Environmental Engineering Reference
In-Depth Information
as temperature, vegetation type and cover,
leaf area index, and land use. This informa-
tion can have important indirect uses, such as
defining geologic structure, identifying areas
that have similar hydrologic characteristics
(Brunner
et al
.,
2007
), and providing a basis for
the upscaling of point measurements of evapo-
transpiration (Smith
et al
.,
2007
).
be estimated with knowledge of surface and
atmospheric temperatures, which can be deter-
mined from radiation readings in the infrared
wavelength range. Soil-heat flux is usually
assumed to be a percentage of calculated net
radiation. Estimation of sensible-heat flux is
more problematic;
H
is often estimated with
a temperature gradient/resistance formula,
where the gradient is a function of the surface
and atmosphere temperatures and the aerody-
namic resistance term is a function of vegeta-
tion density and height (Norman
et al
.,
1995
).
Unfortunately, cloud cover interferes with
some imagery and thus may limit the applica-
bility of some methods.
Liu
et al
. (
2003
) estimated daily evapotrans-
piration on a 1-km grid for all of Canada for
1996 using the Penman-Monteith method
(Equation (
2.34
)). The required model input
was land cover, leaf area index, soil-water hold-
ing capacity (AWC), and daily meteorological
data (net radiation, minimum and maximum
air temperature, humidity, precipitation, and
snowpack). Ten classes of land cover were
defined (e.g. coniferous forest, cropland, and
urban area) using data from the Advanced
Very High Resolution Radiometer (AVHRR)
on the NOAA 14 satellite. Leaf area index was
determined with Landsat Thematic Mapper
(TM) images and surface measurements at
eight sites. AWC was obtained from the Soils
Landscapes of Canada database (http://sis.agr.
gc.ca/cansis/nsdb/slc/intro.html; accessed July
21, 2009). Daily meteorological data for 1996
w e r e ob t a i n e d f r om t h e m e d i u m - r a n g e f or e c a s t
Global Flux Archive of the National Center for
Environmental Prediction (Kalnay
et al
.,
1990
).
Bilinear interpolation was used to assign data
values to each point on the 1-km grid.
Daily Priestley-Taylor potential (Equation
(
2.35
)) and reference (Equation (
2.36
)) evapo-
transpiration are available on a 1-km grid for the
State of Florida from 1995 to present (
Table 2.1
).
The calculations are made with a combination
of satellite-derived and land-based data. Daily
solar radiation is calculated from hourly read-
ings from NOAA's Geostationary Operational
Environmental Satellite (GOES) system. Surface
2.5.1 Precipitation
Satellite-based estimates of daily precipitation
(such as those from NASA's AQUA or Tropical
Rainfall Measurement Missions) are generated
worldwide on an approximately 25-km grid
(
Table 2.1
). The spatial resolution is not as fine
as the 4-km grid provided for the United States
by MPE (
Section 2.3.1
,
Table 2.1
), but unlike
ground-based radar, satellite measurements are
not adversely affected by complex terrain, such
as mountainous areas, or areas with no pre-
cipitation gauges. Continued refinement and
integration of ground-based and satellite-based
methods for estimating precipitation rates may
result in the widespread use of these methods
in future recharge studies at the watershed
scale.
2.5.2 Evapotranspiration
Although currently (2010) there is no data-
base of evapotranspiration estimates gener-
ated from satellite data, this is an area of
active research. Most methods for estimating
latent-heat flux, λ
ET
, are based on the surface
energy-balance equation (Equation (
2.15
)).
Remotely sensed data can be used to estimate
net radiation,
R
n
, sensible-heat flux,
H
, and soil-
heat flux,
G
; and λ
ET
can be determined as the
residual in the equation (Timmermans
et al
.,
2007
). Estimation of net radiation as the sum
of net shortwave and net longwave radiation
is fairly straightforward. Net shortwave radia-
tion requires estimates of incoming radiation
and surface albedo or reflectance; these can be
obtained on a 250-m grid with microwave read-
ings from an instrument such as NASA's mod-
erate resolution imaging spectroradiometer,
MODIS (http://aqua.nasa.gov/science/; accessed
August 25, 2009). Net longwave radiation can
