraulic method, the wetted medium method were used as appropriate approaches in this regard. The results indicated that the average environmental flow in the Tajan River using different methods was different according to the desired method, and on the other hand, the construction of the dam led to the released amount not being in good agreement with the calculated amount. The highest value obtained in the flow continuity transfer curve method was 25 cubic meters per second, so that in these conditions, from a management perspective, it is in biological class C with the approach of maintaining the minimum biological conditions of the river. The average flow value in the presence of the dam during the release period is about 16 cubic meters per second, which is relatively consistent with the values calculated in other methods.
Conclusion
The amount of released flow is about one-third of the minimum environmental flow required by the river downstream. Comparing the statistics of the two stations shows that the changes in water quality parameters at the Sulayman-Tangeh station are more noticeable than at other stations. Also, the changes in parameters at the Rig-Cheshmeh station are significant compared to Sulayman-Tangeh. The study of environmental flow conditions and also flow quality after the construction of the dam indicates that despite the fact that the amount of released flow is about one-third of the environmental requirement obtained from the flow continuity curve method, for various reasons such as high groundwater levels, appropriate rainfall in the basin, temporal distribution of rainfall, etc., no serious problems have been observed, and as a result, no significant changes have been observed in the environmental characteristics and quality characteristics.