Probabilistic Seasonal Streamflow Forecast for SOUTH AFRICA (Issued on 15 June, 2011)

This seasonal streamflow forecast is issued by the Climate Studies, Modelling and Environmental Health (CSM&EH) group of the CSIR's Natural Resources and the Environment (NRE). The forecast is the result of an operational multi-model forecast system, developed by CSM modellers, which comprises of forecasts from two coupled ocean-atmosphere models, downscaled statistically to 1946 quaternary catchments across South Africa. The quaternary catchment streamflow values were produced by the School of BEEH, University of KwaZulu-Natal. The coupled models are: 1. The ECHAM4.5-MOM3-DC2, administered by the International Research Institute for Climate and Society (IRI); and, 2. The ECHAM4.5-GML, also administered by the IRI. The coarse-resolution 850 hPa geopotential height forecasts from the coupled models are statistically downscaled to accumulated streamflows at catchment level by using the Climate Predictability Tool (CPT) developed at the IRI. The forecast consists of two maps: The top map shows the probabilities of exceeding the 85th percentile threshold of the climatological record (extremely high accumulated streamflow values, i.e. flooding), and the bottom map the probabilities of exceeding the 15th percentile threshold (extremely low streamflows). PLEASE TAKE NOTE: These streamflow forecasts are from natural conditions without dams, abstractions, return flows etc; hydrological model: ACRU. Many parts of the more arid and flat areas of South Africa have so-called internal drainage (endorheic areas), with the surface runoff re-infiltrating into the soil and under very wet conditions flowing into pans, but not exiting the catchments. Internal drainage not considered.

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