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



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. An 8 × 20 m hillslope was irrigated at a rate of 3·6 mm h−1. Diel fluctuations in hillslope discharge persisted throughout the experiment. Pre-irrigation time lags between maximum transpiration and minimum hillslope discharge were 6·5 h, whereas lags during steady-state and post-irrigation conditions were 4 and 2 h, respectively. 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. Diel fluctuations in soil moisture were greatest at a depth of 0·9–1·2 m prior to irrigation and became more uniform throughout the soil profile during and post-irrigation. 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. Copyright © 2010 John Wiley & Sons, Ltd.