3.1.1 Analysis of Lake Toba water level
This study examined Lake Toba’s water level over two periods: Period I, before the Siruar Dam was constructed (1957–1978), and Period II, after the Siruar Dam’s operation (1981–2020). The dam was constructed between 1979 and 1980. In Period I, the average water level of Lake Toba was approximately 905.2 masl, while in Period II, it was nearly 904.1 masl. The water level trend in Period I decreased by 3.3 mm per year. The water level trend in Period II tended to decrease by 7.6 mm per year (Fig. 2a). Analysis of the monthly averages demonstrated a significant decrease between Periods I and II. There was a consistent decline in the water level of Lake Toba for each month, based on the data for these two periods. It is also important to note that May had the highest water level while September had the lowest (Fig. 2b).
3.1.2 Analysis of climate change
Detecting and quantifying trends in climate variables are essential for assessing the impact of natural changes. Many studies have analyzed trend of changes in rainfall and temperature in climate variables 1,3,49. The climate parameters analysed include annual average temperature, annual evaporation, and annual rainfall. The annual average temperature in the Lake Toba watershed from 1957–2020 exhibited a rising trend of 0.003°C per year. Period I recorded a rising trend of 0.001°C per year, while Period II showed a significant increase of 0.005°C. The annual average temperature was 20.3°C; the highest was 20.6°C in 2016, and the lowest was 20.1°C in 1999 (Fig. 3.a). The annual evaporation in 1957–2020 shows a declining trend of 1.6 mm per year. In Period I, there is an increasing trend of 2.5 mm per year, while in Period II, there was a significant decrease, namely 0.9 mm per year. The highest annual evaporation was 1301.7 mm, which occurred in 1961, while the lowest annual evaporation was 1142.8 mm, which occurred in 2008 (Fig. 3.b). Annual rainfall in 1957–2020 reported an increasing trend of 8.8 mm per year. In Period I, there was a decreasing trend of 6.2 mm per year and a significant increase of 11.9 mm per year in Period II. The highest annual rainfall of 2055 mm occurred in 2008, while the lowest annual rainfall of 1134 mm was recorded in 1975. The increase in annual rainfall in Period II is significant (Fig. 3.c).
3.1.3 Changes in land use
Analysis of land use in the Lake Toba catchment area using Landsat 5, 7, and 8 imageries depicted visible, significant changes from 1988 to 2020. There was a 29% reduction in the dryland forest area with an increase in plantation forests (234%), settlements (22%) and dryland agriculture (9.9%) (Fig. 4).
3.1.4 Use of Lake Toba water
3.1.4.1 Domestic and non-domestic needs, industrial needs, and agricultural irrigation water needs
Along with the increasing population and the community's economy, the need for water for various sectors continues to increase. Figure 5.a and b, based on data from the Central Statistics Agency for North Sumatra Province, illustrate a trend of increasing population growth in the Lake Toba watershed from 1957–2020, with an increasing population of approximately 4,980 people per year. In Period I, the rate of population increase was 5,098 people per year, while in Period II, it was 4,474 people per year. This aligns with the trend shown in Fig. 5.b, where water use for annual domestic and non-domestic needs in 1957–2020 recorded a rising trend of 0.0069 m3/s per year. There was a rising trend of 0.0074 m3/s per year in Period I. In comparison, Period II recorded a rising trend of 0.0063 m3/s per year, with an average annual domestic and non-domestic demands of 0.40 m3/s.
Lake Toba water is also used for industrial processes. In general, water use for industry is relatively constant with variations of months or seasons. With the increasing number of industries, there is a corresponding increase in water consumption. In this study, the industrial use being analysed is limited to the water used for hydropower production. The use of water for hydropower production began in 1981. Lake Toba and Asahan River outflows were used for the Sigura-Gura and Tangga hydroelectric power plants. The average industrial water use was 100 m3/s 14. The highest use of 133 m3/s occurred in 1984, and the lowest of 42 m3/s in 1999. The trend of water use for industry tended to increase by 0.77 m3/s per year (Fig. 5.c).
Several parameters influence the use of water for agricultural irrigation networks. These parameters are: planted area in hectares, plant type, plant growth rate, planting calendar, climatological conditions (rainfall and evapotranspiration), implementation of irrigation systems, soil conditions and irrigation efficiency. The calculated irrigation water must combine irrigation systems, groups and cropping patterns. For the irrigation areas, the data are available only from 2010 to 2020. Based on these, the average water demand for agricultural irrigation was 16.2 m3/s, with an increasing rate of 0.52 m3/s per year (Fig. 5.d).
3.1.5 Lake Toba Water Balance
Lake Toba's water balance analysis exhibited a downward trend in Period I, with a decline rate of 1.03 m3/s, with a surplus water balance. In Period II, there was a downward trend of 0.36 m3/s, with the water balance tending to be in deficit for several years, including 1983–1986, 1989–1990, 1992, 1996–1998, 2000–2001, 2003–2005, 2009–2010, 2012–2016 and 2019–2020 (Fig. 6).
3.2 Discussion
A better understanding of the decrease in the water level of Lake Toba is fundamental to identifying the leading causes or dominant factors for the declining water level16. There was a significant decrease in 1979–1980 caused by the construction of dams and the addition of Lake Toba's water storage area14. In this period, the data were not analyzed to minimize the impact of a permanent lake water level decline. There has been a significant decline in water level in Periods I and II, with the trend of declining water level in Lake Toba in Period II up to more than twice the magnitude of the downward trend in Period I. This finding corroborates the previous research, which underlined that in the last six decades, there had been a slow but steady decline in the water level of Lake Toba13–15. The decrease in Lake Toba's water level can be caused by El Niño activities13,50 and human activities, as in the case of Poyang Lake51 and Dongting Lake52 in China, where the water levels have declined due to human activities comprising construction and regulation of dams. This is in line with the research53 on Lake Urmia in Iran, which states that human activity, ENSO, and climate change are the leading causes of the decrease and variation of the lake's water level.
Based on The Intergovernmental Panel on Climate Change (IPCC) criteria54, climate change has occurred in the Lake Toba watershed. In Period I, temperature and evaporation tended to increase with an increasing trend of 0.001°C and 2.5 mm per year, respectively. This condition was caused by the increasing global average surface temperature54. In comparison, rainfall in this period tended to decrease with a decreasing trend of 6.2 mm per year. The decrease in rainfall in Period I is more associated with the increased El Niño activity during 1957–1978, with El Niño events occurring almost consistently throughout this period. In detail, El Niño events occurred in 1957–1959, 1963–1966, 1969–1970, 1972–1973 and 1976–1978. This also confirms the results13,50, observed that El Niño had an impact on reducing rainfall in the Lake Toba watershed. In Period II, temperature and rainfall tended to increase with a trend of 0.005°C and 11.9 mm per year, respectively. The increase in temperature was possibly caused by the rising average global surface temperature54, while the increase in rainfall was caused more by the decrease in El Niño activity. Evaporation tended to decrease, but not significantly. This phenomenon correlated with the significant changes in land use, with declining natural forest areas and increasing industrial forest, settlement, and agricultural land areas, so the evaporated water tended to decrease. This is in line with the finding3, confirming that climate change has occurred, and others39,55, who reported environmental degradation and land use changes in the Lake Toba catchment area.
Lake Toba is a tourist destination in North Sumatra, Indonesia38. This has serious consequences in the form of increasing population and water consumption for hotels and other public facilities. The increased population led to significant land clearing for settlements in Toba and Samosir regencies. The built-up land area only reached 7.54 km2 between 1990 and 1997, rising to 68.10 km2 between 2013 and 2018. Therefore, open land increased more than eight times in the last 20 years18. This indicates that population growth strongly correlates with the increasing open land area and demand for clean water from Lake Toba. The consumption of Lake Toba water for generating clean water in general also increased with a rising population56. Population growth also triggers an increase in demand for foods, especially those from agricultural products. Data from the Central Statistics Agency highlights that the Toba Regency is part of 10 food centre districts in North Sumatra Province. Thus, it is necessary to develop irrigation with the water sourced from the Lake Toba catchment area.
The central government policy has determined the Lake Toba region as one of the national strategic areas. Industries for aluminium smelting (PT Inalum) and paper raw material processing (PT Toba Pulp Lestari) have been established there. Lake Toba’s water was used massively to support industrial operations in Period II. The water demand from Lake Toba for industrial needs was around 100 m3/s14, while based on calculations, the water availability from Lake Toba was 99.3 m3/s. Even though the trend of Lake Toba water availability increased in Period II with an increasing rate of 1.08 m3/s per year, the demand for Lake Toba water also increased by 0.77 m3/s per year for the industry and 0.5 m3/s per year for agricultural irrigation. This is in line with previous research results1,57,58, which indicated that human activities in the forms of dam regulation and using water for agricultural irrigation and hydroelectric power, caused the reduction in lake water level.
The decreasing water level of Lake Toba in Period I was mainly due to climatic factors since there was a decrease in rainfall in the Lake Toba catchment area. At the same time, human activity had almost no significant effect. In Period II, rainfall trended to increase; this should have positively impacted Lake Toba's water level. However, in Period II, human activity using water for industry, mainly hydropower and agricultural irrigation exhibited a significantly increasing trend. This led to a decrease in the water level of Lake Toba. Lake Toba's water balance experienced a deficit in Period II, and the trend has decreased until now. Therefore, it is necessary to plan the use of water sourced from Lake Toba by involving the government, Lake Toba water consumers such as the industry and the community, and researchers or research institutions. To create sustainable lake water resources, they must conduct further studies to find the solutions for adaptation and mitigation of climate change impacts and human activities.