Estimating Evapotranspiration in Three Contrasting Forest Ecosystems Using Eddy Covariance, Sapflow, and Soil Water Balance Methods

Evapotranspiration (ET) represents the second largest flux in terrestrial ecosystem water budget. In recent years, much attention has been given to the coherent linkages among hydrological cycle, ecophysiological processes, disturbances, and ecosystem function. However, quantification of ET at various temporal and spatial scales remains challenging (e.g., continuous changes of ET with time of a forest). Large uncertainties and measurement errors exist in fully accounting the ET flux, a process that involves both the physical (atmospheric and soil water control) and biological processes (leaf stomata and stem conductance control). In 2004, we established three research sites to study the climatic and forest management effects on ecosystem carbon and water balances in three contrasting forests: an oak openings in NW Ohio, a recent plantation of loblolly pine in eastern North Carolina, and a 13 year-old loblolly pine stand in eastern NC. The oak-opening ecosystem in a dry, cold environment while the other two in eastern North Carolina's lower coastal plain represent loblolly pine plantations on drained soils. Field installation on each site includes an eddy flux tower to measure ecosystem water exchange at 30-minute interval. Forest canopy interception, soil water content, and groundwater table depth were monitored around the flux tower along with rainfall above the forest canopy to develop water balances at multiple temporal scales. Stand-level transpiration was estimated by scaling up sapflow flux of 6-16 trees. Estimated ET values from the three independent methods were compared to identify major controls of ET. We also applied the MIKE SHE hydrologic model with site specific stand and soil information to simulate ET and compare with the measured data at the daily temporal scale. From the one-year data, we found that: 1) Ecosystem ET had very high natural variability, thus any single method was insufficient to quantify and model it at a high temporal resolution; 2) Tree harvesting greatly reduced total evapotranspiration even in these wet soils; 3) Sapflow and eddy covariance methods likely underestimated ecosystem ET at all three sites.