Will extreme drought impact the reservoir water quality? A 30-year observational study

Climate change affects the security of the global water supply. However, only a few studies on the effects of drought on water quality in Asia have been published. In 2021, Taiwan faced its most severe water crisis in the past 56 years. The Te-Chi reservoir, situated at an elevation exceeding 1420 m, stands out as significantly affected by hydrologic variability. Overcoming geographical challenges, we captured an exceptional climatic period to sample and analyze water quality. Subsequently, we investigated multivariate long-term water quality data incorporating climate and hydrological information to evaluate the effect of climate change on the reservoir. The results showed the annual average temperature had risen by about 1.4 °C while total annual rainfall declined over the past 18 years. 30-year water quality data from 1993 to 2022 was analyzed to examined the long-term water quality change. There was an observed upward trend in the electrical conductivity of the reservoir water. The concentration of dissolved oxygen showed a decreasing trend, which may be attributed to the increase of temperature due to climate change. Besides, as the water level dropped during the drought, we found that the concentration of total phosphorus, total nitrogen and electrical conductivity in Te-Chi reservoir tended to increase, while the transparency of the water decreased. Consequently, we further explored the potential pollution sources during extreme drought by multivariate statistical methods, such as Pearson’s correlations, principal component analysis, and factor analysis. We found that four varimax rotated principal components collectively explain 82% of the variation in water quality data of the Te-Chi reservoir with the reservoir water level, signifying their representativeness in interpreting the monitoring data. We also assessed the contribution of each factor to specific water parameters, which can aid in better management of water resources during such challenging conditions. In conclusion, this study provides evidence and raises awareness on the effect of climate change by long-term water quality data with multiple parameters. The water quality components in the reservoir are affected by extreme weather, which is predicted to occur more frequently in the future and thus could affect energy scarcity, public health and food safety for sustainable reservoir operation. The study underscores adaptive management, mitigation strategies and the future research direction.