The Increasing Role of Seasonal Rainfall in Western U.S. Summer Streamflow

• Published in: Geophysical research letters Vol. 50; no. 9
• Main Authors: Ban, Zhaoxin, Li, Dongyue, Lettenmaier, Dennis P.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 16.05.2023; Wiley
• Subjects: Algorithms; Climate; Climate change; Climate prediction; climate warming; declining snowmelt; Drought; Ecological function; Global warming; Hydrologic models; Hydrology; Mathematical models; Precipitation; Rainfall; Rivers; Runoff; Seasonal rainfall; Seasons; Snow; Snow accumulation; Snowmelt; Snowpack; Stream discharge; Stream flow; streamflow source contribution tracking algorithm; Streamflow variations; Streams; Summer; summer streamflow; Tracking; Warm winters; Water resources; Water supply; western U.S; Winter; winter rainfall; Winter snow; Western states; United States > US
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2023GL102892

Summer streamflow variations strongly affect water supply reliability and ecological functioning of western U.S. (WUS) streams. Traditional snow‐based forecasts of summer streamflow are becoming less accurate with warming‐induced reductions in winter snow accumulation. This reflects a rising importance of competing runoff‐generating processes in controlling summer streamflow variations, primarily an increasing role of rainfall in contrast to snowmelt. Here, based on a snowmelt‐rainfall tracking algorithm applied to two hydrological models, we show that cool‐season rainfall provides an important volumetric contribution to summer streamflow for many WUS streams in the current climate, and this contribution will increase under climate warming, especially in years with warm snow droughts and abnormally dry summers. We also show that seasonal rainfall (warm‐/cool‐seasons) dominates the variability of summer streamflow across ∼70% area of WUS. We show that an increasing warm‐season rainfall contribution to summer streamflow (largely replacing snowmelt) results in reduced summer streamflow predictability. Plain Language Summary Summer streamflow is a critical water resource in the generally dry summers of the western U.S. (WUS), and is routinely forecasted using spring snowpack and/or winter total precipitation as primary predictors. However, climate warming leads to reduced snowpacks, exacerbates summer low flows, and reduces the accuracy of snow‐based summer streamflow forecasts. On the other hand, the role of winter rainfall as a control on summer streamflow increases in a warmer climate. Here, we explicitly quantify the contributions from cool‐season rainfall, warm‐season rainfall, and snowmelt to summer streamflow across the WUS, and how they change under a uniformly 1°C warmer climate. We show that the cool‐season rainfall contribution to summer streamflow increases under warming across WUS, especially in streams that currently have low‐to‐moderate snow contributions to runoff, and in years with anomalously warm winters and/or dry summers. We also show that the warm‐season rainfall contribution to summer streamflow increases widely, especially in the southern interior of WUS in a warmer climate, and that increasing warm‐season rainfall contribution to summer streamflow (largely replacing snowmelt) results in reduced summer streamflow predictability. Key Points The cool‐season rainfall contribution to summer streamflow is greatest in low‐elevation coastal streams with dry summers Climate warming leads to an increased contribution of seasonal rainfall to summer streamflow as spring snowmelt contributions decline Summer streamflow predictability declines with reduced snowmelt and increased warm‐season rainfall contribution in a warmer climate