Environmental Engineering Reference
In-Depth Information
apply and, in general, require only data that
are available from standard sources, such as
National Weather Service weather stations and
US Geological Survey stream-gauging stations.
Although discussed here in terms of mesoscale
studies, these methods may also be applied in
smaller scale studies. In particular, estimating
equations are often preferred to the expen-
sive and time consuming micrometeorological
methods for measuring evapotranspiration.
Watershed models are commonly used for stud-
ying water budgets at this scale (
Section 3.4
).
Discussion of upscaling procedures, a key part
of mesoscale watershed studies, is deferred to
Section 3.7
.
values are determined on the basis of surface-
based weather stations and elevation.
2.4.2 Evapotranspiration
Because of the limited availability of evapo-
transpiration information at the mesoscale,
values of evapotranspiration are often esti-
mated. Many estimating equations have been
developed, and although they may lack the
accuracy of the micrometeorological methods,
they are easier and less expensive to apply.
Rosenberg
et al
. (
1983
) referred to these tech-
niques as
climatological
methods. Jensen
et al
.
(
1990
) reviewed several of these methods. Brief
descriptions of some of the more widely used
equations follow.
Most of the techniques described in this sec-
tion were developed to predict potential eva-
potranspiration,
PET
. According to Rosenberg
et al
. (
1983
), “Potential evapotranspiration is
the evaporation from an extended surface
of a short green crop which fully shades the
ground, exerts little or negligible resistance to
the flow of water, and is always well supplied
with water. Potential evapotranspiration can-
not exceed free water evaporation under the
same weather conditions.” Conditions within a
watershed will not always be consistent with
these requirements, so actual evapotranspira-
tion is often less than
PET
. Actual values can be
estimated from values of
PET
if the ratio of
ET/
PET
, called the crop coefficient (
K
c
), is known.
Doorenbos and Pruitt (
1975
) and Jensen
et al
.
(
1990
) provide tables of
K
c
values for various
crops under different climate conditions and
stages of growth. Both actual and potential
evapotranspiration are important in hydro-
logic studies. Potential evapotranspiration cal-
culations are used in irrigation management
systems. Many hydrologic models require
potential, as opposed to actual, evapotranspi-
ration data (or the data from which
PET
can be
calculated).
Pan evaporation is measured at many
weather stations (
Table 2.1
).
PET
can be esti-
mated from recorded pan evaporation using the
formula:
2.4.1 Precipitation
In the past, maps of mean annual precipita-
tion and precipitation frequency distribution
for the United States were published by the US
Department of Commerce (e.g. Karl and Knight,
1985
; Barnston,
1993
). These widely used maps
were generated from data collected at surface-
based precipitation gauges. Newer techniques
that use radar as well as gauge data provide more
accurate data at finer resolutions. Since 2005,
NOAA's Multisensor Precipitation Estimation
(MPE) program (Seo,
1998
; Seo
et al
.,
1999
;
2000
)
has provided estimates of hourly and daily pre-
cipitation on a 4-km grid for the entire conter-
minous United States by using surface-based
Nexrad Doppler radar in conjunction with real-
time standard rain gauges and satellite imagery
(
Table 2.1
). These estimates are used for pre-
dicting streamflow and providing warnings of
severe weather and floods, especially for large
watersheds; the estimates are also useful in
water-budget studies for estimating recharge.
Refined temporal and spatial resolutions of the
MPE estimates should be available in the future.
The Daymet database (
Table 2.1
; Thornton
et al
.,
1997
) is another source of precipitation data. It
contains daily precipitation, air temperature,
and radiation data for the United States from
1980 to 1997 generated on a 1-km grid using
data from NOAA's surface-based weather sta-
tions. The Prism modeling system (
Table 2.1
,
Daly
et al
.,
1994
;
2008
) provides monthly pre-
cipitation and air temperature on a 4-km grid;
PET
=
k E
p
pan
(2.29)
