Spatial Patterns of March and September Streamflow Trends in Pacific Northwest Streams, 1958–2008

Abstract

Summer streamflow is a vital water resource for municipal and domestic water supplies, irrigation, salmonid habitat, recreation, and water‐related ecosystem services in the Pacific Northwest (PNW) in the United States. This study detects significant negative trends in September absolute streamflow in a majority of 68 stream‐gauging stations located on unregulated streams in the PNW from 1958 to 2008. The proportion of March streamflow to annual streamflow increases in most stations over 1,000 m elevation, with a baseflow index of less than 50, while absolute March streamflow does not increase in most stations. The declining trends of September absolute streamflow are strongly associated with seven‐day low flow, January–March maximum temperature trends, and the size of the basin (19–7,260 km2), while the increasing trends of the fraction of March streamflow are associated with elevation, April 1 snow water equivalent, March precipitation, center timing of streamflow, and October–December minimum temperature trends. Compared with ordinary least squares (OLS) estimated regression models, spatial error regression and geographically weighted regression (GWR) models effectively remove spatial autocorrelation in residuals. The GWR model results show spatial gradients of local R 2 values with consistently higher local R 2 values in the northern Cascades. This finding illustrates that different hydrologic landscape factors, such as geology and seasonal distribution of precipitation, also influence streamflow trends in the PNW. In addition, our spatial analysis model results show that considering various geographic factors help clarify the dynamics of streamflow trends over a large geographical area, supporting a spatial analysis approach over aspatial OLS‐estimated regression models for predicting streamflow trends. Results indicate that transitional rain–snow surface water‐dominated basins are likely to have reduced summer streamflow under warming scenarios. Consequently, a better understanding of the relationships among summer streamflow, precipitation, snowmelt, elevation, and geology can help water managers predict the response of regional summer streamflow to global warming. Patrones espaciales de las tendencias de los caudales de marzo y septiembre en el Pacífico Noroccidental. Los caudales (streamflows) de verano son recursos hídricos vitales para el abastecimiento de agua municipal y domestico así como para el riego agrícola, el hábitat de los salmónidos, la recreación, y para varios servicios de los ecosistemas en el Pacífico Noroccidental (Pacific Nortwest‐PNW) de los Estados Unidos. Este estudio identifica tendencias negativas considerables en los caudales absolutos de septiembre en la mayoría de las 68 estaciones de medición situadas en ríos y arroyos no regulares del PNW entre 1958 y 2008. La proporción del caudal de marzo con respecto al caudal anual aumenta en la mayoría de las estaciones situadas a más de 1000 metros de altitud, que tienen un índice de caudal base (base flow index‐BFI) de menos de 50, pero...

Precipitation Intensity Trend Detection using Hourly and Daily Observations in Portland, Oregon

“…Kendall's tau was selected because it is not based on the assumption of normal distribution of either variable, and is effective in small sample sizes such as our 10-15-year periods. In addition, Kendall's tau has been widely used to detect trends in hydroclimate time-series [42,43]. The two variables used in this test were year of record and monthly precipitation intensity for a given station.…”

Toward a formal definition of water scarcity in natural‐human systems

“…[36] In snowmelt-dominated basins in the western United States, for example, there has been a shift in the timing of peak streamflow to earlier in spring, primarily driven by an increase in winter and spring temperatures [Stewart et al, 2005], and declines in late summer streamflow [Chang et al, 2012]. Formal detection and attribution studies show that human influence is responsible for 60% of the climaterelated trends in historical streamflow and snowpack in the western United States from 1950 to 1999 [Barnett et al, 2008].…”

Changes in runoff in two neighbouring catchments in the Bohemian Forest related to climate and land cover changes

“…Negative trends have been reported for low flows in summer and autumn in some parts of the northern hemisphere (Birsan et al, 2005;Chang et al, 2012;Stahl et al, 2010). Yet opposite trends are evident as well in Great Britain (Dixon et al, 2006) and Northern Scandinavia (Wilson et al, 2010), for example, where precipitation in summer has increased.…”