Return period of low water periods in the river Rhine

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. 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 1 day discharge of 732 m3/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. A 180 day average discharge of 1017 m3/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. The method used can be applied to other rivers to find return periods for low flows, as this is currently not commonly found.