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Abstract:
The WAter Cycle Multi-mission Observation
Strategy – EvapoTranspiration (WACMOS-ET) project has
compiled a forcing data set covering the period 2005–2007
that aims to maximize the exploitation of European Earth
Observations data sets for evapotranspiration (ET) estimation.
The data set was used to run four established ET algorithms:
the Priestley–Taylor Jet Propulsion Laboratory model
(PT-JPL), the Penman–Monteith algorithm from the MODerate
resolution Imaging Spectroradiometer (MODIS) evaporation
product (PM-MOD), the Surface Energy Balance System
(SEBS) and the Global Land Evaporation Amsterdam
Model (GLEAM). In addition, in situ meteorological data
from 24 FLUXNET towers were used to force the models,
with results from both forcing sets compared to tower-based
flux observations. Model performance was assessed on several
timescales using both sub-daily and daily forcings. The
PT-JPL model and GLEAM provide the best performance
for both satellite- and tower-based forcing as well as for the
considered temporal resolutions. Simulations using the PMMOD
were mostly underestimated, while the SEBS performance
was characterized by a systematic overestimation. In general, all four algorithms produce the best results in wet
and moderately wet climate regimes. In dry regimes, the correlation
and the absolute agreement with the reference tower
ET observations were consistently lower. While ET derived
with in situ forcing data agrees best with the tower measurements
(R2 D0.67), the agreement of the satellite-based
ET estimates is only marginally lower (R2 D0.58). Results
also show similar model performance at daily and sub-daily
(3-hourly) resolutions. Overall, our validation experiments
against in situ measurements indicate that there is no single
best-performing algorithm across all biome and forcing
types. An extension of the evaluation to a larger selection of
85 towers (model inputs resampled to a common grid to facilitate global estimates) confirmed the original findings.
