Diel Discharge Cycles as Indicators of Evapotranspiration Rates, with Implications for Groundwater Dynamics

Forest withdrawal of groundwater for evapotranspiration (ET) is a notable example of landscape scale vegetation influence on both groundwater and surface water dynamics. The daily variation of ET demand has the potential to lead to cycles in stream discharge, which although minor in themselves appear to enable a spatially integrated estimation of daily riparian ET rates, as well as the inference of some subsurface flow characteristics. We analyzed diel cycles in stream discharge during baseflow periods in a headwater stream in La Selva Biological Station, Costa Rica, a wet tropical site, which appear to be associated with groundwater withdrawal by the forest for ET. Accounting for a small time lag, the discharge cycle timing correlates strongly with daily variation in vapor pressure differential (VPD), which previous researchers have found to correlate with ET demand and sap flow. This study develops an empirical model for inferring hydraulic properties of the subsurface from variation in the rate of change of nightly stream stage recovery. At high stages the rate of stream stage recovery from ET losses decreases throughout the night, which we interpret to be the result of decreasing hydraulic gradient as groundwater levels recover from ET losses. However, at low stages the rate of stream stage recovery increases throughout the night, suggesting that transmissivity becomes the dominant control on the rate of groundwater flow into the channel. For the six baseflow periods analyzed, the model estimates ET losses ranging from 5.6 to 12.1 mm/day within the riparian area. The modeled total amount of ET losses in each baseflow period correlates well with average VPD for that period, and the temporal distribution of ET losses correlates well with VPD variation. Model results indicate that the area of the riparian zone that influences streamflow increases with stream stage and ranges from 1-2% of the total basin area.