Future projections of streamflow magnitude and timing differ across coastal watersheds of the western United States.

ABSTRACT: Climate change has and will continue to impact the magnitude and timing of streamflow throughout the world. While streamflow timing of western United States watersheds has shifted earlier in mountainous snow‐dominated watersheds due to earlier snowmelt, timing for rain‐dominated coastal watersheds has shifted to later in the year. Despite these dissimilarities, coastal watersheds have received little attention in the literature. This research assesses changes in projected climate and hydrology for five small coastal basins (42–718 km²) in Washington, Oregon, and California. Projections of temperature and precipitation from General Circulation Models (GCMs) for Representative Concentration Pathways 4.5 and 8.5 are coupled with the Soil and Water Assessment Tool hydrologic model to simulate future hydrology for each watershed. The response of climate, streamflow discharge, and timing was analysed for each watershed by comparing the projected mid‐21st century (2035–2064) and late‐21st century (2070–2099) to the historical period (1970–1999). While temperature is projected to increase at each watershed from 1 to 6 °C by the end of the 21st century, precipitation varies widely with the median and mean change across all watersheds and GCMs being close to zero (just 2.0 and 3.6%, respectively). Changes in the magnitude of peak winter streamflow discharge differ across the region compared to the historical period. Streamflow timing also features latitudinal divergence, with the southern California watershed projecting later timing by 13 days and all other watersheds indicating smaller shifts (0–7 days) for the end of the 21st century compared to the historical time period, largely from corresponding shifts in precipitation timing. The projected decreases in peak winter streamflow magnitude, particularly in central and southern California, have critical implications for water supply, which is already strained from extensive drought and high demand. This research makes strides to assess new trends across coastal watersheds. [ABSTRACT FROM AUTHOR]