Title Trends in the start of the wet season over Africa Dom Kniveton 1 Russ Layberry 2 Charlie Williams 3 1 Department of Geography, University of Sussex, UK 2 Environmental Change Institute, University of Oxford, UK 3 Walker Institute, University of Reading, UK

Rationale Lots of work on monthly/seasonal rainfall totals, and relationships with SST, land surface processes, etc However, agricultural users say knowing start and end dates of wet season is more useful than rainfall totals – especially start dates, as crucial for sowing times Fuzzy nature to start date e.g. no formal rule as to how much rain and over what period defines start of the wet season Further complicated by occurrence of dry spells – if occurring after initial period of rainfall, may signify false start of wet season – again vital for sowing times

a)Start of the wet season = after a particular date, d b)Potential start date is defined as first occurrence of at least x mm of rain totalled over t consecutive days c)Potential start could be a false start if a dry spell of n days occurs in the following m days after start date. Example of widely used definition Stern et al. (1982)

Rain gauge network

Values for parameters d (particular date): date with rainfall minimum prior to maximum x (amount of rainfall): 10mm, 20mm and 30mm t (period of rainfall): 2 consecutive days n (number of days constituting dry spell): 10 days m (number of days from d, over which dry spell reclassifies start date as false start): 30 days

Spatial variation of parameter, d

% of days with available data

10mm20mm30mm Average start dates for 3 thresholds of x

Influence of dry spells on start date Difference between possible and real start dates of the wet season

10mm20mm30mm Trends in start dates for 3 thresholds of x Average trends of start dates for Africa (south of 20°N): day yr day yr day yr -1

Correlations: African temp. & start dates

Conclusions Start of wet season = crucial importance for regions with rain fed agriculture – from farmer to intergovernmental organisations Start dates over Africa show large spatial and temporal variability – appears to increase with increased rainfall needed to define start date Climate change: from wet season appears to have started later in year for majority of Africa – up to 4 days / year in some parts, and on average , and days / year for 3 rainfall thresholds used Over early part of this millennium compared to previous 25 years, results equate to a wet season that is starting on average 9, 12 and 21 days later for the 3 rainfall thresholds used Results may also suggest that regional temperature and start date are related, so future global warming may increase later start date trend