The Start of the rains at Moorings

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

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

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

305244183122

60

40

20

0

1923 - 454mm on 49 days

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60

40

20

0

1924 - 1184mm on 109 days

El Nino season La Nina season

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60

40

20

0

1992 - 830mm on 59 days

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60

40

20

0

1993 - 464mm on 34 days

El Nino seasonOrdinary year

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

Data for 1992

Data for 1993

Start of the rains for MooringsFirst occasion from 15 Nov w ith 20mm in a 3-day period

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Da

y n

um

be

r fr

om

1st

Ju

ly

180

170

160

150

140

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

Percent Probability Plot of Start

Day of year from I July190180170160150140

Pe

rce

nt

100

80

60

40

20

0

Dates of start of rains

From 15 Nov with >20mm in 3 days And with no 10-day dry spell in 30 days

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Day

num

ber f

rom

1 J

uly

180

150

Start date as before No dry spell exceeding 12 days in next 30 days

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Day

num

ber f

rom

1 J

uly

180

150

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?

Startgfedcb

X89gfedc

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r fr

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1st

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200

190

180

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20mm start

Startgfedc

X89gfedcb

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Da

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r fr

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1st

Ju

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200

190

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15mm start

Startgfedcb

X89gfedcb

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Da

y n

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r fr

om

1st

Ju

ly

200

190

180

170

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150

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Both starts

Difference in 9 years – late starts are prevented

Now plant by 15 December except in one year

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?