Environmental Engineering Reference
In-Depth Information
Table 2.3. Average monthly water budget in millimeters for Seabrook, NJ (Thornthwaite and Mather,
1955
).
Jan
Feb
Mar
April
May
June
July
Aug
Sept
Oct
Nov
Dec
Year
P
87
93
102
88
92
91
112
113
82
85
70
93
110 8
PET
1
2
16
46
92
131
154
136
97
53
19
3
750
ET
1
2
16
46
92
129
147
13 0
92
53
19
3
730
Δ
S
uz
0
0
0
0
0
-38
-35
-17
-10
32
51
17
0
MS
86
91
86
42
0
0
0
0
0
0
0
73
378
Q
bf
43
46
43
21
0
0
0
0
0
0
0
37
189
ET
i
, was set equal to
PET
for each day, if precipi-
tation and soil-water storage were sufficiently
high; otherwise,
ET
i
was calculated on the basis
of
S
uz
i
and a threshold parameter.
S
uz
max was
set equal to the depth of rooting multiplied
by the available water capacity (AWC). AWC is
the difference in water content between field
capacity and wilting point and varies with soil
texture. Values of AWC were obtained from
the Natural Resources Conservation Service
SSURGO database (http://soils.usda.gov/survey/
geography/ssurgo/; accessed July 27, 2009) and
other published reports for the 65 mapped soil
types in the study area. Rooting depth varied
by land cover class (there were 26 such classes),
and was held constant throughout the year.
Mean annual drainage rates for the 1926
to 1979 period varied widely (
Fig ure 2.11
),
ranging from less than 250 mm/yr to more
than 7500 mm/yr. The pattern of drainage
matches that of precipitation, with increas-
ing rates with increasing elevation. Drainage
rates were highest in the 1926 to 1979 period
and showed a general decreasing trend in sub-
sequent periods (
Fig ure 2.12
). The estimated
drainage for the 2000 to 2004 period was
44% less than that for the first period. The
decreased drainage was attributed mainly
to a reduction in the amount of land in agri-
cultural use (the amount of land planted in
sugar cane decreased by 22% between 1979
and 2004), improved irrigation efficiency
(furrow irrigation of sugar cane was replaced
with drip irrigation during the 1980s), and
decreased rainfall rates from 1990 to 2004.
Engott and Vana (
2007
) conducted a sensitiv-
ity analysis to identify parameters that most
If
S
temp
<
S
max, then
S
=
S
temp, and
D
=
0
uz
uz
uz
uz
i
i
If
S
temp
>
S
max, then
S S
DS
=
max, and
uz
uz
uz
uz
i
=
temp
−
S
max
uz
uz
i
where
S
uz
temp is the preliminary calculated
soil-water storage, the index i indicates values
for the current day, i-1 represents the previous
day's value,
I
rr
is irrigation,
FD
is fog drip,
D
is
drainage beneath the root zone, and
S
uz
and
S
uz
maxi- are again soil-water storage and maxi-
mum soil-water storage, respectively.
Water budgets were calculated for six histori-
cal periods, which extended from 1926 through
2004. Within each period, it was assumed
that there was no change in land use. The
first period ran from 1926 to 1979; subsequent
periods were 5 years in duration. The amount
of land in cropped agriculture decreased with
each period. Historical precipitation data from
33 locations were used. Irrigation rates were
calculated as potential evapotranspiration
minus rainfall infiltration (precipitation minus
direct runoff); frequency of irrigation appli-
cation was based on crop type. Fog drip was
assumed to occur only on the windward (east)
side of West Maui Mountain, where it was set
equal to 20% of rainfall based on the findings
of Scholl
et all
. (
2005
). Direct runoff,
R
off
, was
determined by using streamflow hydrograph
separation techniques (described in
Chapter
4
) and calculating monthly ratios of runoff to
precipitation for four runoff regions.
PET
rates
were determined from pan evaporation data
(Equation (
2.29
)). Crop coefficients for sugar
cane were varied throughout the growing
cycle, but coefficients for all other vegetation
were held constant. Actual evapotranspiration,
