Impacts of Climate Change on the Seasonality of Extremes in the Columbia River Basin

Abstract

The impacts of climate change on the seasonality of extremes i.e. both high and low flows in the Columbia River basin were analyzed using three seasonality indices, namely the seasonality ratio (SR), weighted mean occurrence day (WMOD) and weighted persistence (WP). These indices reflect the streamflow regime, timing and variability in timing of extreme events respectively. The three indices were estimated from: (1) observed streamflow; (2) simulated streamflow using simulated inputs from ten combinations of CMIP5 inputs for the future climate (2040–2070) including the pathway RCP4.5 (3) simulated streamflow using simulated inputs from ten combinations of CMIP5 inputs for the future climate (2040–2070) including the pathway RCP8.5. The hydrological model was calibrated at 1/16 latitude-longitude resolution and the simulated streamflow was routed to the subbasin outlets of interest. These three cases are compared to assess the effects of forcing by different climate models and different pathways on the three indices. The results showed significant differences between three cases indicating a shift in streamflow regime and timing of extreme events such as high and low flows in the Columbia River Basin. The persistence of high flows are similar in all cases. The results will help to understand the effects of climate change on three important seasonality properties: regime, timing and persistence and associated errors. Conclusion Multi-model Averaging of 10 Climate Forcing Data Results: Climate change impacts on high flows (Q90) Acknowledgment  High flows occur between mid-April and June: ~day 100th and 180 respectively in Fig 2  Winter low flows are observed in snow dominated northern and southeastern CRB where precipitation is kept as snow or ice in winter (Fig 2).  High flow persistence is increasing in all six locations by 2040s (Table 1)  No significant difference was found between RCP45 and 85 for WP  SR is increasing in all locations except for Revelstoke  Weighted mean high flow occurrence day (WMOD) is about ten day late as compared to observed WMOD of high flows. The delay is 20 days in Orofino by 2040s based on RCP85 scenario The Seasonality Ratio (SR): This index reveals the streamflow characteristics in summer and winter periods. The definitions of extremes (high or low flow) and the seasons (months for winter and summer) are crucial for the SR results as the underlying hydrological processes for summer and winter streamflows are different. In this study we focus on high flows and use the 90% exceedence probability (Q90) as a threshold for defining summer high flow (Q90s) and winter high flow (Q90w). The SR index is calculated as the ratio of Q90s and Q90w (Eq 4) A value of SR greater than one indicates the presence of a winter high flow regime and a value smaller than one indicates the presence of a summer high flow regime. Weighted Mean Occurrence Day (WMOD) The days at which the discharge is above the Q90 threshold are transformed into Julian dates Di, i.e. the day of the year ranging from 1 to 365 in regular years and 1 to 366 in leap years. The day number of each high flow event (Di) is weighted by the inverse high flow value (1/Qi) on the same day to address the severity of a high flow event as well as its occurrence day. The weighted mean day of occurrence is estimated first in radians to represent the annual cycle correctly. Otherwise, a simple averaging of high flow occurrences in winter months, e.g. January and December, can lead to a large error in the results. The weighted mean of Cartesian coordinates xθ and yθ of a total of high flow days i is defined as The values of θ can vary from 0 to 2π, where a zero value indicates the 1st of January, π/2 represents the 1st of April, pi represents the 1st of July and 3π /2 represents the 1st of October. The main advantage of using circular statistics is that it allows us to correctly average high flow occurrences in the winter half-year period. The WMOD is then obtained by back-transforming the weighted mean angle to a Julian date: (4)