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The Start of the rains at Moorings
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Contents
• The data
• Annual values
• Examples of years
• Defining the start of the season
• Results omitting dry spells
• Adding the dry spells
• Conclusions and further work
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The data
• Moorings 16° South 27° East
• Data – February 1922 to March 2003
• Mean annual 800mm from 64 rain days
• Highest > 1250mm in 1951 and 1957
• Lowest < 400mm in 1991 and 1994
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Number of rain days at start and mid-seasonStart (low er line) is Nov 1 to Dec 15, mid-season is Dec 16 to March 31
Threshold for rain = 0.85mm
20001990198019701960195019401930
60
40
20
0
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305244183122
60
40
20
0
1923 - 454mm on 49 days
305244183122
60
40
20
0
1924 - 1184mm on 109 days
El Nino season La Nina season
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305244183122
60
40
20
0
1992 - 830mm on 59 days
305244183122
60
40
20
0
1993 - 464mm on 34 days
El Nino seasonOrdinary year
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Start of the season
• Following discussion with Amina:
• Use the following criteria:• Planting from mid-November• Plant when more than 20mm in a 3-day period• Replanting needed
– if more than 12 days dry in 30 days– for farmers practicing simple water-harvesting
• Replanting needed – if more than 9 days dry in 30 days– for other farmers
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Data for 1992
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Data for 1993
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Start of the rains for MooringsFirst occasion from 15 Nov w ith 20mm in a 3-day period
20001990198019701960195019401930
Da
y n
um
be
r fr
om
1st
Ju
ly
180
170
160
150
140
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Summary of results
• Graph shows no indication of trend • No obvious evidence of climate change
• With this definition:• Plant within a week in 40% of years
• Could an earlier start be considered?
• Plant by end November in 52 out of 82 years• About 2/3 of years
• Plant by mid-December in all by 5 years• About 19 years out of 20
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Percent Probability Plot of Start
Day of year from I July190180170160150140
Pe
rce
nt
100
80
60
40
20
0
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Dates of start of rains
From 15 Nov with >20mm in 3 days And with no 10-day dry spell in 30 days
20001990198019701960195019401930
Day
num
ber f
rom
1 J
uly
180
150
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Start date as before No dry spell exceeding 12 days in next 30 days
20001990198019701960195019401930
Day
num
ber f
rom
1 J
uly
180
150
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Summary of initial results
• With 10-day dry spell replanting in 15 years• About 1 year in 6
• If 12 days OK (rainwater harvesting) then only replant in 7 years
• About 1 year in 12
A second definition – suppose you need 20mm in November,
but relax to 15mm from 1st December,
Does this make a difference?
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20mm start
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15mm start
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Both starts
Difference in 9 years – late starts are prevented
Now plant by 15 December except in one year
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Conclusions and further work
• Definition of start seems reasonable• Water harvesting is useful – halves the risk
• Some farmers claim to always plant by mid-December
• What definition would correspond to this?• Possibly relax amount to 15mm from 1 December.• This seems to work!
• If earlier planting is considered• Before 15th November• Then what is the conditional risk of a long dry spell?