Evaluation of Lake Toba's water level decline in Indonesia over the past six decades

Lake Toba's water level has significantly declined during the last six decades. Unfortunately, the studies that have been conducted are only sectoral, both in terms of temporal aspects and influencing parameters. Hence, a comprehensive study has been conducted to explain the significant factor causing the Lake Toba's water level decline. Climate change and hydrological issues have become significant concerns. Change point analysis, trend tests, and Structural Equation Model (SEM) were used in this study. Since the results of the analysis show a significant decrease in the water level of Lake Toba in 1984, the discussion of the water level of Lake Toba is divided into two periods, namely Period I (1957–1983) and Period II (1984–2020). This division is also in line with the periods before and after the operation of Siruar Dam. This study revealed that climate change has occurred in the Lake Toba watershed with a significant increasing trend in temperature, evaporation, and rainfall of 0.003 °C, 1.4 mm, and 8.9 mm per year, respectively. Climate factors primarily contributed to the decline in Lake Toba's water level in Period I, whereas the use of water for hydroelectric power plants had a greater impact in Period II. However, with effective regulation of the Siruar Dam during Period II, the water level of Lake Toba has been maintained at a stable level within the predetermined threshold. The study's findings are intended to be useful to the government, stakeholders, and water users in devising mitigation and adaptation measures for the long-term management of Lake Toba's water resources.