Within the past decade or so, the accuracy of evapotranspiration ( ET ) estimates has improved due to new and increasingly sophisticated methods. Yet despite a plethora of choices concerning methods, estimation of wetland ET remains insufficiently characterized due to the complexity of surface characteristics and the diversity of wetland types. In this review, we present models and micrometeorological methods that have been used to estimate wetland ET and discuss their suitability for particular wetland types. Hydrological, soil monitoring and lysimetric methods to determine ET are not discussed. 
Our review shows that, due to the variability and complexity of wetlands, there is no single approach that is the best for estimating wetland ET . Furthermore, there is no single foolproof method to obtain an accurate, independent measure of wetland ET . Because all of the methods reviewed, with the exception of eddy covariance and LIDAR, require measurements of net radiation ( R n ) and soil heat flux ( G ), highly accurate measurements of these energy components are key to improving measurements of wetland ET . 
Many of the major methods used to determine ET can be applied successfully to wetlands of uniform vegetation and adequate fetch, however, certain caveats apply. For example, with accurate R n and G data and small Bowen ratio (β) values, the Bowen ratio energy balance method can give accurate estimates of wetland ET . However, large errors in latent heat flux density can occur near sunrise and sunset when the Bowen ratio β≈-1·0. The eddy covariance method provides a direct measurement of latent heat flux density ( E ) and sensible heat flux density ( H ), yet this method requires considerable expertise and expensive instrumentation to implement. A clear advantage of using the eddy covariance method is that E can be compared with R n -G-H , thereby allowing for an independent test of accuracy. The surface renewal method is inexpensive to replicate and, therefore, shows particular promise for characterizing variability in ET as a result of spatial heterogeneity. LIDAR is another method that has special utility in a heterogeneous wetland environment, because it provides an integrated value for ET from a surface. The main drawback of LIDAR is the high cost of equipment and the need for an independent ET measure to assess accuracy. If R n and G are measured accurately, the Priestley-Taylor equation can be used successfully with site-specific calibration factors to estimate wetland ET . The 'crop' cover coefficient ( K c ) method can provide accurate wetland ET estimates if calibrated for the environmental and climatic characteristics of a particular area. More complicated equations such as the Penman and Penman-Monteith equations also can be used to estimate wetland ET , but surface variability and lack of information on aerodynamic and surface resistances make use of such equations somewhat questionable.

A review of models and micrometeorological methods used to estimate wetland evapotranspiration

Surface energy fluxes of Phragmites australis in a prairie wetland

Evapotranspiration in semi-arid wetlands: relationships between inundation and the macrophyte-cover:open-water ratio

The diurnal course of the xylem water flow in a solitary Salix fragilis L tree in a wet grassland was measured using the tissue heat-balance method There was considerable variation due to meteorological factors Maximum flow rate was 04 kg h-1 m-2 of crown projection area, or 59 kg h-1 kg-1 leaf dry weight The daily total was 24 kg m-2 day-1 or 36 kg kg-1 day-1 Water flow decreased immediately at the tree base and at the branches after start of rain, and in a branch, after cutting it off: the time constant of the system was 600–700 s in both cases The part of the crown oriented to the sun transpired up to ten times as much as the shaded part Over 70% of the total cross-sectional area of the conductive xylem vessels of the trunk was used by the transpiration flow The water content of the trunk tracked the diurnal changes of the xylem water flow rate with a short time-lag During the day, 1% of the trunk volume was temporarily exploited as water reserve, an amount equalling 3% of daily water loss The stereometric configuration of the crown significantly influenced its water loss During the summer period, about 33 mature (polycormic) trees per ha may drain 100% of water consumed by the present-day sedge-grass marsh

https://www.jstor.org/stable/info/4217437

Xylem water flow in a crack willow tree (Salix fragilis L.) in relation to diurnal changes of environment.

Evapotranspiration from sedge-dominated wetland surfaces

Daily course of heat balance components (sensible heat, latent heat of evaporation and ground heat fluxes) and evapotranspiration were computed from meterological data recorded in a sedge-grass marsh in South Bohemia, Czechoslovakia. Bowen-ratio method and tank measurements were used, and several comparisons made with parallel aerodynamic measurements. Two simplified empirical formulae for calculating evapotranspiration are suggested: one based on net radiation, the other based on net radiation and on the difference between the mean relative air humidities at the upper and lower levels of the boundary layer above the stand. The seasonal courses and totals of heat-balance components and evapotranspiration obtained in three years (1977 to 1979) are compared and discussed.

Heat balance components and evapotranspiration from a sedge-grass marsh

Vertical profiles of soil temperature and of the dry and wet bulb temperature of the air, relative humidity and wind velocity were measured within and above stands ofJunco-MolinietumPreising 1951 (facies ofDeschampsia caespitosa) andCaricetum gracilisAlmquist 1929 on selected sunny days during the growing season. Evapotranspiration was computed by means of the heat-budget equation. The principal difference between the two communities has been found in the soil temperature. The stand of theCaricetum gracilis occupies a habitat with a cooler soil and with a higher ground water level than the stand of theJunco-Molinietum coeruleae, facies ofDeschampsia caespitosa. The evapotranspiration per unit ground area was higher by some 20% in theCaricetum gracilis. The evapotranspiration per unit above-ground biomass was much the same in both communities.

Karel Přibáň

Papers

Heat balance components and evapotranspiration from a sedge-grass marsh

Microclimate and evapotranspiration in two wet grassland communities