Hydrology of the Sierra Nevada Network national parks: Status and trends

Andrews, Edmund

The purpose of this project was to inventory and analyze existing hydrologic data, primarily streamflow and snow course data, for the region encompassing Yosemite and Sequoia and Kings National Parks, and Devils Postpile National Monument in order to assist the Sierra Nevada Network (SIEN) Inventory and Monitoring Program in developing a protocol for monitoring streams and rivers. Important hydrological indicators include streamflow magnitude and timing and snow water content. In order to develop a monitoring protocol, it is helpful to first inventory existing regional data and monitoring locations. Because streamflow in the Sierra Nevada is dominated by snowmelt, inventory and analysis of snow course data are critical to understanding variations in streamflow. Results of these analyses can inform monitoring objectives, site selection, and analytical methods. This project’s objectives were to: 1. Review and summarize literature related to Sierra Nevada surface water and snow dynamics. 2. Assemble, document, and analyze existing streamflow and snow course data in and near SIEN parks to inform the selection of sites and metrics for long-term monitoring. 3. Improve understanding of the annual and long-term dynamics of SIEN hydrologic systems through literature review and data analyses in a final report. Major findings include the following: Annual mean discharges at streamgages included in these analyses are generally well-correlated, due to the similarities of geology, topography, and climate among drainage basins. Spring snowmelt runoff timing: A statistically significant decreasing trend of the April-July(AMJJ) /annual runoff ratio was found at 10 of the 20 west slope gaging stations analyzed. Further, at stations on the west slope of the Sierra, the center of mass and the snowmelt onset are occurring earlier in the year on average. On the east slope of the Sierra Nevada, there is a generally increasing trend. The most significant trends in streamflow magnitude among the 20 gaging stations studied were increasing winter low flows. A trend of increasing 7-day winter low flow was found for 15 of the 20 streamflow records. Flow duration indicates the percent of time a given flow has been equaled or exceeded during the period of record. Dimensionless flows equaled or exceeded more than 50 percent of the time are nearly identical for all 20 drainage basins in the south Sierra Nevada. The distinctive differences between southern Sierra Nevada drainage basins are found in the relatively infrequent high flows whereas the frequent low flows are quite similar. Trends in snow water equivalent (SWE) measured at snow courses on or near April 1 are consistent with a general regional warming of 0.8-1.0 oC. Snow courses located well west of the Sierra Nevada crest tend to show trends of decreasing April 1 SWE. Snow courses located at relatively high elevations near the Sierra Nevada crest tend to show trends of increasing April 1 SWE. The crossover elevation from negative trends to positive occurs, on average, at about 8500 feet. Key recommendations for hydrological monitoring in the SIEN are: Essential information about SIEN hydrology is provided by streamgages and snow courses, but many are located outside of the parks. National Park Service participation in the measurement and assessment of snowpack and streamflows throughout the southern Sierra Nevada region will be a cost-effective way to achieve a portion of the SIEN monitoring objectives for hydrology. More monitoring at middle to high elevations would provide hydrologic data with better spatial representation of these topographically diverse parks. Many of the ecologically relevant questions concerning the temporal and spatial distribution of surface water can be evaluated using less costly and more wilderness compatible methods than the streamflow gaging station.

Hydrology of the Sierra Nevada Network national parks: Status and trends

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