Earth’s Future RESEARCH ARTICLE Assessing Mountains as Natural Reservoirs With a Multimetric 10.1002/2017EF000789 Framework Key Points: 1 1 2 • There is a need to evaluate Alan M. Rhoades , Andrew D. Jones , and Paul A. Ullrich snowpack data sets beyond conventional techniques such as 1Climate and Ecosystem Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA, climate average absolute bias and 2Department of Land, Air, and Water Resources, University of California, Davis, CA, USA spatial correlations • Multimetric evaluations elucidate processes that create disagreement in Anthropogenic climate change will continue to diminish the unique role that mountains climate model snowpack estimates Abstract • A consistent high bias in snowmelt perform as natural reservoirs and alter long-held assumptions of water management. Climate models are rates across climate models limits important tools to help constrain uncertainty and understand processes that shape this decline. To ensure their utility for water management that climate model estimates provide stakeholder relevant information, the formulation of multimetric applications model evaluation frameworks informed by stakeholder interactions are critical. In this study, we present one such multimetric framework to evaluate snowpack data sets in the California Sierra Nevada: the snow water Supporting Information: equivalent (SWE) triangle. SWE triangle metrics help to describe snowpack characteristics associated with • Supporting Information S1 total water volume buildup, peak water availability, and the rate of water release. This approach highlights compensating errors that would not be reflected in conventional large-scale spatiotemporal analysis. Correspondence to: To test our multimetric evaluation framework, we evaluate several publicly available snow products A. M. Rhoades, [email protected] including the Sierra Nevada Snow Reanalysis, Livneh (L15), and the North American Land Data Assimilation System version 2 data sets. We then evaluate regional climate model skill within the North American Coordinated Regional Climate Downscaling Experiment. All data sets analyzed show variation across the Citation: Rhoades, A. M., Jones, A. D., various SWE triangle metrics, even within observationally constrained snow products. This spread was & Ullrich, P. A. (2018). Assessing especially shown in spring season melt rates. Melt rate biases were prevalent throughout most regional mountains as natural reservoirs climate model simulations, regardless of snow accumulation dynamics, and will need to be addressed with a multimetric framework. Earth’s Future, 6. to improve their utility for water stakeholders. https://doi.org/10.1002/2017EF000789 Plain Language Summary Mountain snowpack is a key natural water reservoir. Due to anthropogenic climate change, this natural reservoir will likely decline over the next century. This decline Received 27 DEC 2017 Accepted 27 JUL 2018 will be nonlinear in both space and time. As such, climate model estimates will be key in constraining Accepted article online 14 AUG 2018 uncertainty surrounding this decline. These virtual laboratories allow us to understand what could be and not just what has been. The utility of climate models is apparent; however, uncertainties surrounding climate model estimates of mountain snowpack need to be understood. This article assesses uncertainties surrounding both observationally based and climate model estimates of mountain snowpack in the California Sierra Nevada. We use a new multimetric evaluation framework called the snow water equivalent (SWE) triangle. This framework elucidates agreement/disagreement in estimates of peak water volume and timing, accumulation, and melt rates, and the lengths of the accumulation and melt seasons. We found that spread across snowpack data sets were partly driven by differences in the accumulation to peak timing phase of the winter season but was dominated by melt season differences. To expand climate model utility for water management applications, more research is needed to understand why models tended to have earlier peak timing and abrupt snowmelt. 1. Introduction Mountains are unique physical features that act as both natural dams that impede atmospheric moisture ©2018. The Authors. transport and natural reservoirs that extract, store, and release water over time. The western U.S. (WUS) water This is an open access article under the system is largely dependent upon the seasonal accumulation and melt of mountain snowpack (Bales et al., terms of the Creative Commons Attribution-NonCommercial-NoDerivs 2006) with three fourths of the WUS water supply originating from snowmelt (Palmer, 1988). For example, in License, which permits use and the California Sierra Nevada mountain snowpack was found to contribute 72% additional storage compared distribution in any medium, provided with current man-made reservoirs (Dettinger & Anderson, 2015). Thus, water stakeholders are often beholden the original work is properly cited, the use is non-commercial and no to the atmospheric, hydrologic, and land surface processes that shape snowpack dynamics to ensure they modifications or adaptations are made. can meet annual water demands. The interplay between these components of the Earth system coupled with RHOADES ET AL. 1 Earth’s Future 10.1002/2017EF000789 anthropogenic climate change (Pierce et al., 2008) are leading to earlier peak snowpack timing (e.g., for the California Sierra Nevada 6–21 days earlier; Kapnick & Hall, 2010) evidence of enhanced warming with eleva- tion (Pepin et al., 2015), and a slower snowmelt, which could alter hydrologic basin connectivity and decrease annual mean streamflow (Musselman et al., 2017). Snowpack diminution has been consistent across many of the Mediterranean mountain ranges with sensitivity studies showing varying reduction in mean snow water equivalent, SWE, (10–25% per degree Celsius increase) and snow duration (5 to >25 days per degree Cel- sius increase) based on the climate characteristics that drive snow-mass-energy balance (Fayad et al., 2017; López-Moreno et al., 2017). In the WUS, the rate of change in mountain snowpack is unprecedented in the paleoclimatic record (Belmecheri et al., 2016; Pederson et al., 2011). All said, changes in snowpack under- mine the oft-used stationarity assumption (Milly et al., 2008) that has traditionally guided the design of water systems in the WUS and emphasizes the key role of snow drought (Harpold et al., 2017). WUS scientists and water stakeholders have a need to understand seasonal snowpack dynamics and how related processes will be affected under anthropogenic climate change. Hossain et al. (2015) conducted a survey of 90 water experts with at least two decades of experience: 71% of respondents felt that conven- tional water management practices are not adequate to meet 21st century demand, and half of respondents desired more flexible water management strategies and/or better evaluation of the drivers of uncertainty in water resources, especially at the local-to-regional scale. Therefore, as the stationarity assumption in water management becomes less reliable with the onset of climate change, the utilization of models to identify and understand the various processes that shape snowpack supplies will be critical to mitigate future water supply risk. Models can act as virtual laboratories offering process understanding across global, regional, and local scales. This allows hydroclimate scientists and resource managers to pose questions of what could be and not just what has been. However, each model and scale presents its own set of potential biases that can have a cascading effect on modeled snowpack (i.e., an uncertainty chain). For example, at the global scale, the ability of a global climate model to realistically simulate the jet stream, semipermanent high-pressure regions, and atmosphere-ocean-land teleconnections will impact the timing and landfall location of storm systems. This in turn feeds into the regional scale, where land surface cover and topographic representation shape the inten- sity and phase of regional precipitation. Finally, at the local scale radiative and hydrologic processes shape the residence time of water that is then used within the ecology-energy-food-water nexus. Evaluating the credibility of simulated climate and snow dynamics through a decision-relevant lens and attributing sources of bias across multiple processes, scales of analysis, and models poses a significant challenge to the scientific community. The WUS is a well-studied region with a vast literature in observed snowpack dynamics (Bales et al., 2006; Cristea et al., 2017; Henn et al., 2016; Kapnick & Hall, 2010; Margulis, Cortés, Girotto, & Durand, 2016; Mote, 2006; Mote et al., 2005; Painter et al., 2016; Serreze et al., 1999; Trujillo & Molotch, 2014) and modeling approaches (Ashfaq et al., 2013, 2016; Giorgi et al., 2009; Huang et al., 2016; Liu et al., 2017; McCrary et al., 2017; Minder et al., 2016; Naz et al., 2016; Pierce & Cayan, 2013; Rasmussen et al., 2014; Rhoades et al., 2016, 2017, 2018; Sun et al., 2016; Wu et al., 2017) dating back decades (Anderson, 1976; Palmer, 1988). Consequently, a staggering growth in publicly available observational and regional cli- mate data sets have been produced, yet a lag in expert direction regarding their applicability for regional planning (Hall, 2014). Thus, a major need is not only climate data that can account for regional drivers that shape planning decisions,