Geoscience Reference
In-Depth Information
Question 3
(Uses understanding and equations
from Chapters 1, 4, 6, and 7.)
A farmer has a copy of
Terrestrial Hydrometeorology
and therefore has
wide-ranging knowledge of the subject. He has a field that is currently bare soil
near Casa Grande, Arizona which is 32.5°N of the equator. This is the arid site for
which you calculated values of net radiation in 2(d). He recently irrigated the field
prior to planting and the sandy soil is close to saturated. He decides to measure the
evaporation loss from the field but he only has three thermometers and a set of tall
stepladders with which to do so. He installs one thermometer in the soil to measure
the temperature very close to the surface of the soil. He wraps the mercury bulb of
a second thermometer in a small piece of cloth that he is careful to keep moist so
that it measures wet bulb temperature. The third thermometer he uses as a dry
bulb thermometer to measure air temperature.
Starting at midnight on July 13 he measures the wet bulb and dry bulb
temperature 0.5 m above the ground and then quickly runs up the stepladder and
makes the same measurements at 3.0 m from the ground. Every 5 minutes he
repeats this operation throughout the next 24 hours. In his spare time he monitors
the thermometer in the soil and notices that the minimum temperature of 20°C
occurs at 01:00 and the maximum temperature of 24°C occurs at 13:00. He also
monitors the sky and decides that the fractional cloud cover is 0.7 and fairly
constant all day. He computes the hourly-average values of wet and dry bulb
temperature at the top and bottom of the stepladder given in Table 26.1.
Having read Chapter 6 in
Terrestrial Hydrometeorology
, the farmer realizes that if
he assumes the soil is uniform with depth and the diurnal cycle in soil surface
Table 26.1
Values of hourly average dry and wet bulb temperatures measured by the farmer in question 3.
Bottom
Top
Bottom
Top
Time
(hour)
Dry bulb
(°C)
Wet bulb
(°C)
Dry bulb
(°C)
Wet bulb
(°C)
Time
(hour)
Dry bulb
(°C)
Wet bulb
(°C)
Dry bulb
(°C)
Wet bulb
(°C)
0.5
16.786
11.714
15.654
11.177
12.5
33.139
21.096
28.347
16.703
1.5
14.337
11.026
14.609
11.117
13.5
34.790
21.547
29.391
17.043
2.5
12.482
10.157
14.069
10.948
14.5
34.820
21.365
29.932
17.093
3.5
9.963
9.146
14.068
10.948
15.5
34.600
20.918
29.931
16.888
4.5
8.838
8.482
14.609
11.052
16.5
33.255
20.360
29.391
16.733
5.5
13.222
10.473
15.653
11.563
17.5
30.856
18.978
28.346
16.282
6.5
17.093
13.081
17.130
11.996
18.5
27.105
17.012
26.870
15.769
7.5
20.394
15.325
18.938
12.808
19.5
25.255
15.567
25.061
15.125
8.5
23.451
17.181
20.956
13.978
20.5
24.786
15.274
23.043
14.394
9.5
26.654
18.610
23.044
14.851
21.5
22.795
14.562
20.955
13.445
10.5
29.122
19.718
25.062
15.733
22.5
20.439
13.644
18.938
12.685
11.5
31.598
20.625
26.870
16.306
23.5
18.632
12.692
17.129
11.870
