Peak Runoff Timing Is Linked to Global Warming Trajectories

The earth’s hydroclimate is continuing to change, and the corresponding impacts on water resource space‐time distribution need to be understood to mitigate their socioeconomic consequences. A variety of ecosystem services, transport processes, and human activities are synced with the timing of peak annual runoff. To understand the influence of changing hydroclimate on peak runoff dates across the continental United States, we downscaled outputs of 10 Global Circulation Models for different future scenarios. Our results quantify robust spatial patterns of both negative (up to 3–5 weeks) and positive (up to 2–4 weeks) shifts in the dates of peak annual runoff occurrence by the end of this century. In snowmelt‐dominated areas, annual maxima are projected to shift to earlier dates due to the corresponding changes in snow accumulation timing. For regions in which the occurrence of springtime extreme soil wetness shifts to later time, we find that peak annual runoff is also projected to be delayed. These patterns of runoff timing change tend to be more pronounced for projections of higher greenhouse concentration in the future.Plain Language SummaryThe occurrence of peak annual runoff characterizes the major phase of watershed surface hydrology. Many natural dynamics and human activities are synced with the timing of its occurrence, ranging from ecosystem services and channel transport of sediments and contaminants to reservoir refilling and management. The sensitivity of peak annual runoff timing to changing hydroclimate remains unknown. In this work, we identify how peak annual runoff occurrence will change in the future over the continental United States using outputs of several climate models. Spatial patterns of the change show both earlier (by up to 3–5 weeks) and delayed (up to 2–4 weeks) occurrence of peak runoff. We attribute these timing changes to the shifts in snowmelt and springtime soil moisture processes. Specifically, areas in which snowmelt drives watershed hydrology exhibit earlier dates of maximum snow accumulation and peak runoff. In regions where peak runoff is projected to occur later, we find a tendency for later occurrence of full saturation conditions. Earlier and later peak runoff occurrence can potentially lead to competing water use interests and aggravating concerns for aquatic environments and their ecosystem services.Key PointsClimate multimodel ensemble projects change of peak annual runoff timing over the continental United States during the 21st centurySpatial patterns of peak runoff timing earlier onset as well as delay are more pronounced for higher future greenhouse concentrationsSpringtime shifts in the dates of maximum snow accumulation and soil moisture wetness are associated with changes in peak runoff timingPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/169318/1/eft2877_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/169318/2/2021EF002083-sup-0001-Supporting_Information_SI-S01.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/169318/3/eft2877.pd