Environmental Engineering Reference
In-Depth Information
1000
Figure 2.7
Cumulative water-
budget components for July 1982
through June 1983 for a site in
northwestern Illinois (after Healy
et al
.,
1989
).
P
is precipitation,
ET
uz
is evapotranspiration from the
unsaturated zone,
R
off
is runoff, Δ
S
uz
is unsaturated-zone storage change,
and
D
is drainage.
P
ET
uz
R
off
∆
S
uz
D
800
600
400
200
0
Jul
Aug
Sep
Oct
Nov
Dec
JanFeb
Mar
Apr
May
Jun
1982
1983
estimates. The discrepancy could be explained
by a 10% inaccuracy in precipitation measure-
ments or a 15% inaccuracy in evapotranspir-
ation estimates. There is also the question
of spatial variability in processes and prop-
erties and of measurement scale for deter-
mining water-budget components. Runoff
estimates are integrated over the entire area
that drained to the stream gauges (more
than 80% of the site). Likewise, evapotrans-
piration rates are values integrated over a
fairly large portion of the site. Precipitation
did not vary considerably across the site. So
the water-budget estimate of drainage can be
viewed as a value integrated over the entire
site. The Darcy method produced point esti-
mates at two discrete locations. Perhaps more
accurate sitewide estimates would have been
obtained with additional measurement loca-
tions. Uncertainty in hydraulic conductivity
can also contribute to inaccuracy in Darcy
method estimates. In any regard, the accuracy
of either drainage estimate cannot be quanti-
fied. This example illustrates the importance
of applying multiple methods for estimating
recharge.
Owing to the permeable soils at their study site,
surface-water flow, direct runoff, change in
unsaturated-zone storage, and travel time for
water draining through the unsaturated zone
were assumed to be negligible. Recharge was
calculated as the difference between precipita-
tion and evapotranspiration:
ET
(2.27)
R
=−
P
By our definition (
Section 2.2
), Equation (
2.27
)
actually calculates drainage, but Steenhuis
et al
.
(
1985
) referred to this as recharge. Again, the
distinction between drainage and recharge is
one of lag time. An energy-budget approach
was used to estimate evapotranspiration.
Micrometeorological measurements made at
the site included precipitation (three gauges),
wind speeds and air temperatures at 1 and 3 m
heights, dew point temperatures at 1.68 and
2.68 m heights, and net radiation. Data were
recorded at hourly intervals.
Monthly estimates of recharge and precipi-
tation for 8 months in 1980 are shown in
Table
2.2
. Negative recharge rates reflect decreases in
soil-water storage that were not accounted for
in this approach. Also shown in
Table 2.2
are
recharge estimates obtained with the unsatu-
rated-zone Darcy method. For most months,
these values were less than the estimates
obtained by the water-budget method. The dis-
crepancies are again attributed to soil-water
Example: Eastern Long Island
Steenhuis
et al
. (
1985
) estimated groundwater
recharge at a site on eastern Long Island by
analyzing the water budget of a column of soil
extending from land surface to the water table.
