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

Abstract

Abstract: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 (ˇ) 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 ratioˇ³ 1Ð0. 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.

Sazonalidade do Balanço de Energia e Evapotranspiração em Área Arbustiva Alagável no Pantanal Mato-Grossense

“…O MRB tem sido amplamente utilizado desde sua proposição em 1926, porque tem as vantagens de um conceito físico, alguns requisitos de parâ-metros, e um método de cálculo simples (Hu et al, 2013). A comparação de LE e H obtidos pelo MRB e método de covariância turbulenta mostram que, o MRB fornece valores exatos e confiáveis, e que o desvio de leitura dos instrumentos é mínimo em um período inferior a um ano (Drexler et al, 2004;Billesbach e Arkebauer, 2012;Dicken et al, 2013). O cálculo de MRB seguiu as orientações descritas por Perez et al (1999).…”

Estimating the Bowen ratio over the open and ice‐covered ocean

“…The Priestley-Taylor method, especially, is still used frequently [e.g., Eichinger et al, 1996;Kim and Entekhabi, 1997;Drexler et al, 2004;Guo et al, 2007]. Both Priestley and Taylor and Hicks and Hess predicted Bo from a linear function of Boà but only over water surfaces and only for the H s > 0 and H L > 0 regime.…”

“…;Brutsaert, 1982, p. 210;Arya, 1988, p. 191;Stull, 1988, p. 274;Drexler et al, 2004;Guo et al, 2007]. We represent the surface energy budget as…”

Wetland Evapotranspiration: Eddy Covariance Measurement in the Biebrza Valley, Poland

“…At the same time, the complexity of the process of evapotranspiration in wetlands can be evidenced by the fact that in many cases, however, higher evaporation from an open water surface was observed (Campbell and Williamson 1997;Burba et al 1999) in relation to evapotranspiration from wetlands. In addition, a number of studies point to evapotranspiration as the main component of the water balance of marshland areas, and (the energy equivalent of evapotranspiration) the latent heat flux dominates in the surface heat balance (Priban and Ondok 1985;Drexler et al 2004). Therefore, a detailed knowledge of evapotranspiration of wetlands can be crucial for the functioning of these ecosystems or for restoring the land transformed as a result of land reclamation or other human activities to its original condition.…”