An investigation of vegetation - hydrology interactions in watershed 1 at the H.J. Andrews Experimental Forest

For management purposes, it would be useful to be able to predict streamflow response to forest practices in small, unmonitored basins. The primary objective of this study is to investigate the influence of early successional vegetation on summer streamflow levels. The long-term data records from Watershed 1 and the HJ Andrews Experimental Forest provide a case study of vegetation dynamics and streamflow changes during the first three decades following clear-cut harvest. This study documents the vegetation dynamics in Watershed 1, based on long-term vegetation plot data and aerial photos. Hypotheses about the mechanisms by which vegetation influences streamflow levels are presented and explore using a spatially explicit watershed model, MAPSS-W (Daly, 1994). In order to test these hypotheses of vegetation-hydrology interaction, spatially distributed climate, soils and vegetation datasets were developed for MAPSS-W. MAPSS-W was calibrated and evaluated for use in Watersheds 1 and 2 at the HJ Andrews Experimental Forest. Following calibration and evaluation of MAPSS-W, experimental simulations were run to explore hypotheses of vegetation-hydrology interaction. These analysis of long-term vegetation data and results from watershed simulations indicate that changes in summer streamflow levels in Watershed 1 are related to shifts in the dominant vegetation in the watershed. Rapid growth of herbaceous vegetation appears to influence summer streamflow during the first 5-10 years following harvest. Summer streamflow deficits appear to be related to the dominance of deciduous broadleaf vegetation in the watershed during the second decade following harvest. Conifers appear to being to play a significant hydrologic role during the third decade following harvest.