David Young

Date of Graduation

2014

Document Type

Thesis

Degree Type

MS

College

Davis College of Agriculture, Natural Resources and Design

Department

Forest Resource Management

Committee Chair

Nicolas P Zegre

Committee Co-Chair

Pamela J Edwards

Committee Member

Michael P Strager

Abstract

Forest management practices and climate change can alter streamflow in headwater catchments by changing the balance between precipitation and factors that control evapotranspiration. To understand how water resources are changing, the impacts of climate change and forest disturbance must be isolated. Toward this end the Budyko decomposition method is applied to quantify the impacts of climate change and forest disturbance on catchments of the Fernow Experimental Forest. The Budyko framework provides the theoretical basis for the methodology and assumes variation in streamflow can be attributed to the principal controls governing climate (precipitation and potential evapotranspiration) and catchment water balance (precipitation and actual evapotranspiration). Although the Budyko framework is intended to model large-scale water balance with long-term data, this study tests the effectiveness of the method at the headwater scale and at both long-term and several fine temporal scales. This was accomplished by applying the decomposition method to planned forest disturbances and comparing the results to those obtained from the paired-catchment method. Data covering a 60 year period from a reference catchment (WS-4) and three disturbed catchments were analyzed. The disturbed watersheds were subject to forest disturbances followed by three different forms of regeneration: natural hardwood forest regrowth (WS-1), herbicide-suppressed regrowth followed by natural hardwood regrowth (WS-7), and herbicide-suppressed regrowth followed by stand conversion to a coniferous forest (WS-6). Based on this analysis forest disturbance, forest regeneration, and climate change differentially impacted streamflow. Using a Mann-Kendall trend analysis positive trends were found in climate variables: annual average and minimum temperature and growing season precipitation. Using long-term averages, the decomposition method showed that all catchments deviated in both the horizontal and vertical directions reflecting the impacts of climate and disturbance driven changes. For all catchments, the direction of change influenced by climate (dryness index) did not produce a shift in the evaporative index according to the Budyko curve. Instead, disturbance-induced changes in streamflow dampened (WS-1) or masked (WS-6, WS-7) the effects of climate change over the study period. For the reference catchment, incremental increases in streamflow are not explained by climate change alone. Analyzing species composition changes over the study period suggest that late forest succession may be causing increases in streamflow. Using linear regression and correlation the application of the Budyko framework at the headwater scale and at inter-annual timescales was supported by the paired catchment method for WS-6 and WS-7. This result was less influenced by the ability of the Budykos curve to predict actual evapotranspiration from the climate dryness index, then the sensitivity of the Budyko decomposition method to the magnitude and duration of disturbance. This study provides guidance for how the Budyko decomposition method can be applied to headwater catchments and at inter-annual time scales. In addition this study provides a starting point for understanding the future impacts of climate change at the Fernow.

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