UNIVERSITY OF THE FREE STATE
MAIN CAMPUS
LWR 314
DEPARTMENT: SOIL, CROP AND CLIMATE SCIENCES
CONTACT NUMBER : 4012388
EXAMINATION: JULY 2007
PAPER 1
ASSESSOR(S): 1. Mrs. L. De Wet
2. Mr. S. Steyn
MODERATOR(S): Prof. S. Walker
TIME: 3 hours
MARKS: 100
Instructions:
1. Answer all the questions in SECTION A.
2. Answer only 5 questions from SECTION B.
3. Write your student number on the graph paper before handing it in.
SECTION A
COMPULSORY QUESTIONS
[50]
QUESTION 1
LONG TERM RAINFALL
(10)
Use the following rainfall dataset for January, February and March (JFM) to draw
the graph of the probabilities of non-exceedence for rainfall at Bethal in
Mpumalanga. Give the values for the near normal rainfall for this time of year. If
the forecast is for a 40% chance of above normal rainfall - explain to the farmers
what this means and how much rain they can expect to receive.
Table 1.1
Rainfall for January, February and March for Bethal
Year
1825 1826
JFM(mm) 251
197
1827
167
1828
257
1829
271
1830
352
1831
417
1832
324
1833
605
1834
218
QUESTION 2
TEMPERATURE HUMIDITY INDEX
(10)
You have been chosen to travel to Europe to represent the South African
contingent of agricultural advisors invited to give a presentation to potential
buyers on ideal dairy farming locations in South Africa.
a)
Use the data in Tables 2.1 and 2.2 to calculate the temperature humidity
index (THI) for two locations for November to February only.
(8)
b)
Use the results in (a) to support your choice for best location.
(2)
1
Table 2.1
Location
Tmx
RHmn
es
e
Td
THI
Climate data for location A
o
C
%
kPa
kPa
o
C
Table 2.2
o
JAN FEB
27.5 27.2
19.0 20.7
3.7
3.6
0.7
0.7
24.9 20.6 16.3
23.0 28.0 28.6
3.1
2.4 1.9
0.7
0.7 0.5
13.1 12.8 14.6
36.0 33.9 28.6
1.5
1.5
1.7
0.5
0.5
0.5
17.5
26.7
2.0
0.5
20.9
21.7
2.5
0.5
23.7
20.2
2.9
0.6
26.1
20.8
3.4
0.7
1.8
2.4
67
-1.6
54
-1.8
58
-1.8
61
-0.4
2.0
2.8
1.5
62
-1.9
57
-2.7
53
-3.4
55
Climate data for location B
Location
B
Tmx
RHmn
es
e
o
Td
THI
o
1459 m 32 23'S 20 40'E 57y
MAR APR MAY JUN JUL AUG SEP
OCT
NOV DEC
A
C
%
kPa
kPa
o
C
o
o
JAN FEB
1304 m 28 57'S 26 20E 76y
MAR APR MAY JUN JUL AUG SEP
OCT
NOV DEC
30.9 29.4
21.4 26.9
4.5
4.1
1.0
1.1
27.2 23.8 20.6
27.5 25.7 21.8
3.6
2.9 2.4
1.0
0.8 0.5
17.5 17.7 20.6
23.3 20.8 18.5
2.0
2.0
2.4
0.5
0.4
0.4
24.5
18.5
3.1
0.6
26.8
20.6
3.5
0.7
28.4
22.0
3.9
0.9
30.3
20.4
4.3
0.9
6.3
6.9
71
-3.7
57
-1.0
65
2.4
69
4.6
5.1
8.4
3.0
66
-2.0
61
-5.0
57
-4.2
60
QUESTION 3
EVAPOTRANSPIRATION
(10)
Make use of the two figures to explain the influence of maximum and minimum
temperature on the potential evapotranspiration. Compare the Priestly-Taylor and
Penman-Monteith methods giving reasons for the observed variations.
ETo PenMon (mm/d)
8
7
y = 0.1937x + 1.7585
R2 = 0.7327
y = 0.2298x - 2.7206
R2 = 0.5878
6
5
4
3
2
1
0
-2
3
8
13
18
23
28
33
Tm in or Tm ax (C)
2
ETo Priestly-Taylor (mm/d)
8
7
6
5
y = 0.9233x - 0.2285
R2 = 0.8517
4
3
2
1
0
0
1
2
3
4
5
6
7
8
ETo Pen-Mon (m m /d)
QUESTION 4
REMOTE SENSING
(20)
a) Describe how LANDSAT-7 satellite imagery can be used to identify and
monitor the state of agricultural crop production in a certain region.
(7)
b) Study the Radar image in Figure 4.1. What type of weather is occurring
between Bloemfontein and Kimberley at the time of the image? Motivate your
answer by referring to the observed reflectivities, associated rainfall rate,
special echo structures and your theoretical knowledge of rainfall-producing
weather systems.
(5)
Figure 4.1: Radar reflectivity image of the southern Free State area at 14:30 UTC on
2006-10-19 (copyright, SAWS).
3
c) Identify the cloud types marked A to D on the satellite images in Figures 4.2
to 4.4. In each case state the reasoning behind your decision.
(8)
C
A
A
B
A
D
A
Figure 4.2: Visual satellite image of Southern Africa at 06:00 UTC on 2006-10-12
(copyright 2006, EUMETSAT)
Figure 4.3: Water vapour satellite image of Southern Africa at 06:00 UTC on 2006-10-12
(copyright 2006, EUMETSAT)
4
Figure 4.4: Infrared satellite image of Southern Africa at 06:00 UTC on 2006-10-12
(copyright 2006, EUMETSAT)
________________________________________________________________
SECTION B
CHOICE QUESTIONS
[50]
You must answer 5 questions from this section. Choose between:
QUESTION 5
OR
QUESTION 6
QUESTION 7
OR
QUESTION 8
QUESTION 9
OR
QUESTION 10
QUESTION 11
OR
QUESTION 12
QUESTION 13
OR
QUESTION 14
AND
AND
AND
AND
QUESTION 5
IRRIGATION
(10)
You have used SapWat to calculate the irrigation schedule for your wheat crop
that was planted on 1 June. The results you obtained are shown below in the
graph. The lower limit for the soil is at 80 mm and the drained upper limit is 140
mm. Compare the amount of rainfall received and the evapotranspiration (ETc)
5
demand - use the monthly values to explain when you will need to irrigate.
Compare the soil water content under the 2 different types of irrigation schedule
– where 25 mm of water is applied every 7 days versus varying applications in
each of the 4 stages (15 mm every 7 days; then 25 mm every 6 days; then 25
mm every 5 days; then 25 mm every 8 days). Discuss the effect on your wheat
crop and explain which is the better irrigation schedule.
effective rain
180
ETc
160
140
ETc or Rain
(mm/month)
120
100
80
60
40
20
0
Jun
Jul
Aug
Sept
Tim e
Oct
Nov
7d25mm
140
130
120
Soil Water (mm)
4stage
110
100
90
80
70
60
50
0
20
40
60
80
100
Time (days)
120
140
160
OR
QUESTION 6
DROUGHT
(10)
You are asked to manage the drought plan for the farming community around
Gariep Dam. Give details of which key groups you will invite to serve on the
‘Drought Task Force’. List at least 5 of the most important purposes of the
‘Drought Plan’.
6
AND
QUESTION 7
PESTS AND DISEASES
(10)
Prediction results are provided in Table 7.1 for a warning index using
accumulated favourable hours as input. Favourable hours were calculated using
weather variables typical of cool, wet weather. Predicted severity was then
plotted against observed disease severity (Figure 7.1).
Use the data provided and explain how you propose this warning index was
developed. Which weather variables were needed for the disease and how would
you apply the available results?
Table 7.1
Results for prediction of disease severity and accumulated
favourable hours calculated for a location where crop disease
proved to be a problem for farmers
Date
1-Sep-04
2-Sep-04
3-Sep-04
4-Sep-04
5-Sep-04
6-Sep-04
7-Sep-04
8-Sep-04
9-Sep-04
10-Sep-04
11-Sep-04
12-Sep-04
13-Sep-04
14-Sep-04
15-Sep-04
Favourable
hours
294
300
364
403
444
469
509
512
550
558
569
580
594
600
602
Disease severity
Observed Predicted
10.0
19.0
20.0
20.5
30.0
35.9
40.0
45.3
45.0
55.1
50.0
61.2
60.0
70.8
66.0
71.5
69.0
80.7
70.0
82.6
75.0
85.3
80.0
87.9
90.0
91.3
90.0
92.7
100.0
93.2
7
Predicted dis severity (%)
100.0
80.0
60.0
Linear
(Series1)
y = 0.9441x + 9.8612
R2 = 0.9604
40.0
20.0
0.0
0.0
20.0
40.0
60.0
80.0
100.0
Observed dis sev (%)
Figure 7.1 Observed vs predicted disease severity using accumulated
favourable hours from inoculation on 25 July 2004.
OR
QUESTION 8
FROST
(10)
Study Table 8.1 which shows long term maximum (Tmx) and minimum (Tmn)
temperatures and altitudes for three different stations in South Africa for the
winter months June to August.
a)
Use your knowledge of frost to describe frost in general as related to the
environment.
(4)
b)
Discuss frost occurrence at the different stations in Table 8.1 using the
available data and refer to terms such as climate, topography and
surrounding environment to motivate and explain your answers.
(6)
Table 8.1
Locations
Alt (m)
o
Ta ( C)
Jun
Month
Jul
Aug
Long term maximum (Tmx) and minimum (Tmn) temperatures
for Stations A, B, C and D at different altitudes for the months
June to August
Station A
1690
Tmn
-2.6
-2.8
-9.3
Tmx
15.3
16.2
18.1
Station B
1345
Tmn
0.4
0.3
3.1
Tmx
18.8
19
22.1
Station C
1065
Tmn
-0.3
-0.5
2.9
Tmx
21.5
22.1
24.9
Station D
107
Tmn
10.6
9.8
9.7
Tmx
17.6
16.8
17.1
AND
8
QUESTION 9
WIND CHILL INDEX
(10)
a)
State the general limitations valid for both old and new WCI formulas.
(5)
b)
Use the data supplied in Table 9.1 in order to calculate the new WCI for
Cradock for each time step.
(5)
Table 9.1
Weather data for Cradock for 2007-05-16
Time
Tdrybulb
Twetbulb
Wind
Wind
WCI
h
ºC
ºC
km/h
m/s
ºC
04:00
-2
-8
18
5.0
06:00
-4
-7
18
5.0
08:00
1
-8
22
6.2
10:00
6
-8
35
9.8
12:00
10
-9
45
12.6
OR
QUESTION 10
FIRE DANGER INDEX
(10)
a)
Make use of the attached nomogram and the data supplied in Table 10.1
in order to calculate the expected Lowveld Fire Danger Index (FDI) for
Phalaborwa.
(3)
Table 10.1
Tomorrow’s weather forecast for Phalaborwa
Parameter
Tmx
Tmn
RHmx
RHmn
Rain
Wind
Unit
ºC
ºC
%
%
mm
km/h
Forecast value
37
18
56
28
0
26
NOTE: The last rainfall in Phalaborwa was 18 mm and occurred 4 days ago.
b)
The following cut-off values are employed in the Lowveld Fire Danger
Index:
0 - 20:
Blue
21 – 45:
Green
46 – 60:
Yellow
61 – 75:
Orange
≥ 76:
Red
State which fire danger rating you would allocate to Phalaborwa for tomorrow
and discuss the expected fire behaviour and recommended fire control measures
which apply.
(7)
AND
9
QUESTION 11
HEAT UNITS
(10)
Calculate both germination and emergence dates for lentils planted at a location
in South Africa on 4 August. Base temperature for lentils is 1.4 oC; thermal time
(TT) for germination is 30 and TT for emergence is 90.
Table 11.1
Maximum (Tmx) and minimum (Tmn) temperatures for a
location in South Africa where lentils were planted in 2005
Doy
212
213
214
215
216
217
218
219
220
221
222
223
224
Date
31-Jul-05
Tmx
18.2
21.7
21.9
20.4
19.7
20.4
19.5
21.0
22.8
22.8
19.0
20.0
23.3
Tmn
1.5
-0.1
0.8
0.4
0.2
0.4
1.5
2.8
11.9
5.5
1.5
2.6
3.5
TT
STT
OR
QUESTION 12
DISCOMFORT INDEX
(10)
You are an agrometeorological expert and a private company approaches you for
advice concerning the purchase of air-conditioning equipment for their offices in
two different locations. They need to decide which location most urgently needs
air conditioning as they are on a limited budget. Refer to Tables 12.1 and 12.2
and use the data given to
a)
Calculate the discomfort index (DI) for both locations for Dec and Jan.
(6)
b)
Which location most urgently needs air-conditioning? Motivate.
(4)
Table 12.1
Climate data for location X
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
C 24.30 25.90 24.90 22.20 19.40 17.20 16.40 16.60 18.30 20.30 22.60 24.30
% 25.80 26.70 25.20 22.10 19.90 17.10 16.00 16.80 18.70 20.70 22.00 23.90
83
80
73
67
61
59
60
64
69
74
LOCATION X
Tmx
RHmn
DI
o
10
Table 12.2
Climate data for location Y
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
C 32.10 31.70 30.70 29.30 27.30 25.10 25.00 26.10 28.40 29.10 30.00 31.70
% 36.70 38.90 39.30 35.40 32.00 28.40 28.60 29.20 30.80 32.80 35.40 35.90
100
97
93
87
81
81
84
90
92
95
LOCATION Y
Tmx
RHmn
DI
o
________________________________________________________________
AND
QUESTION 13
SEASONAL FORECASTING
(10)
Your farm is near Bethal in Mpumalanga highveld. It is after Christmas 2002 and
little rain has fallen since the winter months. You receive the seasonal forecast
in the map below from the South African Weather Service. Explain it to your
neighbour. Describe some of the activities that you could use on your farm
during 2003. Be sure to include practices connected with the water and land
conservation and crop management.
OR
QUESTION 14
SHORT TERM FORECASTING
(10)
You are appointed as an agricultural consultant and one of your clients farm with
lettuce in the central Free State. Study the short-term forecast (Table 14.1) and
attached synoptic weather chart before answering the following questions:
a) Describe the weather that is expected over the particular area and explain the
problems that it may have in store for the specific farming activity.
(2)
b) Identify and describe the weather system(s) responsible for the weather
described in (a).
(4)
c) Discuss the possible management decisions that can be made in order to
alleviate the negative impacts of the weather or to make optimal use thereof
(whatever the case may be).
(4)
11
Discussion
Issued on 1994-04-23
Dry air is in circulation over the interior with associated fine weather. A cold
front causes strong winds and cloudy conditions with rain along the Cape south
coast, which will spread eastwards to KwaZulu-Natal. A strong Atlantic Ocean
High (AOH) of about 1032 hPa is ridging behind the cold front, which should
force it northwards across the country.
The AOH will ridge to the south of the country on the 24 th with strong onshore
flow over the south and east coast where cloudy conditions with rain are
forecast. The cold front will lie over the Northern Free State, Southern
Mpumalanga and Swaziland with cold conditions to the south of it. Bergwind
conditions are expected along the west coast due to a strong offshore flow.
A cut-off low is developing off the west coast on the 25th, but will only cause
some high level cloud over the coast and adjacent interior at this stage. The
cold front will move out to the east of the country, while the onshore flow and
cloudy conditions along the east coast will spread to the Lowveld. Fine but cool
weather are expected over the remainder of the country.
With the high southeast of the country on the 26th, partly cloudy conditions are
expected to continue over the north-eastern parts of the country due to the
influx of moist air around the periphery of the high. Fine weather are expected
over the remainder of the country, except over the Western and South-western
Cape where the cut-off low will cause an onshore flow with cloudy conditions
and light showers.
The cut-off low will move to the south of the country on the 27 th with a cold front
approaching the country from the west. Cloudy conditions with rain are
expected in the strong onshore flow along the south-west coast. The fine
weather conditions are expected to continue over the rest of the country, with
hot bergwinds in the offshore flow over the Eastern Cape coast to the east of
the coastal low.
12
Table 14.1
5-Day Forecast for Bloemfontein (Free State)
Date
Cloud cover
Weather
Tmin
Tmax
Wind
1994-04-23
Fine
Frost (am)
1
20
Moderate SW
1994-04-24
Partly cloudy
nil
4
19
Moderate S to
SE
1994-04-25
Fine
Frost (am)
-3
22
Light NE
1994-04-26
Fine
Frost (am)
-1
23
Light N to NE
1994-04-27
Partly cloudy
Frost (am)
2
23
Moderate NW
13
FORMULA SHEET
FORMULEBLAD
p  RT
R  287 Jkg 1 K 1
E  T 4
E  T 4
 1
2897
 max 
T
Rn  S t  r S t  S e   tSe  Ld  Le  Lb  0
  5.67  10 8 Wm 2 K 4
S  aS  rS  tS  vS
St  Sb  S d
 va   va'   Ta  Tw 
aC p
 
 0.5 gm 3 K 1  0.066 kPaK 1
Lv
e  es   Ta  Tw 
1 Atmosphere  1013.3 hPa
C p  1010 Jkg 1  C 1
Lv  2.50  10 6 Jkg 1
e

RH  a  100  va'  100
es
 va
es  0.6108  e
 17.2694Tw 


 237.3Tw 
e  A B
SD  es  ea   va'   va  VT
Td  273.16 
A  0.6108  e
B  0.066Ta  Tw 1  0.00115Tw 
ea
0.6108
ea
1  0.0579 ln
0.6108
273.16  2.0765 ln
 psy  a psy P
 17.2694Ta 


 237.3Ta 
ea  e 0 Tw    psy Ta  Tw 
for ventilated Asmann type psychrometer apsy = 0.622×10-3
900

u 2 es  ea 
0.408Rn  G 
ET0 
 T  273
   1  0.34u 2 
   1  0.34u 2 
es at Ta
Rn  Rns  Rnl
All radiation parameters are in MJm-2d-1
Rns  1 0.23Rs
0.23 is the albedo for a well-watered short grass surface
n

Rs   a s  bs  Ra where no calibrated values are available, a s  0.25
N

bs  0.50


Rs 0  0.75  2  10 5 z Ra


T K 4  Tmin K 4 


Rs
Rnl   max
 0.35 
 0.34  0.14 ea 1.35
2
Rs 0




ETc  K c  K s  ET0
ET  P  I  S  SF  CR  R  DP
 T  Tmn

TT   mx
 Tb   t
2


FDI  BGI  BI  WF   RCF
 RH mn

DI  OBI  2  Tmx   
 Tmx   24
 100

THI  Tmx  0.36Td  41.2
old WCI  nou WVI  (0.2136 V  0.2637  0.0183V )(1.8T  59.4)  33



new WCI  nuwe WVI  13.12  0.6215T  11.37 V 0.16  0.3965T V 0.16

14
15