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.