Forest harvesting has been shown to cause various changes in water quantity and water quality parameters, highlighting the need for comprehensive forest practice rules. Studies show a myriad of impacts to ecosystems as a result of watershed level changes, such as forest harvesting. Being able to better understand the impact that forest harvesting can have on stream temperature is especially critical in locations where federally threatened or endangered fish species are located. The overall goal of this research project is to assess responses in stream temperature to various riparian and forest harvest treatments in a maritime, mountainous environment. The results of this study aim to inform decision makers with additional information pertaining to the effects of forest harvest on water temperature. Modeling is done as a part of the third Caspar Creek Paired Experimental Watershed study. Located in Mendocino County, the site provides a place for California researchers and decision makers to learn about the cumulative watershed effects of forest management operations on peak flows, sediment production, anadromous fish, macro-invertebrate communities, nutrient cycling and more. Historic data was used to calibrate the Distributed Hydrology Soil Vegetation Model (DHSVM) and River Basin Model (RBM) to measured stream temperatures in the South Fork of Caspar Creek (SFC) for hydrologic years 2010-2016. Critical summer time periods, when temperatures are highest and flows are low, are the primary concern for this work. The key modeling scenarios evaluated were (1) varying percentages of Watercourse and Lake Protection Zones (WLPZ) canopy cover, (2) the 2018-2019 SFC forest harvest and (3) an experimental design converting dominant riparian vegetation along 300-yard stream reaches. Modeling results showed that stream temperatures begin to rise above third-growth conditions when canopy cover is reduced to 25% and 0% retention levels. Larger increases in Maximum Weekly Maximum Temperature (MWMT) values, compared to Maximum Weekly Average Temperature (MWAT) values, were seen across all scenarios. There was essentially no difference between altering buffer areas along only class I streams, compared to along all stream classes. At the 0% canopy retention, MWMT values consistently rose above recommended thermal limits for Coho salmon (Oncorhynchus kisutch) and state regulations prohibiting more than a 5 degree F increase in waters. Clearcutting the entire watershed produced less of an effect than simulations clearing on only the riparian area, suggesting that groundwater inflows act to mitigate stream temperature rises in the SFC. The 2018-2019 harvest showed a relatively consistent increase in MWAT values (avg. 0.11 degree C) and more varied increases in MWMT values (avg. 0.32 degree C). Simulations converting dominant riparian vegetation by clearing could not be considered conclusive due to sensitivity analyses suggesting potentially unrealistic tracking of downstream temperatures. Additional sensitivity analyses suggest that tree height and the monthly extinction coefficient (a function of Leaf Area Index) are most influential on stream temperature changes in SFC. This is consistent with other modeling studies and suggests stream temperature management focus on tall, dense buffers as opposed to wider buffer widths.
| Identifer | oai:union.ndltd.org:CALPOLY/oai:digitalcommons.calpoly.edu:theses-3450 |
| Date | 01 June 2019 |
| Creators | Ridgeway, Julia B |
| Publisher | DigitalCommons@CalPoly |
| Source Sets | California Polytechnic State University |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Master's Theses |
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