Seasonal Stremflow Forecasting Service

The Bureau of Meteorology now provides a Seasonal Streamflow Forecasting Service for the south east Murray Darling Basin freely on its website. These products are helping water managers and users make better informed decisions. Seasonal Streamflow Forecasts estimate how much water is likely to flow past a specific location in a stream over the next three months. They are based on probabilities that is, the likelihood or chance of water flowing into a stream based on relationships with recent climate and catchment conditions. Forecasts are issued around the seventh day of every month – covering that month and the following two months. We use statistical modelling that provides 5000 forecast possibilities for each site. These forecasts are presented as probability distributions and exceedance probability charts and maps. The forecast service with be extended over time to more catchments across Australia. Our Website features four key products; Tercile summary map, Tercile Forecast, Historical and probability distribution, and the historical and exceedance probability. Seasonal Streamflow Forecasts are presented as a tercile or three part pie chart on a map. The pie chart represents the probability or likelihood of getting, low, near median and high flow for each site. This tercile summary map for south-east Murray Darling includes catchments such as the Murrumbidgee, Upper Murray, Ovens, Goulburn and Kiewa. It shows that the current forecast is favouring high flows at most sites, apart from two sites in the Goulbourn that are more likely to receive near median flow. Shading and greyscale of the pie charts, indicates the reliability of the forecasts. This appears in the legend at the bottom of the map. This graphic describes the shift in probability of each flow category for the forecast period. Based on historical data the probabilities of high, near median and low flows for each location are defined as 33%. The chart on the bottom right, shows the probability distribution of the historical streamflow data. It is similar to a bell curve or normal probability distribution. The chart on the top right shows how the probability of each flow category has changed from the historical data for the forecast period. The tercile forecast on the left shows in this case that there is a 78% percent chance of high flow, 20% chance of near median flow and a 2% chance of low flow. This forecast covers the same location and the same period as the previous tercile forecast. Forecast and historical distributions shown on the right can be compared with historical data presented on the left with a common vertical axis. The historical data includes the stream flow recorded last year for the forecast three month period, and the minimum and maximum stream flow total recorded for the same historical period. It also includes the ten year and long term average for this period at the site. The historical and exceedance probability product is similar to the previous product but this time the forecast is presented using the concept of exceedance probability. This shows the likelihood of a particular stream flow value being exceeded. This can be compared to the historical data on the left. We value your feedback and encourage you to submit comments or requests for more information at www.bom.gov.au/water/ssf