Assessment of the determinants of salinity in the Gotvand Dam reservoir and the contribution of outlet structures to its regulation

Introduction
Salinity accumulation and the consequent deterioration of water quality in reservoir systems represent a critical challenge in water resources management, particularly in arid and semi-arid regions where evaporation rates are high and freshwater availability is limited. In such environments, even moderate increases in salinity can significantly reduce the usability of stored water for agricultural, domestic, and industrial purposes. The problem becomes more severe in reservoirs located downstream of evaporite geological formations, where continuous dissolution of salt-bearing strata introduces persistent and often difficult-to-control saline inflows. These processes not only degrade water quality but also complicate reservoir operation, ecological stability, and downstream water allocation. The Gotvand Dam reservoir in southwestern Iran is a well-documented and particularly severe example of this phenomenon. Since its design and construction phases, the reservoir has been recognized as vulnerable to salinity intrusion due to its proximity to the Gachsaran Formation, specifically the Anbal salt section. Following impoundment, rapid salinization of reservoir water confirmed concerns raised in early feasibility studies, making it one of the most significant salinity-impacted reservoirs in the region. Previous studies have primarily focused on identifying salinity sources and simulating its distribution; however, there remains a lack of long-term, field-based evaluations of salinity control strategies. This study aims to address this gap by investigating the temporal evolution of salinity within the reservoir and evaluating the effectiveness of outlet structures, particularly the bottom outlet and GRP pipe, in controlling salinity over a 13-year operational period. By integrating extensive field measurements with mass balance and stratification analyses, this research aims to provide a comprehensive understanding of both natural processes and operational interventions influencing reservoir salinity dynamics.
Materials and Methods
The study was based on an extensive and continuous dataset collected over 13 years from August 2011 to October 2024, comprising more than 1,400 sampling days. The collected data included inflow and outflow discharge measurements, salinity concentrations of released water, vertical salinity profiles at multiple depths, reservoir water level fluctuations, and detailed operational records of outlet structures. Salinity observations were primarily conducted at the deepest point near the dam body, which was confirmed through preliminary analysis to be representative of the overall vertical salinity structure of the reservoir. This location provided a reliable proxy for assessing stratification dynamics and the effectiveness of withdrawal operations across different depth layers. The reservoir system receives its primary inflow from regulated releases of the upstream Masjed Soleyman Dam, supplemented by contributions from intermediate sub-basin areas. Outflows from the reservoir occur through four main structures: the hydropower intake, the spillway, a bottom outlet located at an elevation of 123 meters, and a GRP pipe installed at an elevation of 90 meters. To analyze salinity behavior, the study employed a combination of time-series analysis, vertical stratification assessment, and salt mass balance modeling. Stratification assessments were used to evaluate vertical salinity gradients and the stability of density-driven layering under different hydrological and thermal conditions. The mass balance approach quantified salt inputs, outputs, and storage variations within the reservoir system, enabling a system-scale understanding of salinity accumulation and removal processes. Furthermore, the performance of outlet structures was evaluated under different operational scenarios by correlating changes in salinity profiles with discharge regimes.
Results and Discussion
The results indicated that reservoir salinity dynamics were governed by a combination of hydrological, thermal, and operational factors. Water level fluctuations, inflow characteristics, and seasonal temperature variations significantly influenced salinity stratification. During warm periods, inverse stratification was observed in upper layers due to evaporation and inflow of warmer saline layers, whereas colder seasons exhibited more uniform salinity profiles near the surface. Flood events played a dual role by both inducing vertical mixing and introducing additional salt loads, particularly from intermediate sub-basin interacting with saline formations. However, the intensity of salt dissolution from the Anbal formation appeared to decrease over time, suggesting a reduction in readily soluble salt sources. A key finding of this study was the differentiated performance of outlet structures in salinity management. The bottom outlet had proven highly effective in removing saline water from intermediate layers (approximately 120–160 m elevation), thereby reducing salinity gradients and preventing upward migration of saline layers toward the power intake level. Sustained operation of this outlet (even at moderate discharges of 5–10 m³/s) significantly stabilized salinity conditions in these layers. In contrast, the GRP pipe, designed to evacuate highly saline water from deeper layers, exhibited limited effectiveness due to its low discharge capacity. While it can locally reduce salinity near its intake elevation (90–100 m), its overall impact on reservoir-scale salinity control was negligible. Moreover, its operation contributed to increased salinity in downstream systems. The salt mass balance analysis further corroborated these findings. While the total salt load entering the reservoir from upstream flows was substantial, its concentration remained relatively low. Conversely, salt inflow from the Gachsaran Formation showed a decreasing trend over time, except during major flood events. The total salt storage in the reservoir increased sharply during the initial impoundment phase but stabilized in subsequent years, reflecting the effectiveness of controlled outlet operations, particularly after 2019.
Conclusion
This study provided robust, long-term empirical evidence on salinity dynamics and management in a reservoir affected by evaporite formations. The findings demonstrated that while natural factors such as hydrology and temperature influenced salinity distribution, operational strategies played a decisive role in its control. Among the evaluated measures, selective withdrawal through the bottom outlet emerged as the most effective strategy for managing salinity, particularly in intermediate layers. In contrast, the GRP pipe had limited practical utility despite its intended purpose. The results highlighted that optimizing the operational regime of the bottom outlet was essential for sustainable salinity management in the Gotvand Dam reservoir. Furthermore, the study underscored the importance of continuous monitoring and improved data availability, especially for intermediate inflows, to enhance the reliability of future assessments and support informed reservoir management decisions.