Hillslope position drives patterns of forest water relations in a steep headwater catchment

Evapotranspiration is a major determinant of streamflow in forested basins.  However, the effect topography has on forest water relations has rarely been examined.  Quantifying the variation in forest water use with regards to slope position is critical to predicting the hydrologic impacts of various forestry operations and removal of vegetation and is central to understanding controls on water quantity and quality.  We measured transpiration, soil moisture, and foliar pre-dawn water potential in 4 plots across a ridge to ridge transect throughout the summers of 2005 and 2006 in a small headwater catchment in western Oregon. Additionally, we measured deuterium and ¹⁸O of xylem water and soil water to track changes in transpiration source water throughout the summers.  In 2006, transpiration in upslope plots was 42% greater on average for days 160 to 260 than that of valley bottom plots (3.8 mm day^-1 vs. 2.6 mm day^-1, respectively).  Average transpiration for the same period was 25% higher in south-facing plots in comparison to north-facing plots.  Minimum pre-dawn water potential values ranged from -0.8 to -1.3 MPA on day 240 with north-facing plots having the lowest values.  Preliminary results suggest that topographic gradients influencing soil depth, soil texture, and microclimate result in large variation in forest water use over very small distances.