Trends and Sensitivities of Low Streamflow Extremes in Pacific Northwest

Abstract:

Historic low stream flow extremes from 42 gauges in the Pacific Northwest are sensitive to both annual stream flow totals as well as center of stream flow timing. The total annual stream flow is primarily affected by precipitation amount, while stream flow center of timing is affected by both precipitation amount and temperature. Using statistical methods and historic data, we make inferences about the effects of temperature and precipitation variations on low flow metrics. Path analysis essentially normalizes the center of timing to remove the annual stream flow amount effects on timing. Annual weekly minimums are generally more sensitive to total annual runoff than to the center of timing of the annual hydrograph. When normalized, the effect of center of timing is lower. The relative sensitivity of low flows to precipitation and temperature has important implications for projections of hydrologic drought in the Pacific Northwest because global circulation models produce robust estimates of temperature changes but disagree on projected precipitation amounts. Trends in low flow metrics, including 7q10 summer, are largely negative. Trends in the 25th percentile of annual flow show declines and center of stream flow timing is occurring earlier.