Assessing the trend in low flows and the impact of climate change using the Standardized Precipitation–Evapotranspiration Index and total runoff: A case study of Bintan Island, Indonesia

Abstract Understanding the characteristics of low flows over time is critical to sustainable water resource management in Bintan, a small tropical island in Indonesia. Low flows are driven by precipitation and controlled by the soil and geological parameters of a catchment. A method that integrated the Standardized Precipitation–Evapotranspiration Index (SPEI) and the calculated total runoff (TRO) was developed in order to analyze the change in low-flow intensity over time. This method included steps for the selection of an appropriate SPEI timescale and the development of low-flow intensity classes based on the TRO, and was used to assess the trend in low-flow intensity change in Bintan Island for 1981–2020 and the impact of climate change on low-flow intensity for 2031–2070. The results showed that low-flow intensity in the second half of the 1981–2020 period was 45% less than in the first half, with an estimated increase in low-flow volume of 405 mm (22.4%). The climate-change projection for 2031–2070 indicated an increase in low-flow intensity of 14% compared with 1981–2020, with an estimated 12% decrease in low-flow volume. The integrated method indicated a trend similar to that of a method using only the SPEI, but with better performance in estimating the low-flow volume change, thus providing a better understanding of how the low-flow intensity will change over time.