A natural resource condition assessment for Sequoia and Kings Canyon National Parks: Appendix 7b – hydrology of Sierra Nevada Network parks

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, all metrics derived from long-term monitoring at fixed station locations. 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 seasonal and long-term variations in streamflow. Results of these analyses can help to inform monitoring objectives, site selection, and analytical methods. This project‘s objectives were to: 1. Review and summarize existing literature related to Sierra Nevada surface water and snow dynamics, with particular attention to interpretation of hydrologic changes in response to potential climate change scenarios. 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 a literature review and analyses and compile data in a final report. Recommendations are made regarding hydrological monitoring approaches, sites, and equipment. The datasets analyzed for this study included: Twenty streamflow gaging station records from the U.S. Geological Survey‘s National Water Information System database that were determined to be essentially unimpaired and suitable length for all of the proposed analyses, seven additional gaging station records suitable or available for only partial analyses, and 68 snow courses with 25 years or more of record located within or adjacent to the SIEN. Analyses followed well-established, conventional methods for examining trends in hydrological data. 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. Even streamgages separated by a substantial distance and on opposite slopes of the Sierra Nevada have quite high correlations. All streamgages considered in this study appear to be part of a well-defined hydrologic region.  Spring snowmelt runoff timing was examined with these three metrics: 1) the percent of the annual discharge coming during the April through July (AMJJ) period; 2) the date to the runoff center of mass; and 3) the onset of snowmelt runoff. A statistically significant decreasing trend of the 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, the date on which half of the total annual discharge has occurred, and the snowmelt onset are occurring earlier in the year on average. In contrast to the decreasing trend of AMJJ/annual runoff on the west side, 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, and for eight of the records, the trends are statistically significant at the 95% confidence limit. ***Not all findings included in abstract due to space limitations***

A natural resource condition assessment for Sequoia and Kings Canyon National Parks: Appendix 7b – hydrology of Sierra Nevada Network parks

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