Environmental Engineering Reference
In-Depth Information
Daily estimates of reference evapotranspira-
tion are available at approximately 120 loca-
tions in the California Irrigation Management
Information System (CIMIS) net work (
Table 2.1
)
and at 70 locations in the Pacific Northwest of
the United States as part of the Agrimet net-
work operated by the Bureau of Reclamation,
in conjunction with the Agricultural Research
Service (
Table 2.1
).
8
Penman
P-T
ET
o
6
4
2
2.4.3 Change in storage
Much of the discussion contained in
Section
2.3.3
for measuring changes in storage at the
local scale also applies at the mesoscale. Snow
courses are used to determine water storage
in transects of snowpacks. Aircraft-mounted
gamma-ray spectrometry (Glynn
et al
.,
1988
)
and microwave measurements (Chang
et al
.,
1997
) offer more rapid means to conduct snow
surveys; 500 km transects can typically be cov-
ered in a single day. In addition, Fassnacht
et al
.
(
2003
) demonstrated how to interpolate snow-
pack data from SNOTEL sites onto a 1-km grid.
The temporal gravity technique described in
Section 2.3.3
has been applied over transects
as long as 20 km; longer transects are feasible.
Thornthwaite and Mather (
1955
;
1957
) proposed
a bookkeeping technique for determining
total change in storage in a watershed. Their
approach is illustrated in a following example.
0
0
100
200
300
400
Julian date, 1978
Figure 2.9
Daily estimates of potential
evapotranspiration by the Penman (Equation (
2.32
)) and
Priestley-Taylor (P-T) (Equation (
2.35
)) methods and
reference evapotranspiration (ET
o
) by Equation (
2.36
) for
Kimberly, Idaho, for 1978 as calculated by the computer
program Ref-ET (University of Idaho,
2003
). Data used in
the calculations included daily precipitation, minimum and
maximum air and soil temperatures, relative humidity, and
solar radiation. Similar trends are apparent over the course
of the year for all three estimates, although considerable
variability exists on a daily basis.
can be calculated by multiplying
ET
o
by the
crop coefficients listed in Allen
et al
. (
1998
).
An attractive feature of these climatologi-
cal methods is that they can usually be applied
with data that are available at standard NOAA
weather stations (e.g. air temperature, pre-
cipitation, relative humidity, wind speed, and
solar radiation); no additional data collection is
required. Some parameters in the above equa-
tions, such as net radiation and soil-heat flux,
are seldom measured at NOAA stations, but
equations for estimating these variables can be
found in Allen
et al
. (
1998
), Jensen
et al
. (
1990
),
and Rosenberg
et al
. (
1983
). The similarities in
the above equations facilitate calculation of
multiple estimates of
PET
or
ET
o
in a spread-
sheet or in computer programs such as Ref-ET
(University of Idaho,
2003
). Ref-ET will calculate
PET
or
ET
o
by as many as 15 different equations
(
Figure 2.9
) using available data and estimating
values for parameters that were not measured.
No national network of evapotranspiration
sites exists within the United States, but sta-
tions are maintained in certain regions to assist
water managers and irrigation schedulers.
2.4.4 Surface flow
Estimates of surface-water flow may be needed
in recharge studies for predicting direct run-
off in the absence of any streamflow gauges or
data. A common technique for predicting dir-
ect runoff is the US Soil Conservation Service
curve number method (Natural Resources
Conservation Service,
2004
). The dimensionless
curve number,
CN
, is determined on the basis
of soil type, land use, and antecedent soil-water
content. Direct runoff,
R
off
, in inches, is esti-
mated as:
R
=− +
(
P SP S
.2 ) /(
.8 )
(2.37)
2
off
where
P
is precipitation (in inches) and
S
= 1000 /
CN
- 10. Numerous variations of Equation (
2.37
)
appear in the literature (Garen and Moore,
2005a
,
b
).
