Environmental Engineering Reference
In-Depth Information
profile. Analytical models are often more easily
applied than numerical models, but numerical
models have a greater ability to simulate com-
plex hydrologic systems because they are less
limited by assumptions.
Temperatures within the geothermal zone
represent long-term average conditions, reflect-
ive of a large block of the subsurface. These tem-
peratures respond only slowly to any changes in
surface conditions. Temperature-depth profiles
within the geothermal zone have been used to
predict past ground-surface temperatures and
to identify periods of climate change (Clow,
1992
; Huang
et al
.,
2000
; Beltrami
et al
.,
2005
).
Estimated drainage rates within the geother-
mal zone reflect diffuse, steady drainage.
Temperatures within the surficial zone
fluctuate on diurnal and seasonal patterns.
Temperature-based estimates of focused
recharge from streams are generally for a sin-
gle point in space. Estimates based on diurnal
temperature patterns are considered point esti-
mates in time; those based on seasonal patterns
8.5 Discussion
Analytical and numerical models that simulate
heat flow are useful tools for estimating drain-
age rates within the subsurface. Models are
calibrated so that results mimic patterns of
measured temperatures with the objective
of identifying the advective component of
heat flow. A key reason for the success of this
approach is that thermal properties of geo-
logic material vary within a range that is much
smaller than the range over which hydraulic
conductivity varies (Constantz
et al
.,
2008
). So
models can be used to determine the values of
hydraulic conductivity (or flux) that produce the
best agreement with measured temperatures.
For estimation of focused drainage, model
results are compared with temporal fluctua-
tions in temperatures, usually diurnal or sea-
sonal. For estimation of diffuse drainage, steady
water and heat flow are assumed, and model
results are compared with a temperature-depth
40
30
20
10
No flow
Flow
Discontinuous flow
0
30
1
357
9
11
13
15
17
19
21
23
25
27
29
1
3
5
7
March
April
May
2002
0 meter
0.2 meter
0.5 meter
1.0 meter
1.38 meters
Figure 8.11
Streambed temperatures at various depths
beneath Trout Creek over periods of no channel flow,
continuous flow, and discontinuous flow (after Prudic
et al
.,
2007
).
