The GEWEX LandFlux project: evaluation of model evaporation using tower-based and globally gridded forcing data
Determining the spatial distribution and temporal development of evaporation at regional and global scales is required to improve our understanding of the coupled water and energy cycles and to better monitor any changes in observed trends and variability of linked hydrological processes. With recen...
| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2016-01-01
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| Series: | Geoscientific Model Development |
| Online Access: | http://www.geosci-model-dev.net/9/283/2016/gmd-9-283-2016.pdf |
| Summary: | Determining the spatial distribution and temporal development of evaporation
at regional and global scales is required to improve our understanding of
the coupled water and energy cycles and to better monitor any changes in
observed trends and variability of linked hydrological processes. With
recent international efforts guiding the development of long-term and
globally distributed flux estimates, continued product assessments are
required to inform upon the selection of suitable model structures and also
to establish the appropriateness of these multi-model simulations for global
application. In support of the objectives of the Global Energy and Water Cycle Exchanges (GEWEX) LandFlux project,
four commonly used evaporation models are evaluated against data from
tower-based eddy-covariance observations, distributed across a range of
biomes and climate zones. The selected schemes include the Surface Energy Balance System (SEBS) approach, the Priestley–Taylor Jet Propulsion
Laboratory (PT-JPL) model, the Penman–Monteith-based Mu model (PM-Mu) and
the Global Land Evaporation Amsterdam Model (GLEAM). Here we seek to examine
the fidelity of global evaporation simulations by examining the multi-model
response to varying sources of forcing data. To do this, we perform parallel
and collocated model simulations using tower-based data together with a
global-scale grid-based forcing product. Through quantifying the multi-model
response to high-quality tower data, a better understanding of the
subsequent model response to the coarse-scale globally gridded data that
underlies the LandFlux product can be obtained, while also providing a
relative evaluation and assessment of model performance.
<br><br>
Using surface flux observations from 45 globally distributed
eddy-covariance stations as independent metrics of performance, the
tower-based analysis indicated that PT-JPL provided the highest overall
statistical performance (0.72; 61 W m<sup>−2</sup>; 0.65), followed closely by GLEAM (0.68; 64 W m<sup>−2</sup>; 0.62), with
values in parentheses representing the <i>R</i><sup>2</sup>, RMSD and Nash–Sutcliffe efficiency
(NSE), respectively. PM-Mu (0.51; 78 W m<sup>−2</sup>; 0.45) tended to underestimate fluxes, while SEBS
(0.72; 101 W m<sup>−2</sup>; 0.24) overestimated values relative to observations. A focused analysis
across specific biome types and climate zones showed considerable
variability in the performance of all models, with no single model
consistently able to outperform any other. Results also indicated that the
global gridded data tended to reduce the performance for all of the studied
models when compared to the tower data, likely a response to scale mismatch
and issues related to forcing quality. Rather than relying on any single
model simulation, the spatial and temporal variability at both the tower-
and grid-scale highlighted the potential benefits of developing an ensemble
or blended evaporation product for global-scale LandFlux applications.
Challenges related to the robust assessment of the LandFlux product are also
discussed. |
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| ISSN: | 1991-959X 1991-9603 |