A hydrogeologic framework for characterizing summer streamflow sensitivity to climate warming in the Pacific Northwest, USA

Abstract

Abstract. Summer streamflows in the Pacific Northwest are largely derived from melting snow and groundwater discharge. As the climate warms, diminishing snowpack and earlier snowmelt will cause reductions in summer streamflow. Most regional-scale assessments of climate change impacts on streamflow use downscaled temperature and precipitation projections from general circulation models (GCMs) coupled with large-scale hydrologic models. Here we develop and apply an analytical hydrogeologic framework for characterizing summer streamflow sensitivity to a change in the timing and magnitude of recharge in a spatially explicit fashion. In particular, we incorporate the role of deep groundwater, which large-scale hydrologic models generally fail to capture, into streamflow sensitivity assessments. We validate our analytical streamflow sensitivities against two empirical measures of sensitivity derived using historical observations of temperature, precipitation, and streamflow from 217 watersheds. In general, empirically and analytically derived streamflow sensitivity values correspond. Although the selected watersheds cover a range of hydrologic regimes (e.g., rain-dominated, mixture of rain and snow, and snowdominated), sensitivity validation was primarily driven by the snow-dominated watersheds, which are subjected to a wider range of change in recharge timing and magnitude as a result of increased temperature. Overall, two patterns emerge from this analysis: first, areas with high streamflow sensitivity also have higher summer streamflows as compared to lowsensitivity areas. Second, the level of sensitivity and spatial extent of highly sensitive areas diminishes over time as the summer progresses. Results of this analysis point to a robust, practical, and scalable approach that can help assess risk at the landscape scale, complement the downscaling approach, be applied to any climate scenario of interest, and provide a framework to assist land and water managers in adapting to an uncertain and potentially challenging future.

Characterizing Runoff and Water Yield for Headwater Catchments in the Southern Sierra Nevada

“…Similar to BFI, several methods exist for recession analysis and there is a considerable debate on appropriate techniques for recession analysis (Tallaksen, ; Vogel and Kroll, ; Smakhtin, ; Sujono et al ., ; Posavec et al ., ). Estimates of k are comparable using some techniques (Sujono et al ., ; Safeeq et al ., ) but not others (Vogel and Kroll, ). Safeeq et al .…”

“…As temperatures continue to warm, much of the region is expected to experience a shift from 1 solid (i.e., snow) to liquid (i.e., rain) phase precipitation (Knowles et al, 2006;Klos et al, 2014;Safeeq et al, 2015b). More precipitation falling as rain instead of snow will affect total snow accumulation and the timing of snowmelt and runoff, potentially leading to lower streamflow in late spring and summer (Berghuijs et al, 2014;Safeeq et al, 2014). However, within a specific climate region, the actual magnitude and timing of streamflow decline will vary based on watershed characteristics, such as hydrogeology and snowpack dynamics (Tague and Grant, 2009;Mayer and Naman, 2011;Safeeq et al, 2013Safeeq et al, , 2014.…”

“…More precipitation falling as rain instead of snow will affect total snow accumulation and the timing of snowmelt and runoff, potentially leading to lower streamflow in late spring and summer (Berghuijs et al, 2014;Safeeq et al, 2014). However, within a specific climate region, the actual magnitude and timing of streamflow decline will vary based on watershed characteristics, such as hydrogeology and snowpack dynamics (Tague and Grant, 2009;Mayer and Naman, 2011;Safeeq et al, 2013Safeeq et al, , 2014. Hence, quantitative characterization of streamflow (both magnitude and rate) and water yield from headwater catchments with respect to their physiographic (e.g., elevation, vegetation, soil) and climatic (precipitation and temperature) differences is an essential first step in making predictions about future water availability.…”

Climate Elasticity of Low Flows in the Maritime Western U.S. Mountains

“…Understanding spatial variations in streamflow sensitivity to climate is important for anticipating water scarcity at the local scale, especially in the diverse climatic and physiographic landscape of the western United States (Safeeq et al, ; Tague & Dugger, ). Three factors have been identified as important mediators of spatial variability of both annual and summer streamflow sensitivity to climate elsewhere or separately: (1) the humidity index P / PET , which reflects the degree to which the mean annual catchment water balance is energy limited ( P / PET > 1) versus water limited ( P / PET < 1); (2) the degree to which seasonal moisture supply is in phase with seasonal energy demand, for which the fraction of annual precipitation that falls as snow versus rain and/or the timing of snowmelt may be proxies; and (3) differences in the capacity of catchments to store water, and the rate at which catchment water storage is translated into streamflow (i.e.…”

“…In addition, catchments where seasonal moisture supply is in phase with seasonal energy demand, or where rainfall is a larger portion of annual precipitation than snowfall, are generally more sensitive to climatic variability (Milly, ; Sankarasubramanian et al, ). Drainage rates are known to mediate the climatic sensitivity of summer streamflow in the western United States (Safeeq et al, , ; Tague & Grant, ), but to our knowledge the relationship between drainage rates and summer streamflow elasticity has not been quantified (other definitions of sensitivity were used). In most cases, these factors have been examined in the context of case studies, often hydrologic model based, or for a small number of catchments (e.g., Godsey et al, ; Harman et al, ; Jefferson et al, ; Markovich et al, ; Tague & Grant, ), with some exceptions (e.g., Safeeq et al, ).…”