Glossary

This glossary defines the key terms used on the seasonal streamflow forecasts site (listed in alphabetical order).

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25% quartile

The 25% quartile of a set of numbers is the value in which 25% of the numbers are below it and 75% of the numbers are above it when the numbers are arranged in ascending (increasing) order.

Synonym: first quartile

75% quartile

The 75% quartile of a set of numbers is the value in which 75% of the numbers are below it and 25% of the numbers are above it when the numbers are arranged in ascending (increasing) order.

Synonym: third quartile

Bayesian joint probability (BJP)

A modelling system that uses statistical methods to produce predictions of seasonal streamflows. BJP was developed by the Commonwealth Scientific and Industrial Research Organisation (CSIRO). See the Seasonal Streamflow Forecast Research papers section for more information.

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catchment

The land area draining to a point of interest, such as a water storage or monitoring site on a watercourse.

continuous ranked probability score (CRPS)

A skill score calculated in the original measurement space. Both the forecast and the observation are converted to cumulative density functions. In the case of the observation, this is a step function. The area between the two functions is the single CRPS value, which is averaged across all forecasts. Alt text

El Niño

Refers to the extensive warming of the central and eastern Pacific Ocean that leads to a major shift in weather patterns across the Pacific. In Australia (particularly eastern Australia), El Niño events are associated with an increased probability of drier conditions. See the Weather and Climate page on El Niño for more information.

El Niño Southern Oscillation (ENSO)

The oscillation between the El Niño climate phase and the La Niña climate phase, or opposite phase, usually over several years. See the Weather and Climate page on ENSO for more information.

ensemble forecast

A collection or distribution of many forecasts for the same period and location. They can be generated from many models or from one model with many parameter sets.

exceedance probability

The forecast probability that a particular streamflow value will be exceeded.

For example there is a 60% forecast probability that a total streamflow volume of 25 gigalitres for Acheron River at Taggerty will be exceeded in the forecast period October–December 2009.

first quartile

The first quartile of a set of numbers is the value in which 25% of the numbers are below it and 75% of the numbers are above it when the numbers are arranged in ascending (increasing) order.

Synonym: 25% quartile

flow

The flow of water in streams, rivers and other channels.

Synonym: streamflow

hindcast

A retrospective forecast using observed information to validate or test a mathematical model. Known inputs for past events are entered into the model to see how well the output matches the known results.

historical reference or historical probability distribution

A probability distribution created by sampling the historical streamflow data record. It is the baseline reference used in determination of skill scores from hindcasting.

Indian Ocean Dipole (IOD)

A major contributor to rainfall variability over Australia. When the dipole is in a positive phase, the sea-surface temperature (SST) around Indonesia is cooler than average while those in the western Indian Ocean are warmer than average. The positive phase increases easterly winds across the Indian Ocean, while convection in areas near Australia reduces. This results in suppressed rainfall over the Australian region. During a negative phase, warmer than average SST near Indonesia and cooler than average SST in the western Indian Ocean, result in more westerly winds across the Indian Ocean, greater convection near Australia and enhanced rainfall in the Australian region. See the Weather and Climate page on Indian Ocean for more information.

interquartile range

A measure of statistical dispersion, being equal to the difference between the third and first quartiles. Also called the midspread or middle 50.

La Niña

The extensive cooling of the central and eastern Pacific Ocean. In Australia (particularly eastern Australia), La Niña events are associated with an increased probability of wetter conditions. See the Weather and Climate page on La Niña for more information.

monsoon

The northern Australian monsoon season generally lasts from December to March. It is associated with the inflow of moist west to northwesterly winds into the monsoon trough, producing convective cloud and heavy rainfall over northern Australia. These moisture-laden winds originate from the Indian Ocean and southern Asian waters. The northern Australian wet season encompasses the monsoon months but can extend several months either side. See the Weather and Climate page on the Australian monsoon for more information.

pie chart

A circular chart that is divided into sectors. A pie chart is used to display tercile forecasts with three sectors representing low flow, near median flow and high flow.

Related FAQ

probabilistic forecast

A probabilistic streamflow forecast shows the range of possible streamflow values and how likely it is that they will occur.

quantile

Denotes one of a series of threshold values that divides a set of ordered data into groups with an equal number of data points in each. The terms tercile, quartile, quintile, decile and percentile refer to quantiles that divide the distribution of data into 3, 4, 5, 10 and 100 equal parts, respectively.

root mean squared error (RMSE) score

The square error (SE) is the difference between the observed value and median of the forecast. The RMSE score is the square root of the average SE for a set of forecasts.

root mean squared error in probability (RMSEP) score

The square error in probability (SEP) is the difference between the historical probabilities of the observed value and median of the forecast. The RMSEP score is the square root of the average SEP for a set of forecasts.

Seasonal Streamflow Forecast

A three-month outlook of total streamflow volumes at a site or total inflows into a water storage.

skill score

A statistical score used to objectively assess the performance (skill) of a model. A score can be presented absolutely or formulated as a skill score.

Skill Score =(Smod - Sref) / (Sperf - Sref)

Where

Smod = Score for the model forecast

Sref = Score for a reference forecast, usually climatology

Sperf = Score for a perfect forecast, usually 0 or 1

Southern Oscillation Index (SOI)

An index calculated from the monthly or seasonal fluctuations in the air pressure difference between Tahiti and Darwin. See the Climate page on SOI for more information.

streamflow

The flow of water in streams, rivers and other channels.

Synonym: flow

tercile

A set of data arranged in order with values that partition the data into three groups, each containing one-third of the total data. To define these terciles, the historical data is arranged in order from lowest to highest and then the data is partitioned into three groups. The lowest third of the data values are defined as the lowest tercile, the middle third of the values are the middle tercile and the upper third of the values are the upper tercile.

For example, if you had 100 data values the lowest tercile would contain the 1st–33rd data values, the middle tercile the 34th–67th values and the upper tercile the 68th–100th values.

Related FAQ

third quartile

The third quartile of a set of numbers is the value in which 75% of the numbers are below it and 25% of the numbers are above it when the numbers are arranged in ascending (increasing) order.

Synonym: 75% quartile


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