Pacific Northwest Research Station

Title: Mechanistic assessment of hillslope transpiration controls of diel subsurface flow: a steady-state irrigation approach

Author: Barnard, H.R.; Graham, C.B.; van Verseveld, W.J.; Brooks, J.R.; Bond, B.J.; McDonnell, J.J.

Date: 2010

Source: Ecohydrology. 3: 133-142

Description: Mechanistic assessment of how transpiration influences subsurface flow is necessary to advance understanding of catchment hydrology. We conducted a 24-day, steady-state irrigation experiment to quantify the relationships among soil moisture, transpiration and hillslope subsurface flow. Our objectives were to: (1) examine the time lag between maximum transpiration and minimum hillslope discharge with regard to soil moisture; (2) quantify the relationship between diel hillslope discharge and daily transpiration; and (3) identify the soil depth from which trees extract water for transpiration. The greatest correlation between transpiration and hillslope discharge occurred during the post-irrigation period, when the diel reduction in hillslope discharge totalled 90% of total measured daily transpiration. Daily transpiration of trees within the irrigated area remained relatively constant throughout the experiment. This study clearly demonstrates that when soil moisture is high, hillslope trees can be an important factor in diel fluctuations in stream discharge. We advance a conceptual model for the site whereby the relationship between transpiration and hillslope discharge is a function of soil moisture status and drainable porosity.

Keywords: transpiration, hillslope hydrology, diel discharge, soil moisture, Douglas-fir, ecohydrology

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Citation

Barnard, H.R.; Graham, C.B.; van Verseveld, W.J.; Brooks, J.R.; Bond, B.J.; McDonnell, J.J.  2010.  Mechanistic assessment of hillslope transpiration controls of diel subsurface flow: a steady-state irrigation approach.   Ecohydrology. 3: 133-142.