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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, D11118, doi:10.1029/2006JD008088, 2007

Detection, attribution, and sensitivity of trends toward earlier streamflow in the Sierra Nevada

E. P. Maurer

Civil Engineering Department, Santa Clara University, Santa Clara, California, USA


I. T. Stewart

Environmental Studies Program, Santa Clara University, Santa Clara, California, USA


C. Bonfils

School of Natural Sciences, University of California, Merced, California, USA


P. B. Duffy

School of Natural Sciences, University of California, Merced, California, USA


D. Cayan

Climate Research Division, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California, USA


Abstract

Observed changes in the timing of snowmelt dominated streamflow in the western United States are often linked to anthropogenic or other external causes. We assess whether observed streamflow timing changes can be statistically attributed to external forcing, or whether they still lie within the bounds of natural (internal) variability for four large Sierra Nevada (CA) basins, at inflow points to major reservoirs. Streamflow timing is measured by “center timing” (CT), the day when half the annual flow has passed a given point. We use a physically based hydrology model driven by meteorological input from a global climate model to quantify the natural variability in CT trends. Estimated 50-year trends in CT due to natural climate variability often exceed estimated actual CT trends from 1950 to 1999. Thus, although observed trends in CT to date may be statistically significant, they cannot yet be statistically attributed to external influences on climate. We estimate that projected CT changes at the four major reservoir inflows will, with 90% confidence, exceed those from natural variability within 1–4 decades or 4–8 decades, depending on rates of future greenhouse gas emissions. To identify areas most likely to exhibit CT changes in response to rising temperatures, we calculate changes in CT under temperature increases from 1 to 5°. We find that areas with average winter temperatures between −2°C and −4°C are most likely to respond with significant CT shifts. Correspondingly, elevations from 2000 to 2800 m are most sensitive to temperature increases, with CT changes exceeding 45 days (earlier) relative to 1961–1990.

Received 29 September 2006; accepted 30 March 2007; published 13 June 2007.

Keywords: streamflow; snowmelt; runoff.

Index Terms: 1807 Hydrology: Climate impacts; 1863 Hydrology: Snow and ice (0736, 0738, 0776, 1827); 1860 Hydrology: Streamflow; 1630 Global Change: Impacts of global change (1225); 1621 Global Change: Cryospheric change (0776).

Subscriber Access to Full Article (Nonsubscribers may purchase for $9.00, Includes print PDF, file size: 597182 bytes)

Citation: Maurer, E. P., I. T. Stewart, C. Bonfils, P. B. Duffy, and D. Cayan (2007), Detection, attribution, and sensitivity of trends toward earlier streamflow in the Sierra Nevada, J. Geophys. Res., 112, D11118, doi:10.1029/2006JD008088.