Evaporation and the sub-canopy energy environment in a flooded forest

Authors: ALLEN, SCOTT - Louisiana State University; Reba, Michele; EDWARDS, BRANDON - Louisiana State University; KEIM, RICHARD - Louisiana State University

Submitted to: Hydrological Processes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/21/2017
Publication Date: 6/28/2017
Publication URL: http://handle.nal.usda.gov/10113/5762956
Citation: Allen, S.T., Reba, M.L., Edwards, B.L., Keim, R.F. 2017. Evaporation and the sub-canopy energy environment in a flooded forest. Hydrological Processes. 31(16):2860-2871. https://doi.org/10.1002/hyp.11227.
DOI: https://doi.org/10.1002/hyp.11227

Interpretive Summary: A forested wetland is seasonally or permanently flooded. Forested areas and flooded areas have been studied independently but little research has been done at locations where these two systems co-exist. Research on how forested wetland systems partition energy has been under studied. A forested wetland in southeast Louisiana, characterized by bottomland hardwood tree species, was instrumented with sensors to measure energy balance including net radiation, air and water temperature, humidity, and wind. The highest evaporation rates were measured in October and November due to an increase in net radiation as the loss of leaves in the fall allow for more direct radiation into the sub-canopy. Loss of water during this period ranged from 1.8 to 2.0 mm per day. Findings from this research will improve landscape modeling of regions with flooded forests due to improved accounting of canopy effects and heat storage of the floodwater.

Technical Abstract: The combination of canopy cover and a free water surface makes the sub-canopy environment of flooded forested wetlands unlike other aquatic or terrestrial systems. The sub-canopy vapor flux and energy budget are not well understood in wetlands, but they importantly control water level and understory climate. Combined energy balance and eddy covariance approaches were used to quantify sub-canopy energy fluxes in a permanently flooded forested wetland in southeastern Louisiana, USA. Over the five-month measurement period (June-November), the sub-canopy airspace was humid, maintaining saturation vapor pressure for 28% of the total record. This humidity, coupled with the thermal inertia of the water, altered both diel and seasonal energy exchanges. During fall, there were stronger vapor pressure gradients compared to summer when, even during the day, vapor pressure gradients were often towards the water surface. Even with low wind speeds, magnitudes of daytime vapor pressure deficits and aerodynamic conductance could potentially explain a large fraction of the evaporation; however, evaporation varied almost exclusively with energy availability. Monthly mean Bowen ratios were typically close to zero and never exceeded 0.22. Monthly mean evaporation ranged from 0.9 to 2.0 mm day-1, peaking in October due to high net radiation flux with increased transmission of solar radiation through a senescing canopy. Fall senescence and concurrent seasonal temperature shifts resulted in a release of heat stored in floodwater, altering the seasonality of energy available to evaporation. These same conditions also resulted in peak sensible heat fluxes in fall, but sensible heat fluxes were consistently low. Evaporation rates in this wetland exceed those typical of upland forests, but perhaps the distinct difference from upland forests are constant free water availability and seasonal energy shifts due to energy storage in floodwater.