Investigation of main parameters causing decline in water level of Lake Toba during last six decades

Lake Toba’s water level has declined in the last six decades. Hence, a comprehensive study has been conducted, which is expected to explain the leading causes of Lake Toba's water level decline. The Lake Toba water level discussion is divided into two periods, Period I (1957–1978) and Period II (1981–2020), before and after Siruar Dam’s operation. Climate change and human activities have become significant concerns. Trend tests, quantile mapping bias correction, and water balance were used in this study. The result of the trend test is an increasing trend of rainfall and temperature, while evaporation demonstrates a decreasing trend. In addition, there has been a rising trend in the consumption of lake water for domestic and non-domestic needs, agricultural irrigation, and industrial use. Significant changes in land use have also occurred, especially the decline in dryland forests and an increase in industrial forests, settlements, and dryland agriculture. In Period I, the dominant cause of the water level decrease in Lake Toba was climate. In contrast, the leading cause in Period II was human activities such as the water needs of hydroelectric power plants.