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Path analyses of historical streamflow data from the Pacific Northwest indicate that the precipitation amount has been the dominant control on the magnitude of low streamflow extremes compared to the air temperature-affected timing of snowmelt runoff. The relative sensitivities of low streamflow to precipitation and temperature changes have important implications for adaptation planning because global circulation models produce relatively robust estimates of air temperature changes but have large uncertainties in projected precipitation amounts in the Pacific Northwest U.S. Quantile regression analyses indicate that low streamflow extremes from the majority of catchments in this study have declined from 1948 to 2013, which may significantly affect terrestrial and aquatic ecosystems, and water resource management. Trends in the 25th percentile of mean annual streamflow have declined and the center of timing has occurred earlier. We quantify the relative influences of total precipitation and air temperature on the annual low streamflow extremes from 42 stream gauges using mean annual streamflow as a proxy for precipitation amount effects and streamflow center of timing as a proxy for temperature effects on low flow metrics, including 7q10 summer (the minimum 7 day flow during summer with a 10 year return period), mean August, mean September, mean summer, 7q10 winter, and mean winter flow metrics. These methods have the benefit of using only readily available streamflow data, which makes our results robust against systematic errors in high elevation distributed precipitation data. Winter low flow metrics are weakly tied to both mean annual streamflow and center of timing. [ABSTRACT FROM AUTHOR]

ABSTRACT Climate change effects on wildfire occurrence have been attributed primarily to increases in temperatures causing earlier snowpack ablation and longer fire seasons. Variability in precipitation is also an important control on snowpack accumulation and, therefore, on timing of meltwater inputs. We evaluate the correlation of total area burned and area burned severely to snowmelt-induced streamflow timing and total annual streamflow metrics across the Pacific Northwest region from 1984-2005. Principal component scores on total annual water year flow and date of 50th percentile flow (PC1T) in the Pacific Northwest were used as predictors of satellite-inferred area burned and area burned severely in forested settings. Both annual area burned and burned severely are significantly correlated with mean annual flow and streamflow timing. PC1T alone explains 24% of the variability in annual area burned. Path analysis suggests that a substantial amount of the variability in annual area burned, previously attributed solely to temperature effects on melt timing, may be primarily driven by trends in precipitation and total annual streamflow. Principal component analysis scores on mean annual streamflow explain as much as 46% of the variability in annual area burned from 1984-2005. Thus, although streamflow timing may be a better single correlate of annual wildfire activity, timing is, in turn, strongly dependent on precipitation. These results suggest that recent fire activity in forests of this region are influenced more by precipitation variability than temperature-induced shifts in snowmelt timing, with significant implications for our ability to predict wildfire activity in the future. Published 2011. This article is a U.S. Government work and is in the public domain in the USA. [ABSTRACT FROM AUTHOR]