Date of Award

8-2016

Degree Name

MS in Forestry Sciences

Department

Natural Resources Management

Advisor

Christopher G. Surfleet

Abstract

Stream temperature impacts have resulted in increased restrictions on land management such as timber harvest and have created considerable uncertainty for future planning and management of redwood forestlands. Challenges remain in the assessment of downstream cumulative effects given the complexity of stream temperature dynamics. The goal of this research is to identify the risk of downstream temperature heating based on the summer low flow residence times, stream morphological characteristics, stream water storage, and heat budget exchanges. Stream temperature, hydrologic, climatic, and channel morphological data were collected on two, approximately 800 m stream reaches on Little Creek and Scotts Creek located in mixed coast redwood and Douglas-fir forests of Santa Cruz County, California. Spatially and temporally explicit stream temperature measurements were collected using distributed temperature sensing. A fluorescent dye tracer was used to gather information on summer streamflow including the quantification of residence time and hyporheic exchange. A heat budget approach was used to quantify individual heat flux components and to examine the processes of stream heating and cooling. Comparisons of observed and modeled temperatures between the two sites and the relative influences of individual heat budget components indicated that the magnitude and spatial frequency of subsurface-surface water interactions, along with incoming net radiation, played a substantial role in how heat was transferred through each system. Solar radiation exposure from stream shading and modeled groundwater inflows were important explanatory variables in the magnitude and spatial distribution of stream temperatures for the two streams located in the same watershed subjected to similar meteorological conditions. The measurement and evaluation of a stream’s hydrologic characteristics, stream shading, and aspect ratio were statistically significant measurements (α <.05) associated with downstream temperature change for Little Creek. Only weak statistical relationships were found for Scotts Creek. Weak relationships may have been attributed to very low streamflow due to drought conditions creating longer water residence times on Scotts Creek. Heat budget modelling results indicated temperature increases in both study sites downstream of the hypothetical riparian canopy removal. The increases in stream temperature decreased with increased downstream distance from the canopy removal due to increased stream shading and advective cooling from hyporheic water exchanges. Potential increases in groundwater inflows following hypothetical canopy reduction scenarios reduced the effect of downstream temperature increases with greater reductions in stream temperature cooling with increased groundwater inputs.

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