Environmental Engineering Reference
In-Depth Information
albedo was also obtained from the GOES sys-
tem. Incoming and outgoing longwave radiation
was estimated from air and soil-surface tem-
peratures. Temperature, relative humidity, and
wind speed at each grid point were determined
by interpolation (inverse-distance weighting)
of data obtained from land-based NOAA and
University of Florida weather stations.
used to distinguish between water on land
surface and water in the subsurface. In areas
where surface-water storage varies slightly over
time, Rodell
et al
. (
2004
;
2007
) and Strassberg
et al
. (
2007
) demonstrated that GRACE satel-
lite data could be used to monitor changes in
groundwater storage. Rodell
et al
. (
2004
) used
a water-budget method, with change in stor-
age determined from GRACE data, to generate
estimates of evapotranspiration rates for the
Mississippi River watershed.
2.5.3 Change in storage
Passive and active microwave sensors are the
most widely used satellite instruments for esti-
mating surface-water storage and soil-water
content (Krajewski
et al
.,
2006
). The emission
(passive sensing) or backscattering (active sens-
ing) of energy in the microwave region is a
function of the dielectric constant of the soil
and water. The dielectric constant varies with
soil-water content. As part of NASA's AQUA
satellite mission, readings from the Advanced
Microwave Sensing Radiometer (AMSR-E) pro-
vide daily estimates of snow depth, snow-cov-
ered area, snow-water equivalent, and soil-water
content (top 50 mm) worldwide on a 25-km
grid (http://nsidc.org/data/amsre/; accessed
August 25, 2009). The RADARSAT-1 satellite's
active microwave sensor is another source for
soil-water content data (Shi and Dozier,
1997
;
http://www.asc-csa.gc.ca/eng/satellites/radar-
sat1/; accessed August 25, 2009). Microwave
readings are not affected by weather, but they
are affected by land cover, soil properties, sur-
face slope, and aspect. If compensations can be
made for these interferences, reliable estimates
of soil-water content in the top 50 mm of soil
can be obtained (Makkeasorn
et al
.,
2006
). Land-
surface models, such as NOAH, are used to relate
changes in water storage at the land surface to
changes throughout the entire soil profile.
The Gravity Recovery and Climate Exper-
iment (GRACE) satellites provide monthly
measurements of the Earth's gravity field. As
discussed in
Section 2.3.3
, changes in the grav-
ity field over land surface are mainly related to
changes in the amount of water stored on the
surface and in the subsurface. Therefore, data
from GRACE can be used to estimate water
storage changes, although the data cannot be
2.5.4 Indirect use of remotely sensed
data
In addition to supporting water-budget
analyses, remotely sensed data are useful in
a number of indirect ways, such as locating
areas of ground- and surface-water exchange
(Becker,
2006
), mapping areas of groundwater
recharge and discharge (Tweed
et al
.,
2007
),
and identifying areas of apparently uniform
evapotranspiration rates (Smith
et al
.,
2007
).
As part of a water-budget study of the Basin
and Range carbonate-rock aquifer system,
estimates of evapotranspiration were devel-
oped for an area of approximately 37 500 km
2
in Nevada and Utah with the aid of satellite
imagery (Moreo
et al
.,
2007
; Laczniak
et al
.,
2008
). Areas of similar vegetation type and
density and soil characteristics, called evapo-
transpiration units, were identified by using
the Landsat Thematic Mapper. The Landsat
images have a resolution of approximately
30 m and cover six spectral bands. The identi-
fication procedure included, among other cri-
teria, a soil-vegetation index (modified from Qi
et al
.,
1994
). The 10 evapotranspiration units
ranged from areas of no vegetation (including
open water, dry playa, and moist bare soil) to
areas of increasing density of grasses, rushes,
and phreatophytic shrubs (Smith
et al
.,
2007
).
Field measurements of evapotranspiration
rates were obtained over 1 year by the eddy
correlation method in representative locations
in each unit. Evapotranspiration rates for the
entire study area were determined by multi-
plying measured evapotranspiration rates for
each unit by the mapped area of each unit.
