Scientific consensus generally predicts increasing drought threat in areas that are already water-stressed, though it is unclear exactly how climate change will impact future hydrology and drought conditions. In the Sierra Nevada, climate change is expected to produce decreased winter snowpack, wetter, more unpredictable winter flows (including winter flooding), earlier snowmelt, reduced summer base flows, and drying of perennial streams.

Snow is a primary driver of hydrologic processes in the Sierra Nevada. Snowpack functions as a water storage system until spring snowmelt, helping to offset this ecoregion’s typically dry summer season. Thus, drought stress is precipitated largely by deficient winter snowfall or inadequate snowpack. Monitoring by the National Park Service in this region predicts that temperature warming will shift snowmelt timing earlier by approximately one week per °C (Rice & Bales 2013). Runoff from snowmelt may contribute to up to 80% of annual streamflow in this region (Rice et al. 2011), and the high mountains of the Sierra Nevada ultimately serve as water sources for downstream agriculture and municipal uses. 

Hydrologic refugia in the Sierra Nevada might be defined as watersheds or catchments resistant to climate change (or more generally, as sites where water availability is greater than that of the surrounding landscape). These hydrologic refugia may support at-risk areas with fragile ecosystems (like meadows, old growth forest, or alpine environments). 

In the Sierra Nevada, hydrologic refugia might be identified using a watershed ranking index based on annual mass balance of water, including input, output, and storage. Watersheds may be classified as refugia based on having a positive mass water balance.

Focal Resources identified at November 8, 2019 workshop:

• Watersheds whose function will be resistant to climate change, particularly within:

  • Alpine environments

  • At-risk areas with fragile ecosystems (e.g., meadows)

Potential Management Actions:

Based on the focal resources, workshop participants identified potential management actions such as:

• Introduce beavers to maintain stage height throughout the year

  • Research justification: Using beaver dams to restore incised stream ecosystems; Beaver dams without beavers

  • Example from Rocky Mountain National Park: Simulated Beaver Structures

• Manage for important forest components: timber and canopy cover

• Manage stream bank and water table

Data Gaps/Future Work:

• Need smaller scale data for each watershed.

Current efforts, data, and partnerships that could support identification and mapping of hydrologic refugia: 

• California Basin Characterization Model

• NPS Sierra Nevada Inventory and Monitoring Network: River Monitoring

• California Department of Water Resources (DWR)

  • E.g., Snow Surveys

• NASA

  • E.g. MODIS snowpack data

  • Airborne Snow Observatory (ASO)

• Sierra Nevada Conservancy (SNC)

• Data from downstream water users (cities, counties, agriculture)

References

• Rice, R, Bales, RC, Painter, TH, & Dozier, J. 2011. Snow water equivalent along elevation gradients in the Merced and Tuolumne River basins of the Sierra Nevada. Water Resources Research 47:W08515.

• Rice R & Bales RC. 2013. An assessment of snowcover in major river basins of Sierra Nevada Network parks and potential approaches for long-term monitoring. Natural Resource Technical Report. NPS/SIEN/NRTR—2013/800. National Park Service. Fort Collins, Colorado.

News

• Addressing need for conservation of springs in drying climate. 2020. https://news.nau.edu/springer-climate-change-refugia/#.XterK2hKhPY