Evaporation and the subcanopy energy environment in a flooded forest

Abstract

The combination of tree canopy cover and a free water surface makes the subcanopy environment of flooded forested wetlands unlike other aquatic or terrestrial systems. Subcanopy vapour fluxes and energy budgets represent key controls on water level and understorey climate but are not well understood. In a permanently flooded forest in south‐eastern Louisiana, USA, an energy balance approach was used to address (a) whether evaporation from floodwater under a forest canopy is solely energy limited and (b) how energy availability was modulated by radiation and changes in floodwater heat storage. A 5‐month continuous measurement period (June–November) was used to sample across seasonal changes in canopy activity and temperature regimes. Over this period, the subcanopy airspace was humid, maintaining saturation vapour pressure for 28% of the total record. High humidity coupled with the thermal inertia of surface water altered both seasonal and diel energy exchanges, including atypical phenomena such as frequent day‐time vapour pressure gradients towards the water surface. Throughout the study period, nearly all available energy was partitioned to evaporation, with minimal sensible heat exchange. Monthly mean evaporation ranged from 0.7 to 1.7 mm/day, peaking in fall when canopy senescence allowed greater radiation transmission; contemporaneous seasonal temperature shifts and a net release of stored heat from the surface water resulted in energy availability exceeding net radiation by 30% in October and November. Relatively stable energy partitioning matches Priestley–Taylor assumptions for a general model of evaporation in this ecosystem.

Evapotranspiration Characteristics Distinct to Mangrove Ecosystems Are Revealed by Multiple‐Site Observations and a Modified Two‐Source Model

“…As a whole, the cumulative annual sums of half‐hourly G w at YX were negative, indicating a net release of stored tidal water heat energy to the mangrove ecosystem, while cumulative values at the GQ and LZ mangrove ecosystems were positive. The main difference in G w terms between the GQ and YX ecosystems happened in autumn and winter when values of G w were largest but other components of the surface energy budget were relatively small, consistent with the observations of Allen et al () and Shoemaker et al (). Our addition of G w into equation improved energy balance closure in the three mangrove ecosystems by 2% to 3%.…”

“…First, one of the most important differences between mangrove ecosystems and fully terrestrial ecosystems is the impact of tidal fluctuations, as enthalpy exchange with tidal waters (G w ) may affect closure of the surface energy balance as indicated by equation ( 7) (Sobrado, 2007;. Allen et al (2017) and Shoemaker et al (2005) also found that the contributions of vertical exchange with surface water were an essential component of the surface energy budget in wetlands, particularly on dry winter days when surface energy exchanges were seasonally weak. Contributions of tidal water (G w ) to the surface energy budget consist of (1) vertical exchange between the tidal water and the air above or soil below (i.e., the net heating of surface water) and ( 2) lateral transport during ebb-and-flood tides (G adv ).…”

Forested versus herbaceous wetlands: Can management mitigate ecohydrologic regime shifts from invasive emerald ash borer?

“…Vegetation structure can further influence ET through effects to energy partitioning. For instance, canopy structure filters energy inputs for understory transpiration and direct evaporation of standing and/or soil water (Allen et al, 2017). Additionally, canopy structure likely influences snowfall interception and sublimation and wind redistribution that can result in less snow pack under ash relative to clearcut treatments (Molotch et al, 2007;LaMalfa and Ryle, 2008;Veatch et al, 2009).…”

Hydrologic variability in black ash wetlands: Implications for vulnerability to emerald ash borer

“…Under inundated conditions (defined throughout as water level > 0), open water evaporation can become an additional and dominant ET component, often exceeding transpiration rates (Allen, Reba, Edwards, & Keim, 2017). Our previous work in lowland flats suggests that this water level control may determine the extent of ET reduction following tree mortality.…”