Severe drought on the Navajo Nation has negatively affected Tribal crops, food supplies, economic conditions, and water storage since 2002, when Navajo Nation Commissioners on Emergency Management issued the first Navajo Nation Drought Emergency Declaration. Chuska Mountain snowpack is the major source of Navajo water storage and supply, and snowmelt from the mountain fills lakes and creeks for irrigation and boosts soil moisture into the summer. Navajo water managers report that their snow-fed surface waters have been in decline over recent decades, and  climate models suggest that, under a potentially warming climate in the future, snowpack may be limited further, arrive later, and/or melt earlier.

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Long-term information on the extent and severity of past snow surpluses and deficits could help Navajo water managers better understand current conditions and gauge expectations for the future. However, observational data for the Chuska Mountains are limited and too short to fully discern how recent trends in snowpack fit into long-term hydroclimatic variability. To address this gap, USGS scientists and the University of Arizona partnered with the Navajo Nation Water Management Branch to extend the limited instrumental snowpack record using tree-ring data. The goal of this study was to place local snowpack into a centuries-long context while ensuring that the information produced from the study was tailored to be useful for the Navajo Water Management Branch.

An important consideration during study development was the trade-off between using a snowpack dataset from the Chuska Mountains, which was short in length (1985–2014), or a longer dataset from Williams Ski Run (1967–2014) in the San Francisco Peaks, a similar but more distant location. While a longer dataset could produce more statistically robust results, it may be less meaningful for Navajo water managers who wish to obtain knowledge about their specific water resources. For this reason, two separate tree-ring based records were produced using the shorter and the longer instrumental data (Fig. 1). The scientists found that the growth of conifer species in this region is partly controlled by water volume contained in spring snowpack. As snow melts, the water that is released stimulates growth. Large volumes of water in snow can sustain growth well into summer, and low snowpack will limit growth for the year. This indicates that tree rings provide a reliable proxy to estimate snowpack in this region, and the method was then applied to multi-century tree-ring records to reconstruct past snowpack.

The Chuska Mountains reconstruction covered 359 years (from 1656 to 2015 AD), and the Williams Ski Run record spanned 321 years (1694-2015 AD) (Fig. 2). Severe multi-year snow droughts occurred in the region roughly once per century over the last 300 years. During these droughts, snowpack was 76-82% of the long-term average in the Chuska mountains and 58-69% in the San Francisco Peaks. In some decades (e.g. 1950s) snow drought lasted nearly 30 years. The Navajo experienced extreme water scarcity during the 2000s snow drought which lasted a shorter 8 years with a snowpack deficit similar to other centuries. Comparison of snowpack records from the Chuskas and San Francisco Peaks highlights the significant local variability within the region and demonstrates that the Chuska reconstruction better satisfies Navajo water manager needs for legitimate and relevant climate information that directly reflects local climate history and resources.

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This USGS-Tribal partnership supports water-resources planning by Tribes, who have limited access to quantitative climate data while experiencing disproportionate impacts of climate change. In this successful collaboration, tribal representatives outlined specific research questions, goals, and outcomes that they need for planning purposes. By introducing new methods and research approaches, USGS scientists provided long-term data on natural snowpack variability that both advance the science and provide benchmark data on snowpack extremes to Navajo water managers to support water monitoring and planning efforts.

The paper, “Comparing tree-ring based reconstructions of snowpack variability at different scales for the Navajo Nation,” was recently published in Climate Services.

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