Investigating the Start of the Rainy Season and the Best Sowing Dates for St. George, Antigua

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1 Investigating the Start of the Rainy Season and the Best Sowing Dates for St. George, Antigua By Dale Destin Climatologist Antigua and Barbuda Meteorological Service Climate Section May 29,

2 Table of Contents Map of Antigua... 3 Background... 4 Objectives... 4 Methodology... 4 Data... 5 Results... 6 Calculation of Risk The Super Brave Farmer The Brave Farmer The Bold Farmer The Cautious Farmer The Super Cautious Farmer Recommendations Appendix

htm Parishes 1 St. George 2 St. John, 3 St. Mary, 4 St.

3 Courtesy: Parishes 1 St. George 2 St. John, 3 St. Mary, 4 St. Paul, 5 St. Peter, 6 St. Phillip Map of Antigua Courtesy: 3

4 Background This report was prepared for the Ministry of Agriculture of Antigua and Barbuda in support of the farmers who operate in the parish of St. George. This parish is located in the northeast section of Antigua. Most of St. George lies within 3 miles of the V. C. Bird International Airport which is located at Latitude 17 8'N, longitude 61 47'W. The area is relatively flat, less than 100 metres above sea level. The area has a Tropical Marine Wet and Dry Climate. The dry season is January to June and the wet season is July to December. Objectives This study seeks to answer the following questions: For a super brave farmer, willing to sow early to catch the preseason rains, i.e. beginning of May, what would be the distribution of sowing dates, and what is the risk of early crop failure due to dry spells (10 days without rain after the first rain)? For a brave farmer, willing to sow early to catch the preseason rains, i.e. beginning of June, what would be the distribution of sowing dates, and what is the risk of early crop failure due to dry spells (10 days without rain after the first rain)? For a bold farmer, willing to sow at the start of the wet season, i.e. beginning of July, what would be the distribution of sowing dates, and what would be the risk of early crop failure due to dry spells (10 days without rain after the first rain)? For a cautious farmer, willing to sow in the middle of July, what would be the distribution of sowing dates, and what would be the risk of early crop failure due to dry spells (10 days without rain after the first rain)? For a super cautious farmer, willing to sow in the middle of July, what would be the distribution of sowing dates, and what would be the risk of early crop failure due to dry spells (10 days without rain after the first rain)? How often (i.e. in what proportion of years) will a super brave and brave farmers have a longer growing season than the bold farmer. How often will the bold farmer have a longer growing season than a cautious farmer? Methodology The objectives will be achieved by the use of the statistical tool INSTAT. With INSTAT, I will for each question in the objective section, obtain the start of the rains for each year. This would provide the distribution of sowing dates, with and without dry spells. For sowing dates that are equal irrespective of the dry spell, these are successful planting dates, otherwise there is 4

5 failure. It is the proportion of failures to successes that will, in the final analysis, determine what decisions are made in accordance with the objectives. Additionally, along the way to achieving the objectives, descriptive statistics will be used to give a clear picture of the rainfall pattern at St. George, Antigua. Here are the definitions used to extract the required events: I. First date from May 1 getting more than 20 mm in 1, 2, or 3 days. II. First date from June 1 getting more than 20 mm in 1, 2, or 3 days. III. First date from July 1 getting more than 20 mm in 1, 2, or 3 days. IV. First date from July 15 getting more than 20 mm in 1, 2, or 3 days. V. First date from August 1 getting more than 20 mm in 1, 2, or 3 days. VI. As (i) but with no 10 day (or longer) dry spell in the next 30 days. VII. As (ii) but with no 10 day (or longer) dry spell in the next 30 days. VIII. As (iii) but with no 10 day (or longer) dry spell in the next 30 days. IX. As (iv) but with no 10 day (or longer) dry spell in the next 30 days. X. As (v) but with no 10 day (or longer) dry spell in the next 30 days. The risk of early crop failure will be determined by whether there is a 10-day dry spell after the initial rain. Failure is assumed if there is a 10-day dry spell after the initial rain. For season length comparisons, this will be based on the success of each farmer as compared to the other, as determined by their planting strategies. A successful season is one in which there was no early crop failure i.e. no 10-day dry spell in the first 30 days after the initial rain. A rain or rainy day is one with at least 1 mm of rainfall. Thus, a 10-day dry spell is one with no day of the 10 days having 1 mm or more of rainfall. Data Forty years of daily rainfall data for V. C. Bird International Airport, St. George, Antigua was used, 1971 to This data is available at the Antigua and Barbuda Meteorological Service which is located at the above mentioned airport. The data is in EXCEL format, one column of 366 rows per year. As the climatologist and the person task with data management, there are systems in place to insure the highest quality data possible; the non-leap year coding used for day 60 was There is one missing month December The register, into which these data were inputted, has gone missing. After importing the data into INSTAT, three negative values were observed during further inspection via preliminary descriptive statistics. These values were set as missing; however, with available time, the correct value will be inputted. Notwithstanding these minor things, more than 99% of the data were available and were in good order. 5

6 Results Graph 1: Annual rainfall total for St. George, Antigua Graph 2: Comparison of the start of the rains based on the definitions 6

7 Graph 3: Mean daily rainfall at St. George, Antigua Graph 4: Summary values of monthly rainfall at St. George, Antigua 7

8 Table 1: Summary values of monthly rainfall at St. George, Antigua Min. 25% Median Mean 75% Max. SDE Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Table 2: Results obtained according to the definitions in the methodology Super Brave Farmer Brave Farmer Bold Farmer Cautious Farmer 8 Super Cautious Farmer Year I VI (stdry1) II VII (stdry2) III VIII (stdry3) IV IX (stdry4) V X (stdry5)

9 Year I VI (stdry1) II VII (stdry2) III VIII (stdry3) IV IX (stdry4) V X (stdry5) Earliest Mean Latest Std Earliest 01-May 01-May 01-Jun Never 01-Jul Never 15-Jul Never 01-Aug Never Mean 25-May 21-Jun 04-Jul 09-Jul 22-Jul 21-Jul 02-Aug 01-Aug 20-Aug 18-Aug Latest 14-Aug 28-Aug 28-Aug 04-Sep 28-Aug 04-Sep 14-Sep 14-Sep 21-Oct 18-Dec Note that definition v above in the second to last column is the same as strt_aug in graph 2. 9

10 Figure 1: Statistics of columns generated by the definitions in the methodology section Simple Models - Normal Distribution, One Sample Normal model, one sample Column i Minimum 122 Maximum 227 Range 105 Mean Std. deviation Standard error of mean = with 39 d.f. 95% confidence interval for mean to Simple Models - Normal Distribution, One Sample Normal model, one sample Column ii Minimum 153 Maximum 241 Range 88 Mean Std. deviation Standard error of mean = with 39 d.f. 95% confidence interval for mean to Simple Models - Normal Distribution, One Sample Normal model, one sample Column iii Minimum 183 Maximum 241 Range 58 Mean Std. deviation Standard error of mean = with 39 d.f. 95% confidence interval for mean to Simple Models - Normal Distribution, One Sample Normal model, one sample Column iv Minimum 197 Maximum 258 Range 61 Mean Std. deviation Standard error of mean = with 39 d.f. 95% confidence interval for mean to Simple Models - Normal Distribution, One Sample Normal model, one sample Column v Minimum 214 Maximum 295 Range 81 Mean Std. deviation Standard error of mean = with 39 d.f. 95% confidence interval for mean to

11 Calculation of Risk The Super Brave Farmer From the Methodology section, the super brave farmer proposes to plant at the beginning of May. If this is done, the following is observed: Figure 2: Probability of success (right) and failure (left) of the super brave farmer One proportion - binomial model Successes 18 Proportion Approx s.e. of proportion = to to One proportion - binomial model Successes 22 Proportion Approx s.e. of proportion = to to The left hand side of the above table represent the risks of failure for the super brave farmer, while the right hand side represent his chances of success. Assuming the sample to be exactly normal, failure would be observed every 2 to 3 years (1.63 to 3.41 years) at the 95% degree of confidence. There is as much as a 62% chance of failure and a minimum failure rate of 29%. For a given 10 year period, there would be three to six failures. Alternatively, with respect the successful planting by the brave farmer, this would happen as little as 39% of the time and a maximum of 71% of the time. This is equivalent to success every 1 to 3 years (1.41 to 2.60 years); success four to seven times every 10 years. The Brave Farmer From the Methodology section, the brave farmer proposes to plant at the beginning of June. If this is done, the following is observed: 11

12 Figure 2: Probability of success (right) and failure (left) of the Brave Farmer One proportion - binomial model One proportion - binomial model Successes 11 Proportion Approx s.e. of proportion = to to Successes 29 Proportion Approx s.e. of proportion = to to The left hand side of the above table represent the risks of failure for the brave farmer, while the right hand side represent his chances of success. Assuming the sample to be exactly normal, failure would be observed every 2 to 7 years (2.28 to 6.85 years) at the 95% degree of confidence. There is as much as a 44% chance of failure and a minimum of 15% chance. Further, in a 10 year period, there would be one to four failures. Alternatively, with respect the successful planting by the brave farmer, this would happen as little as 56% of the time and a maximum of 85% of the time. This is equivalent to success every 1 to 2 years (1.17 to 1.78 years); success six to nine time every 10 years. The Bold Farmer From the Methodology section, the bold farmer proposes to plant at the beginning of July. If this is done, the following is observed: Figure 2: Probability of success (right) and failure (left) of the Bold Farmer One proportion - binomial model One proportion - binomial model Successes 6 Proportion Approx s.e. of proportion = to to Successes 34 Proportion Approx s.e. of proportion = to to

13 The left hand side of the above table represent the risks of failure for the bold farmer, while the right hand side represent his chances of success. Assuming the sample to be exactly normal, failure would be observed every 3 to 18 years (3.36 to years) at the 95% degree of confidence. There is a maximum of 30% chance of failure and a minimum of 6% chance. Further, in a 10 year period, there would be one to three failures. Alternatively, with respect the successful planting by the bold farmer, this would happen 70 to 94% of the time. This is equivalent to success just about every year (1.06 to 1.42 years); success of seven to nine every 10 years. The Cautious Farmer From the Methodology section, the cautious farmer proposes to plant at the beginning of July. If this is done, the following is observed: Figure 2: Probability of success (right) and failure (left) of the Cautious Farmer One proportion - binomial model One proportion - binomial model Successes 6 Proportion Approx s.e. of proportion = to to Successes 34 Proportion Approx s.e. of proportion = to to The left hand side of the above table represent the risks of failure for the cautious farmer, while the right hand side represent his chances of success. Assuming the sample to be exactly normal, failure would be observed every 3 to 18 years (3.36 to years) at the 95% degree of confidence. There is a maximum of 30% chance of failure and a minimum of 6% chance. Further, in a 10 year period, there would be one to three failures. Alternatively, with respect the successful planting by the cautious farmer, this would happen 70 to 94% of the time. This is equivalent to success just about every year (1.06 to 1.42 years); success of seven to nine every 10 years 13

14 The Super Cautious Farmer From the Methodology section, the super cautious farmer proposes to plant at the beginning of July. If this is done, the following is observed: Figure 2: Probability of success (right) and failure (left) of the Super Cautious Farmer One proportion - binomial model Successes 4 Proportion Approx s.e. of proportion = to to One proportion - binomial model Successes 36 Proportion Approx s.e. of proportion = to to The left hand side of the above table represent the risks of failure for the super cautious farmer, while the right hand side represent his chances of success. Assuming the sample to be exactly normal, failure would be observed every 4 to 36 years (4.22 to years) at the 95% degree of confidence. There is a maximum of 24% chance of failure and a minimum of 3% chance. Further, in a 10 year period, there would be virtually no failures. Alternatively, with respect to the successful planting by the super cautious farmer, this would happen 76 to 97% of the time. This is equivalent to success just about every year (1.03 to 1.31 years); success of eight to 10 every 10 years. 14

15 Figure 3: How often (i.e. in what proportion of years) a Super Brave Farmer (i) would plants before the Bold Farmer; the Brave Farmer (ii) plants before the Bold Farmer; the Bold Farmer (iii) plants before the Cautious Farmer; the Cautious Farmer (iv) plants before the Super Cautious Farmer. (i) One proportion - binomial model Successes 35 Proportion Approx s.e. of proportion = to to (iii) One proportion - binomial model Successes 18 Proportion Approx s.e. of proportion = to to (ii) One proportion - binomial model Successes 22 Proportion Approx s.e. of proportion = to to (iv) One proportion - binomial model Successes 21 Proportion Approx s.e. of proportion = to to The above information speaks to the question of how often the various farmers would plant before each other. The super brave farmer that plants at the beginning of May would plant 88% (± 11%) of the time before the bold farmer. This amounts to a planting season of 58 days longer than the bold farmer (± 9 days) operating at the 95% confidence interval. The brave farmer that plants at the beginning of June would plant 55% (± 16%) of the time before the bold farmer. This amounts to a planting season of 18 days longer than the bold farmer (± 8 days) operating at the 95% confidence interval. The bold farmer that plants at the beginning of July would plant 45% (± 16%) of the time before the cautious farmer. This amounts to a planting season of 11 days longer than the bold farmer (± 6 days) operating at the 95% confidence interval. 15

16 The cautious farmer that plants at July 1 would plant 53% (± 16%) of the time before the super cautious farmer. This amounts to a planting season of 18 days longer than the super cautious (± 6 days) operating at the 95% confidence interval. Recommendations The following are my recommendations and associated rational: I. Planting be done at the beginning of July, the start of the rainy season, where on average there is an 85% success rate (± 12%) when planting at the beginning of July. Failure would only occur one to three times per 10 years. As can be seen from the information in the results section, success rates improve to even more acceptable levels through August 1, with failure rates being negligible. II. Further studies are done with respect to the predictability of the rainfall for May to determine if greater success cannot be had during certain phases of ENSO or some other predictor of precipitation. As can be seen, May has a higher average rainfall than June and July; however, its variability is more that twice the variability of these months. This suggests that there is a tendency for extreme rainfall events for this month and perhaps these extremes are predictable. Predicting the lower extremes would likely reduce the failure rate and increase success; allowing for possibly three crops per year. 16

17 Appendix Monthly rainfall totals 1971 to 2010 Year Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec *

18 Daily rainfall data May to Aug, 1971 to 1973 Daily data for: 1971 Mon May Jun Jul Aug Daily data for: 1972 May Jun Jul Aug Daily data for: 1973 May Jun Jul Aug Day m Mean (Overall: 1.9) Total (Overall: 229.1) Minimum (Overall: 0.0) Maximum (Overall: 42.9) Number greater than 0.99 (Overall: 35) Number of missing values (Overall: 1) Mean (Overall: 2.6) Total (Overall: 312.4) Minimum(Overall: 0.0) Maximum(Overall: 49.5) Number greater than 0.99 (Overall: 46) m Mean (Overall: 1.0) Total (Overall: 114.6) Minimum(Overall: 0.0) Maximum(Overall: 13.2) Number greater than 0.99 (Overall: 32) Number of missing values (Overall: 1) (Overall: 1)

19 Daily rainfall data May to Aug, 1974 to 1976 Daily data for: 1974 Mon May Jun Jul Aug Daily data for: 1975 May Jun Jul Aug Daily data for: 1976 May Jun Jul Aug Day m Mean (Overall: 1.7) Total (Overall: 204.0) Minimum (Overall: 0.0) Maximum (Overall: 41.9) Number greater than 0.99 (Overall: 45) Number of missing values (Overall: 1) m Mean (Overall: 1.3) Total (Overall: 155.7) Minimum(Overall: 0.0) Maximum(Overall: 21.3) Number greater than 0.99 (Overall: 32) Number of missing values (Overall: 1) Mean (Overall: 1.6) Total (Overall: 194.1) Minimum(Overall: 0.0) Maximum(Overall: 17.8) Number greater than 0.99 (Overall: 40)

20 Daily data rainfall May to Aug, 2008 to 2010 Daily data for: 2008 Mon May Jun Jul Aug Daily data for: 2009 May Jun Jul Aug Daily data for: 2010 May Jun Jul Aug Day Mean (Overall: 1.6) Total(Overall: 190.3) Minimum (Overall: 0.0) Maximum (Overall: 47.2) Number greater than 0.99 (Overall: 40) m Mean (Overall: 1.3) Total (Overall: 152.3) Minimum(Overall: 0.0) Maximum(Overall: 24.1) Number greater than 0.99 Overall: 34) Number of missing values (Overall: 1) m Mean (Overall: 2.1) Total (Overall: 242.6) Minimum (Overall: 0.0) Maximum(Overall: 93.9) Number greater than 0.99 (Overall: 23) Number of missing values (Overall: 1)

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