Author(s): Saskia Van Brenk; Lieke Lokin; Jord Warmink; Suzanne Hulscher

Linked Author(s): Lieke Lokin, Jord Warmink

Keywords: Climate change; Droughts; Return periods

Abstract: The water level measured in the Rhine river has never been as low as in the year 2018. August 2018 even passed the previous record year 1976 for a few days in precipitation deficit. Due to climate change, such low water events are expected to increase in the future. However, how extreme this event was is currently unknown. The goal of this study is to quantify the return period for the 2018 low flows and determine the effect of climate change on low flow return periods on the Rhine at Lobith. Low flow return periods are important for shipping applications or risk assessments concerning water availability and to prevent salinisation. Discharge data from 1901 to 2020 are used and gained from Rijkswaterstaat, the Dutch water authority. The block method is used to define annual minimal discharges. Low Flow Frequency Curves (LFFC) are constructed by fitting the Generalized Extreme Value distribution on these annual minima to be able to extrapolate to larger return periods. Duration of low flows is also a factor of the severity of the situation. Therefore, several durations are taken into account: 1, 7, 30, 90 and 180 days. An average discharge over this duration gives the severity of the low flow. Results of the 95% confidence interval show that a Weibull fit the best fit for all the different durations. Furthermore, the LFFCs show little difference between the 1 and 7 day fit, but generally the larger the duration the higher the discharges. A discharge of 1000 m^3/s is considered for the shipping applications and 1200 m^3/s for the salinisation problem. These have return periods of 2.3 and 1.3 years for a duration of 1 day and once every 34.0 and 8.5 years for a duration of 180 days. A 1 day discharge of 732 m^3/s, which was the annual minimum of 2018, is likely to occur once every 17.6 years. However, due to climate change in 2085 this can occur once every 6.5 to 22.6 years based on the KNMI'14 scenarios and the GRADE model. In 2085, a 1 day event that will occur once every 17.6 years will have a discharge between 655 and 753 m^3/s. A 180 day average discharge of 1017 m^3/s, which was the annual minimum of 2018, is likely to occur once every 29.5 years. However, due to climate change in 2085 this can occur once every 35.3 to 179.5 years based on the KNMI'14 scenarios. In 2085, a 180 days event that will occur once every 17.6 years will have a discharge between 1035 and 1195 m^3/s. The method used can be applied to other rivers to find return periods for low flows, as this is currently not commonly found.

DOI: https://doi.org/10.3850/IAHR-39WC2521711920221485

Year: 2022