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

El Nino seasonLa Nina season

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

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

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 1 st December, Does this make a difference?

20mm start

15mm start

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 15 th November Then what is the conditional risk of a long dry spell?