Evapotranspiration Estimates from Remote Sensing for Irrigation Water Management - Satellite-based Applications on Climate Change - page 191

in the thermal band. It also is applied with MODIS satellite data, which have a
1,000-m resolution in the thermal band for larger areas where the courser resolution
is sufficient. METRIC is particularly relevant for the Morocco study area because
of the World Bank MA project's need for monthly ET estimates of irrigation
schemes targeted for improvement and rehabilitation, the existence of a long-term
meteorological record at key sites, and an archive of quality Landsat data including
the essential thermal bands. The surface energy balance in METRIC has advantages
over conventional ET estimations based on crop coefficient curves or vegetation
indices, in that specific crop or vegetation types are not required, and the energy
balance can detect reduced ET under water shortages, saline soils, or other
constraints.
METRIC employs calibration principles and techniques used by the pioneering
model SEBAL (Surface Energy Balance Algorithms for Land), an important energy
balance model developed by Bastiaanssen et al. ( 1998 ). In particular, METRIC uses
a calibrated inverse-modeling approach at extreme conditions (Allen et al. 2007a ),
which was pioneered in SEBAL for estimating the near-surface temperature gradi-
ent (dT) as an indexed function of radiometric surface temperature, thereby
eliminating the need for absolutely accurate aerodynamic surface temperature or
the need for air temperature measurements for estimating sensible heat flux at the
surface. METRIC employs internal calibration of the satellite-based energy balance
at two extreme conditions (dry and wet), using locally available weather data.
Professionals familiar with energy balance, aerodynamics, and basic radiation
physics design METRIC algorithms for applications. The primary inputs for the
model are shortwave and longwave (thermal) satellite images (e.g., Landsat and
MODIS), a digital elevation model, and ground-based weather data measured
within or near the area of interest. ET images developed from Landsat provide
means to quantify ET on a field basis in terms of both rate and spatial distribution.
The auto-calibration in METRIC for each image uses an alfalfa-based reference ET
( ET r ) that is derived from hourly weather data. The internal calibration and the use
of the indexed temperature gradient reduce or eliminate the need for atmospheric
correction of T s and reflectance (albedo) measurements using radiative transfer
models. The internal calibration also reduces impacts of biases in estimation of
aerodynamic stability correction and surface roughness (Allen et al. 2007a ).
METRIC uses weather-based reference ET so that both calibration and extrapo-
lation of instantaneous ET to 24-h and longer periods compensate for regional
advection effects where ET can exceed daily net radiation.
13.4.2 METRIC Development History
The innovative component of SEBAL and METRIC is that the energy balance
modeling uses a d T that is indexed to radiometric surface temperature, which has
eliminated the need for absolute surface temperature calibration—a major impedi-
ment to operational satellite ET. The theoretical and computational basis of SEBAL