Australian Water Information Dictionary

Water forecasts: Dynamic streamflow

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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

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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

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Australian Water Availability Project (AWAP)

An operational data assimilation and modelling project that monitors the state and trend of the Australian terrestrial water balance at a resolution of 5 km. Using both measurements and modelling, the past history and present state of soil moisture are determined. The water fluxes that contribute to changes in soil moisture are incorporated in this project. These water fluxes include rainfall, transpiration, soil evaporation, surface runoff and deep drainage.

Source: Australian Water Availability Project

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autocorrelation function (ACF)

Describes the correlation between values of that process as a function of the time separation between them.

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autoregressive moving average (ARMA)

A combined process of statistical time series analysis in terms of two polynomials, one for the auto-regression and the second for the moving-average.

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Bayesian

Refers to methods in probability or statistics, e.g. Bayesian Joint Probability, named after Thomas Bayes (c. 1702-61), who provided the first mathematical treatment of Bayesian inference.

Source: S E Fienberg, 2006, When Did Bayesian Inference Become "Bayesian"?, Bayesian Analysis, 1 (1), 1-40.

Related: Bayesian total error analysis (BATEA)

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Bayesian total error analysis (BATEA)

A generalised Bayesian framework for environmental model calibration and validation. BATEA permits the use of explicit probabilistic error models which are used to describe the uncertainty associated with observed data, notably, in forcing inputs and outputs, e.g. rainfall and discharge data.

Source: G Kuczera, D Kavetski, S Franks, M Thyer 2006, Towards a Bayesian total error analysis of conceptual rainfall-runoff models: Characterising model error using storm-dependent parameters, Journal of Hydrology, 331:1-2,161-177.

Related: Bayesian

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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.

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continuous ranked probability score (CRPS)

A measure of forecast accuracy 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 squared area between the two functions is the single CRPS value, which is averaged across all forecasts. Alt text

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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 Climate Glossary webpage on El Niño for more information.

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El Niño Southern Oscillation (ENSO)

The oscillation between the El Niño climate phase and the La Niña phase, usually over several years. See the Australian Climate Influence webpage on ENSO for more information.

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ensemble forecast

A set of multiple forecasts for the same period and location. The spread of multiple forecasts provides information about forecast uncertainty.

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exceedance probability

The forecast probability that a particular variable such as streamflow volume will be exceeded.

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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

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hindcast

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

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historical reference or historical probability distribution

A probability distribution created by sampling the historical streamflow data record. It is the reference forecast used when skill scores are calculated from hindcast modelling results.

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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.

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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.

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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 Climate Glossary webpage on La Niña for more information.

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Modèle du Génie Rural à 4 Paramètres Journalier (GR4J)

A daily lumped four-parameter rainfall-runoff model, belonging to the family of soil moisture accounting models.

Source: Perrin, C., Michel, C., & Andréassian, V. 2003, Improvement of a parsimonious model for streamflow simulation. Journal of Hydrology, 279(1), 275-289.

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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.

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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.

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Predictive Ocean Atmosphere Model for Australia (POAMA)

A state-of-the-art seasonal to inter-annual forecast system consisting of coupled ocean-atmosphere general circulation models (GCMs) along with sophisticated data assimilation and land-surface initialisation systems.

Source: POAMA

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probabilistic forecast

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

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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.

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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.

Related: root mean squared error in probability (RMSEP) score

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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.

Related: root mean squared error (RMSE) score

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Seasonal Streamflow Forecast

A forecast of total streamflow volumes predicted at a particular gauging station, or for the total of inflowing watercourses into a water storage. The forecast can be for a specific period, ranging from one to several months.

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skill score

A statistical score used to assess the relative performance (skill) of a model against a reference forecast (e.g. climatology).

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

Where

Smod = Score for the model forecast

Sref = Score for a reference forecast, e.g. climatology

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

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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.

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streamflow

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

Synonym: flow

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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 equal 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. A tercile is a specific form of quantile.

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.

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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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