Interpreting the data
Month and year-to-date rainfall and streamflow
The colour of the marker represents the decile category.
- Well above average (blue) refers to a rainfall amount which is within the highest 10% of the historical record. This is referred to as decile 10.
- Above average (light blue) refers to a rainfall amount which falls within deciles 8 and 9.
- Average conditions (white) are rainfall amounts in deciles 4, 5, 6 and 7 – from 30 to 70% of the historical record.
- Below average (yellow) represents a rainfall amount in deciles 2 and 3.
- Well below average (orange) represents a rainfall amount which is within the lowest 10% of the historical record, or decile 1.
Integrated Water Supply Scheme dams
The rainfall station at Jarrahdale is used here to represent rainfall conditions contributing to inflow into the Integrated Water Supply Scheme (IWSS) dams. Subject to rainfall patterns, from 1 May 2017 around 400 mm of rainfall at Jarrahdale is needed to start inflow to the northern dams.
The rainfall station at Perth Airport is used here to represent the rainfall conditions contributing to recharge of Gnangara Mound. This year we need around 600 mm of rainfall to maintain Gnangara groundwater levels at the same level they were last year. With less than this amount groundwater levels are likely to decline, depending on the rainfall pattern and when and where it rains.
The groundwater levels across the Gnangara Mound are influenced by recharge from rainfall, groundwater abstraction and the extent to which different land uses intercept rainfall. Read more about Gnangara Mound.
Rainfall and streamflow tracking
The block colours show the range in cumulative monthly rainfall or streamflow based on 1975-2016 records.
- High (dark blue) refers to the highest on record to the 70th percentile.
- Average (light blue) refers to the 70th to 30th percentile.
- Low (yellow) refers to the 30th to 10th percentile.
- Very low (orange) refers to the 10th percentile to the lowest on record.
The solid black line shows recorded rainfall or streamflow for the year to date.
The black dashed lines show scenarios, based on recorded values from 1975 to 2016, from what has happened so far this year.
- The wet scenario corresponds to the 90th percentile.
- The median scenario corresponds to the 50th percentile.
- The dry scenario corresponds to the 10th percentile.
A percentile is a measure used in statistics to rank one data point compared to the rest of the data. For example the 70th percentile means that 70% of the data is lower and 30% of the data is higher.