This standard defines an information model and XML encoding for exchanging the following three hydrological information resources:
- Conversion tables, or conversion curves, that are used for the conversion of related hydrological phenomenon.
- Gauging observations– the observations performed to develop conversion table relationships.
- Cross sections - survey observations made of the geometric structure of features, such as river channels, storages etc.
Metadata and vocabularies are defined that together provide a means for parties to exchange these concepts using common semantics.
This standard is the second part of the WaterML2.0 suite of standards, building on part 1 that addresses the exchange of time series.
The following are keywords to be used by search engines and document catalogues.
ogcdoc, waterml, ratings, gaugings, o&m, conversion, cross section
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. The Open Geospatial Consortium shall not be held responsible for identifying any or all such patent rights.
Recipients of this document are requested to submit, with their comments, notification of any relevant patent claims or other intellectual property rights of which they may be aware that might be infringed by any implementation of the standard set forth in this document, and to provide supporting documentation.
The following organizations submitted this Document to the Open Geospatial Consortium (OGC):
- Commonwealth Scientific and Industrial Research Organization (CSIRO), Australia
- Australian Bureau of Meteorology, Australia
- Centre for Ecology and Hydrology, UK
- United States Geological Survey (USGS)
- Aquatic Informatics
All questions regarding this submission should be directed to the editor or the submitters:
|Peter Taylor||Commonwealth Scientific and Industrial Research Organisation (CSIRO), Australia||Yes|
|Paul Sheahan||Bureau of Meteorology, Australia||Yes|
|Dominic Lowe||Bureau of Meteorology, Australia||Yes|
|Michael Natschke||KISTERS, Germany||Yes|
|Peter Heweston||KISTERS, Australia||Yes|
|Mathew Fry||Centre for Ecology and Hydrology, UK||Yes|
|David Briar||United States Geological Survey, USA||Yes|
|Stuart Hamilton||Aquatic Informatics, Canada||Yes|
|Brian Gouge||Aquatic Informatics, Canada||Yes|
This document defines an information model to describe hydrological ratings, gauging observations and survey observations. It is part 2 of the WaterML2.0 ‘suite’ of standards; the first part covered time-series observations and monitoring points. This standard re-uses types from part 1.
This work has been conducted by members of the WaterML 2.0 Standards Working Group and the joint WMO/OGC Hydrology Domain Working Group.
This standard defines a UML conceptual model and XML encoding schema for describing hydrological conversions, gauging observations and survey observations.
Requirements for two standardization target types are considered:
- UML models
- XML instances (e.g. XML documents)
Conformance with this standard shall be checked using all the relevant tests specified in Annex A (normative) and Annex B (normative) of this document. The framework, concepts, and methodology for testing, and the criteria to be achieved to claim conformance are specified in the OGC Compliance Testing Policies and Procedures and the OGC Compliance Testing web site.
In order to conform to this OGC™interface standard, a software implementation shall choose to implement:
- Any one of the conformance levels specified in Annex A (normative) or Annex B (normative).
All requirements-classes and conformance-classes described in this document are owned by the standard(s) identified.
3. Normative References
The following normative documents contain provisions that, through referenced in this text, constitute provisions of this document. For dated references, subsequent amendments to, or revisions of, any of these publications do not apply. For undated references, the latest edition of the normative document referred to applies.
- OGC 08-131r3 – The Specification Model – A Standard for Modular Specification
- ISO 19103:2005 – Geographic information - Conceptual schema language
- ISO 8601:2004 - Data elements and interchange formats – Information interchange – Representation of dates and times
- OGC Abstract Specification Topic 1 – Feature geometry (aka ISO 19107)
- OGC Abstract Specification Topic 2 – Spatial Referencing by Coordinates (aka ISO 19111:2007)
- OGC Abstract Specification Topic 6 – Schema for Coverage geometry and functions (aka ISO 19123:2005)
- OGC Abstract Specification Topic 11 – Geographic information — Metadata (aka ISO 19115:2014)
- OGC Abstract Specification Topic 20 – Observations and Measurements (aka ISO 19156:2011)
- OGC 07-036 Geography Markup Language (aka ISO 19136:2007)
- OGC WaterML2.0 part 1 – timeseries. OGC 10-126r4. http://www.opengeospatial.org/standards/waterml
- OGC Observations and Measurements v2.0 XML OGC Document 10-025r1. http://www.opengis.net/doc/IS/OMXML/2.0
- OGC SWE Common Data Model Encoding Standard v2.0 OGC Document 08-094r1 http://www.opengis.net/doc/IS/SWECommon/2.0
- Unified Code for Units of Measure (UCUM) – Version 1.8, July 2009
- Unified Modeling Language (UML). Version 2.3. May 2010.
- Extensible Markup Language (XML) – Version 1.0 (Fourth Edition), August 2006
- XML Schema – Version 1.0 (Second Edition), October 2004
4. Terms and Definitions
This document uses the terms defined in Sub-clause 5.3 of [OGC 06-121r8], which is based on the ISO/IEC Directives, Part 2, Rules for the structure and drafting of International Standards. In particular, the word “shall” (not “must”) is the verb form used to indicate a requirement to be strictly followed to conform to this standard.
For the purposes of this document, the following additional terms and definitions apply. There is some variation in the specific use of some technical terms within the hydrological domain. We have attempted to follow common usage, referring where possible to the WMO Glossary of hydrological concepts (http://webworld.unesco.org/water/ihp/db/glossary/glu/aglu.htm). English version: http://webworld.unesco.org/water/ihp/db/glossary/glu/HINDEN.HTM.
- 4.1 discharge
volume of liquid flowing through a cross-section in a unit time.
water level, typically measured at a stream-monitoring site.
Synonym: gauge height, level.
profile of a stream normal to the mean direction of flow, usually bounded by the potential inundation extent of the stream.
physical properties of a channel which determine the relationship between stage and discharge at a location in the channel. (WMO)
NOTE 1 Physical properties refers to the shape of the natural or anthropogenic feature which is controlling the stream level at the monitoring station.
physical, paired, simultaneous measurement of two related phenomena for the purposes of developing or validating the relationship between them.
EXAMPLE Simultaneous measurements of streamflow and aggregate mean gauge height.
EXAMPLE Simultaneous measurements of velocity and flow.
NOTE 1 Also referred to as Gauging, often - depending on local usage - implying stage and discharge observations specifically.
NOTE 2 In some situations outside of this specification the term calibration, or even rating, is used to describe such a measurement.
primary location for conducting observations. (OGC WaterML2.0 part 1)
NOTE Also often referred to as Station, Gauging Station and Site.
physical phenomenon that may be directly measured or estimated.
EXAMPLE Stage, discharge, rainfall, evapotranspiration
NOTE 1 Phenomena are often associated with features, where they may be termed property or property type.
NOTE 2 Other domain synonyms or close matches include variable, observed propertyand parameter.
relationship between two phenomena at a moment in time as defined by an equation, table of paired values or other form.
EXAMPLE A stage to discharge conversion defined by a rating table describes the relationship between the water level and the volumetric flow rate of a stream.
specific Conversion that relates stage to discharge.
NOTE Within hydrology this is commonly referred to as a Rating. A plot of a Rating Conversion is often called a rating curve.
table representation of a Conversion.
NOTE A conversion table is, most often, a discretised representation of a rating curve; hence there will be a 1:1 relation between a curve and its table. The tabular form of the rating curve facilitates use and exchange.
NOTE Within hydrology this is commonly referred to as a Rating Tablewhen representing a stage to discharge conversion.
equation representation of a Conversion.
NOTE The equation may be presented as a curve, which within hydrology is termed a rating curve.
A conversion group defines the applicable conversions for time periods at a specific location.
EXAMPLE A collection of stage to discharge conversion tables that have their period of application defined across time.
property that is being measured to estimate another property using a Conversion.
EXAMPLE stage, velocity, turbidity
NOTE 1 Also known as the independent (or input) property
property that is being estimated using a Conversion.
EXAMPLE discharge, cross-sectional velocity, total suspended sediment
NOTE 1 Also known as the dependent (or output) property
5.1 Abbreviated terms
The following symbols and abbreviated are used in this standard;
- Geography Markup Language
- Observations and Measurements
- Open Geospatial Consortium
- Sensor Web Enablement
- Unified Modeling Language
- World Meteorological Organisation
- Extensible Markup Language
- W3C XML Schema Definition Language
5.2 Schema language
The XML implementation specified in this Standard is described using the XML Schema language (XSD) [XML Schema Part 1: Structures, XML Schema Part 2: Datatypes] and Schematron [ISO/IEC 19757-3, Information technology — Document Schema Definition Languages (DSDL) — Part 3: Rule-based validation — Schematron].
5.3 UML notation
The diagrams that appear in this standard are presented using the Unified Modeling Language (UML) static structure diagram.
Note:Within the context of this standard, the following color scheme is used to identify the package in which the class exists. This is just for informative purposes.
Tan: Defined within this standard
Blue: WaterML2.0 part 1 - timeseries
Red: ISO19156 – Observations & Measurements
Green: ISO19115 – Metadata
This standard defines a number of properties that require the use of codes/vocabulary items. In some cases, a list of terms is provided. Where no codes are provided, it is expected that a list will be developed in the future, or a local code list may be used. A summary of the vocabularies is shown in Table 1. The joint OGC/WMO Hydrology Domain Working Group (HydroDWG) is responsible for managing the content of these vocabularies. Once agreement is reached for definitions, the HydroDWG should submit updates to the OGC Naming Authority.
|Code list||Package(s)||Code items defined|
Conversions & Gaugings
Conversions & Gaugings
7. Non-Normative (Informative) Material
WaterML2.0 is an initiative of the OGC to develop international standards and address interoperability of hydrological information systems. The first part of WaterML2.0 focused on a standard information model for time series of hydrological observations. The scope was defined through identification of common requirements and priority areas for data hydrological data exchange. The development involved a harmonisation process whereby existing formats were compared and contrasted with an aim of capturing the key elements for time series data exchange. Early versions of the standard were tested through a number of OGC Interoperability Experiments, each testing against a different set of use cases.
WaterML2.0 Part 2 focuses on another key aspect of hydrological data: rating conversions, gauging observations and river cross-sections. These are part of most surface water monitoring programs and are used in daily operations, including reporting, analysis, modelling and forecasting. This type of data is becoming increasingly important to exchange and share outside the scope of single organisations. The full history of rating conversions through time, and the gauging observations used to develop ratings, is needed to judge their fitness for purpose, and increasingly for the quantification of uncertainty for risk-based modelling and analysis.
7.1 Conversions are a generalised concept
The practice of using an observed or theoretical relationship to make indirect observations through conversion of a more readily measured phenomenon has wide application within hydrology. The most prevalent usage of conversions within hydrology is the application of rating conversions.
This standard enables the definition of simple relationships between any phenomena, though the primary focus in its creation has been to ensure its appropriateness for stage to discharge conversion. The following discussion describes stage to discharge relationships, their derivation, and their uncertainty in more detail in order to aid understanding of the use cases and the resulting data model.
Conversions may be applied in a compound manner with the sequential application of conversions being required to determine the final derived value, e.g. the North American practice of correcting stage data via a conversion prior to the derivation of discharge. This standard defines the method for the communication of the conversions but does not define the workflow process for compound conversion usage.
7.2 Key hydrological concepts
Hydrometry is the practice of measuring river flow, which is important to almost all areas of hydrology. River flows are generally measured indirectly – relying on the conversion of a record of stage to discharge using an often empirically derived rating conversion.
A rating conversion contains the information needed to correctly derive the dependent phenomenon (output property) from the independent one (input property). The rating conversion has validity during one, or more, specified periods of duration. The rating conversion period of validity may overlap with the period of validity of another rating requiring a transition.
Figure 2 shows an example plot of a rating conversion curve (on a log-log scale) with the individual gauging points and rating period of application. WMO recommends a curve should “…include at least 12 to 15 measurements, all made during the period of analysis” and these should be “…well distributed over the range of gauge heights experienced.”
184.108.40.206 Construction of a Rating-conversion
A rating conversion is an articulation of the complex physical relationship between stage and discharge. The shape, extent and form of a rating conversion are typically calibrated with gauging observations. Rating conversions are often extrapolated beyond the range of gauging observations.
Construction of a rating conversion requires consideration of factors such as the shape of the channel cross-section, the hydro-morphology of the channel, including the texture of the channel surfaces, and the hydraulics of the channel upstream and downstream of the monitoring point.
Some monitoring points implement an artificial structure to control the stage to discharge relationship over a limited range of stage e.g. a weir or flume. A control structure may be constructed to a specification for which the stage to discharge relationship is known. In this case gauging observations are commonly used to validate the theoretical relationship, and to define the relationship for the stage ranges where the theoretical relationship does not apply e.g. when the capacity of the control structure is exceeded.
220.127.116.11 Maintenance of a Rating-conversion
The stage to discharge relationship can change over time due to a large number of factors including erosion of the channel bed or banks, deposition of sediment either at, or up- and downstream of, the cross-section, growth of weed or accumulation of algae. These changes can be sudden (e.g. erosion of the bed and banks during a flood event) or gradual, seasonal (e.g. weed growth) or irregular in time.
The changing nature of the relationship drives a need for ongoing review and re-definition of the relationship. Monitoring points generally have a history of rating conversions that have a time-limited period of validity; the latest rating conversion is used to define the current stage to discharge relationship.
18.104.22.168 Rating-conversion uncertainty
Uncertainty is an inherent aspect of rating conversion development and application. Some sources of rating conversion uncertainty at a monitoring point can be characterized by expert-knowledge, for example the quality of a structure, or the likelihood of weed-growth or erosion within a river section.
In most cases it is ultimately the quality of the fit of the rating to the gauging observations, and the ability of the gauging observations to represent the range of stage at a monitoring point, which is used to quantify the uncertainty.
Increased sharing of ratings and gaugings will assist data users to complete their own evaluation of uncertainty in any dataset.
22.214.171.124 WaterML2.0 Part 2 expresses the data as a points table
Rating conversions are defined in many different forms in hydrometric data systems. The two primary methods for defining rating conversions are a table of stage/discharge pairs and one or more equations. Assessment of exchange standards to support equation-based conversions was out of scope for this work. This standard is scoped to enable the interoperable exchange of rating conversions though the use of a linearly interpretable points table.
126.96.36.199 Gauging observation
A gauging observation is the simultaneous measurement of the independent and dependent properties that are the subject of a conversion. The observations are used in the development and validation of a conversion.
188.8.131.52 Stage-discharge gauging observation
Stage-discharge gauging observations are the simultaneous direct measurements of instantaneous stage and discharge.
The technique used for measuring both phenomena can vary. The most common method for determining an estimated discharge measurement is the velocity-area method, which is roughly described as follows:
- The velocity of water is measured in different segments of the river cross-section.
- The volume for each segment is calculated based on the segments measured area multiplied by the segments mean velocity, determined from multiple velocity observations.
- The volume of each segment is summed to give an estimate of total discharge.
Stream-discharge gauging observations are undertaken to quantify the discharge at the monitoring point. The location of the actual observation is likely to be up or downstream of the monitoring point. The observation location will be chosen based on the prevailing river conditions and assessment of the most appropriate place for conducting a stage-discharge gauging observation.
184.108.40.206 Changing technologies
The method used to determine the velocity at each point varies – more traditional techniques involve the use of an impeller driven current meter that is lowered into the river, sometimes from a boat or directly by an operator standing in the river.
The use of acoustic methods, such those provided by Doppler instruments, are increasing in use due to their practicality and availability of commercial instruments.
For example, Acoustic Doppler Current Profilers (ADCP) can simultaneously measure the velocity of suspended particles in the river, depth and the river cross section path to estimate discharge. Multiple transects can be taken to provide a more accurate estimate. The increased speed of the measurement technique allows for more precise recording of discharge, allowing for greater definition of the rating curve through an increased number of observations in the same time period compared to older methods.
River channel sections are used in the construction and maintenance of rating tables. The types of sections in common use are cross sections and long sections.
220.127.116.11 Cross section
Cross sections are taken to define the stage/area relationship and identify variation of riverbank profile.
Cross sections are usually taken to define the shape of stage to discharge relationship controlling features. As the stage to discharge relationship controlling feature may change with increasing stage, multiple section may be required e.g. at lower stages there may be an artificial weir, above the weir a downstream bend may be a controlling feature.
18.104.22.168 Long section
Long sections typically follow the riverbed along the thalweg. Long sections are used to understand the slope of the river channel and its contribution to the rivers kinetic energy.
22.214.171.124 Use of gauging observation cross sections
The process of completing a stage-discharge gauging observation involves the measurement of the river cross section at the gauging section. Whilst this information is useful, it has limited value to rating conversion construction and maintenance as it does not define a stage–discharge relationship at the controlling feature and it only defines the riverbank profile for the area of current inundation.
7.3 Use cases/scenarios
Sharing of rating curves is required in multiple scenarios, generally stemming from the need to perform the calculation of the derived phenomenon or to analyse the derived data with a view to understanding its inherent uncertainty or quality. The following use cases were used in the requirements analysis process and produced a set of requirements that are described later in this document.
7.3.1 Data scrutiny
A primary use case for exposing detailed descriptions of rating tables is for closer scrutiny of derived data sets. A number of regularly used hydrological time-series – the most obvious being river discharge/flow – are derived using techniques that are approximations of relationships between other more readily measurable phenomena. It is becoming increasingly important for these data to be treated carefully due to the inherent assumptions made in the conversion process.
Five scenarios were used when analyzing requirements from this perspective:
- Uncertainty research;
- Geomorphic process, cross sections;
- Engineering design, e.g. designing a flood barrier;
- Analysis and assessment of input data for hydrological models; and
- Evaluation of fitness for purpose of data derived from ratings.
7.3.2 Exchange of specific rating table
It is often desirable to have instant access to a specific rating table relationship for a specific purpose. Some indicative examples include:
- Time sensitive (floods, events, emergencies);
- Inundation modelling (reverse the conversion and get the input variable from the output variable, e.g., get state from discharge);
- Asite visit or comparing a gauging point against the curve;
- Exchange of the most recent shift curve. (E.g. for reservoir optimization where the rating for inflow is subject to frequent shifts); and
- Providing ratings to operational systems (such as SCADA).
7.3.3 Exchange of full rating history
Often a centralised repository or reporting agency requires access to a full history of rating tables to run derived calculations for specific sites at any point in time. This requires a full history of rating tables as they have evolved through time. This typically excludes development versions of rating tables that have not been approved for use or release.
7.3.4 Suspended sediment and load calculations
Sediment-transport relationships (for calculating concentration, loads etc.) are used in numerous scenarios requiring an understanding of expected sediment build up or effects on the general environment. These relationships are often required by hydraulic or civil engineers for particular analyses or case studies.
7.3.5 Transfer between disparate information systems
This use case covers exchange between systems that do not have a common information model/schema for representation rating conversion. While this is a generic use case that may occur within the above scenarios, it is an important one for operation and interoperability of distributed information systems. Examples include:
- Exchange of ratings amongst databases using different schema;
- Migration of ratings into a new database using a different schema; and
- Archive of ratings in a form that will preserve information without dependence on any particular software.
7.3.6 Research services
General hydrological studies benefit from open access to hydrological data that may be used in educational scenarios. Rating tables and gauging observations are fundamental concepts within hydrological operations and having access to real world data in common formats supports learning these base concepts.
8. UML Model (normative)
8.1 Requirements class: Collection
The WaterML2.0 Part 2 Collection type shall specialize the WaterML2.0 part 1 Collection by adding support for rating, gauging and cross-section members.