OpenET Satellite-based ET Intercomparisons with Ground-based
Measurements: Phase II Results
Abstract
OpenET is a software system that makes satellite-based multi-model
estimates of evapotranspiration (ET) accessible at multiple spatial and
temporal scales over the U.S. Large-scale ET estimates fill a critical
data-gap for irrigation management, water resources management, and
hydrological modeling and research. We present the methods and results
of the second phase of an intercomparison and accuracy assessment
between OpenET satellite-based models (ALEXI/DisALEXI, eeMETRIC, PT-JPL,
geeSEBAL, SIMS and SSEBop) and a benchmark ground-based ET dataset with
data from nearly 200 eddy covariance towers across the contiguous U.S.
Processing steps for the benchmark dataset included gap-filling, energy
balance closure correction, calculation of closed and unclosed daily ET,
and multiple levels of data QA/QC. The dataset was split into three
groups, phase I and II of the intercomparison and a reserve dataset for
future studies. To sample satellite-based ET pixels, static flux
footprints were generated at each station based on dominant wind speed
and direction. Where data allowed, two dimensional flux footprints that
are weighted by hourly ETo were developed and used for ET pixel
sampling. A wide range of visual and statistical comparisons between
satellite and ground-based ET were conducted at each station and against
stations grouped by land cover type. Based on key performance metrics
including bias, coefficient of determination, and root mean square
error, model results show promising agreement at many flux sites
considering the inherent uncertainty in station data. Remote sensing
models show the highest agreement with closed station ET in irrigated
annual cropland settings whereas locations of native vegetation with
high aridity and some forested stations show relatively less agreement.
The benchmark ET dataset was used to explore different approaches to
computing a single ensemble estimate from the six model ensemble, with
the goal of reducing the influence of model outliers and selection of
weighting and data sampling schemes to reduce the influence of flux
stations with sparse or extensive data records. We present the results
from the model intercomparison and accuracy assessment and discuss model
performance relative to accuracy requirements from the OpenET user
community.