1 / 13

Title

Trends in the start of the wet season over Africa. Title. 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.

gordonhernandez
Download Presentation

Title

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Trends in the start of the wet season over Africa Title Dom Kniveton1 Russ Layberry2 Charlie Williams3 1Department of Geography, University of Sussex, UK 2Environmental Change Institute, University of Oxford, UK 3Walker Institute, University of Reading, UK

  2. 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

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

  4. Rain gauge network

  5. 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

  6. Spatial variation of parameter, d

  7. % of days with available data

  8. Average start dates for 3 thresholds of x 10mm 20mm 30mm

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

  10. Trends in start dates for 3 thresholds of x 10mm 20mm 30mm Average trends of start dates for Africa (south of 20°N): 0.28-0.37 day yr-1 0.43-0.48 day yr-1 0.65-1.0 day yr-1

  11. Correlations: African temp. & start dates

  12. 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 1978-2002 wet season appears to have started later in year for majority of Africa – up to 4 days / year in some parts, and on average 0.28-0.37, 0.43-0.48 and 0.65-1.0 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

More Related