Trends in Global Flood and Streamflow Timing Based on Local Water Year

Abstract

Analysis of flood and streamflow timing has recently gained prominence as a tool for attribution of climatic changes to flooding. Such studies generally apply circular statistics to the day of maximum flow in a calendar year and use nonparametric linear trend tests to investigate changes in flooding on a local or regional scale. Here we investigate both the center timing of streamflow and the day of maximum flow using a local water year. For each station, the start of the water year is defined as the month of lowest average monthly streamflow. This definition of water year prevents ambiguity in the direction of computed trends and enables flood and streamflow timing to be described by a normal distribution. Using the assumption of normality, we calculate the historical trend in both flood and streamflow timing using linear regression. While shifts in flood and streamflow timing are consistent with climate change and are shifting in a similar direction, shifts in the timing of the annual maxima flood are approximately three times that of streamflow timing. The results here have implications for water resources and environmental management where streamflow and flood timing are critical to planning. The applicability of the normal approximation to flood and streamflow timing will enable future analyses to use parametric statistics.

Evaluation of the Drivers Responsible for Flooding in Africa

“…The monthly time series of runoff are available from the ADHI database (Tramblay et al, 2021). This method ensures a more consistent definition of the water year (Wasko et al, 2020b).…”

Changes in Mechanisms and Characteristics of Western U.S. Floods Over the Last Sixty Years

“…On the other hand, climate‐driven changes in flooding have not been widely observed (Archfield et al., 2016; Do et al., 2017; Hall et al., 2014; Hodgkins et al., 2017; Lins & Slack, 1999), and in general, there is no consensus as to whether floods are changing in response to precipitation changes (Sharma et al., 2018). While there is an expectation, based on the first principle, that flood dates in snow‐affected river basins should shift earlier in the year (Bindoff et al., 2013), the later shift of flood timing is also observed in southern Australia and part of the Europe (Bloschl et al., 2017; Wasko et al., 2020).…”

“…While numerous studies have explored how rainfall and snow regime changes affect floods (Archfield et al., 2016; Do et al., 2017; Wasko et al., 2020; Wasko & Sharma, 2017), it is not well understood how changes in flood characteristics (frequency, magnitude, and timing) are linked to flood generating mechanisms. Previous hydrological research has generally focused on specific flood events, with area and event‐specific characterization of hydrometeorological variables and physiographic factors (Bloschl et al., 2013; Ralph et al., 2006).…”

“…based on the first principle, that flood dates in snow-affected river basins should shift earlier in the year (Bindoff et al, 2013), the later shift of flood timing is also observed in southern Australia and part of the Europe (Bloschl et al, 2017;Wasko et al, 2020).…”

Reply to RC1

“…Thank you for pointing this out. References toWasko et al (2020a),Barnett et al (2008), andWasko et al (2020b) will be added (as well as the others). Alongside Gu et al (2017) we will also addMarelle et al (2018) andBrönnimann et al (2018).…”