This section presents the validation results of the QM bias correction model, where the corrected data will be used to generate the trends in the Lake Toba region. The trend analysis for climate change detection uses the MK statistical test (Z), Sen slope (Q) and significance level (SL). Positive and negative Z values indicate an up and down trend, respectively. The value of Q describes the magnitude or rate of trend change per year in a time series. The SL value shows how statistically significant or strong the trend signal is in the time series. Trend strength is divided into four categories based on the level of significance: (1) very weak trend (α > 0.1), (2) weak trend (0.05 < α < 0.1), (3) strong trend (0.01 < α < 0.05) and (4) very strong trend (α < 0.01), as shown in Figs. 3 and 4. Meanwhile, the impact period (1981–2020) is compared to the base period (1951–1980) to identify any changes, which are analysed per decade. The monthly and seasonal scale analyses presented in this study cover the period of the first dry season (December through February), the first rainy season (March through May), the second dry season (June through August) and the second rainy season (September through November).
Quantile mapping bias correction
Table 1 shows the distribution of observed air temperature and ERA5-Land temperature, as well as observational rainfall and ERA5-Land rainfall. The observed temperature and ERA5-Land temperature tend to fit the generalised extreme value distribution approach, except in January, during which they are closer to the logistics distribution approach, while in April, October and December they tend to fit the Normal distribution approach. In the observation of rainfall, the tendency is to use the Weibull distribution, except in September and December, during which they tend to match the Gamma distribution approach and generalised extreme value. While in ERA5-Land rainfall, the tendency is to match the generalised extreme value distribution, in January they show the Log-normal distribution, in February the Inverse Gaussian distribution, in May and August the Gamma distribution and in June and September the generalised extreme value distribution. Moreover, October and December show the Log-logistics distribution. The differences in the results of distribution identification for each month are caused by the differences in the slope and symmetry of data distribution. Thus, a distribution pattern that can describe the observation data and ERA5-Land is needed.
Table 1
Identification of the data distribution of observation temperature and temperature of ERA5-Land and observation rainfall and rainfall of ERA5-Land
No | Temperature | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
1 | Observasi | LOG | GEV | GEV | NOR | GEV | GEV | GEV | GEV | GEV | NOR | GEV | NOR |
2 | ERA5-Land | GEV | GEV | GEV | GEV | GEV | GEV | GEV | GEV | GEV | GEV | GEV | GEV |
| Rainfall |
1 | Observasi | WB | WB | WB | WB | WB | WB | WB | WB | GAM | WB | WB | GEV |
2 | ERA5Land | LN | ING | GEV | GEV | GAM | WB | GEV | GAM | GEV | LL | GEV | LL |
The accuracy of the model is determined by the quantile value. Therefore, it is necessary to validate the value by using the MAE value. The quantile values of 2, 4, 6, 8, 10, 14, 16, 18, 20, 40, 50, 60, 70, 80, 90 and 100 were tested to obtain the optimum MAE value. The most optimum MAE value for temperature was found to be at quantile 4 (Fig. 2a). As for rainfall, the optimum MAE value was observed at quantile 80, which is constant up to quantile 100 (Fig. 2b). In the case of temperature, the data do not show a large variation, so a small quantile value is sufficient. This differs from the rainfall that has a very high and heterogeneous variation. Consequently, a large quantile value is used to accommodate such conditions.
The MAE value determines the model’s accuracy so that the results of validation can verify whether this model is suitable or not. The validation of the corrected temperature model shows a better result compared to the model data that have not been corrected. As shown in Fig. 2c, the corrected MAE value is smaller than the uncorrected one. The same finding is also obtained in the case of rainfall validation (Fig. 2d).
Temperature trend and temperature change
Figure 3 shows the results of SL, Z and Q, the monthly temperatures in the Lake Toba region from 1981–2020. The spatial distribution of trends in the study area generally shows an increasing trend. An SL of 67% shows a strong to very strong increasing trend that occurred in December, January, March, May, September, October and November. In general, the trend of a weak temperature increase of 33% is spatially distributed in April and June. Meanwhile, in February, July and August, the trend of a weak temperature increase was generally distributed only around the Lake Toba catchment area. The rate of increasing temperature trends based on MK and Q shows a 50% rate of increase in temperature of 0.007–0.009°C per year and a 33% rate of increase in temperature of 0.004–0.006°C per year, while the rate of increase is 0.001–0.003°C and > 0.009°C per year, each by 13% and 3%, with an average increase rate of 0.006°C per year. Generally, a strong trend of increasing temperature occurs in the first dry and rainy seasons, while the second dry seasons tend to have a weak increasing trend. Meanwhile, the rate of temperature increase in the Lake Toba catchment area did not rise significantly compared to outside of the Lake Toba catchment area, especially in the first and second dry seasons.
Analyses of changes in temperature were carried out over the decades of 1981–1990, 1991–2000, 2001–2010 and 2011–2022, relative to the period of 1951–1980, to show the impact of temperature changes in the Lake Toba area (Fig. 4). In general, the temperature change, with an increasing trend every decade, is observed at the study site. The magnitudes of the temperature changes were 0.09°C, 0.06°C, 0.15°C and 0.24°C, respectively. The spatial distribution shown in Fig. 5 also indicates a similar tendency, that is, in the first two decades there was an increase of 0.1–0.2°C, except that the southern part of the Lake Toba catchment area showed a temperature decrease of about 0.002°C in the second decade. In the third decade, the temperature generally increases by 0.11–0.2°C, except in the southern part of the Lake Toba catchment area, where there is an increase of 0.1°C. While the last decade was the hottest, with a significant increase in temperature ranging from 0.21–0.3°C, in some areas of Lake Toba, there was a lower temperature increase (i.e. 0.11–0.2°C).
Precipitation trends and changes in precipitation
The spatial distribution of monthly rainfall trends in the study area, shown in Fig. 6, generally shows an increasing trend. The trends of increasing rainfall are very weak, weak, strong and very strong, at 39%, 8%, 8% and 15%, respectively, while the decreasing trend is very weak at 30%. The trend of increasing rainfall is distributed in January, February, April, June, August, October, November and December, while the decreasing trend of rainfall is distributed in March, May, July and September. The rates of increase in rainfall of 0.1–1 mm, 1.1–2 mm, 2.1–3 mm and > 3 mm per year are 31%, 30%, 7% and 3%, respectively. Meanwhile, the rate of decrease in rainfall of (-1)-0 mm per year is 30%, and the average increase rate is 0.71 mm per year. Generally, the trend of increasing rainfall occurs in the dry period and the second rainy season.
The change in rainfall over a decade, as shown in Fig. 4, indicates a change in rainfall in the Lake Toba area. There was a change in rainfall around the study area, with the increasing trends for each decade of 6%, 17%, 17% and 22%, respectively. The spatial distribution shown in Fig. 7 also shows the same tendency, that is, an increase in rainfall of 0–10% in the early decades and an increase of 11–30% in the second and third decades. Generally, an increase of 21–30% is distributed in the western part of the Lake Toba catchment area in the second decade, while its distribution extends to the east and along the Bukit Barisan Mountains, stretching from north to south in the third decade. The fourth decade has the highest increase in rainfall, where the spatial distribution of the increase in rainfall of 21–40% is distributed in the Lake Toba catchment area and along the Bukit Barisan Mountains.