OGC Standard

OGC WaterML 2: Part 4 - GroundWaterML 2 (GWML2)
Boyan Brodaric Editor
Version: 2.2.1
Additional Formats: XML PDF DOC
OGC Standard


Document number:16-032r3
Document type:OGC Standard
Document subtype:Implementation
Document stage:Approved
Document language:English


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I.  Abstract

This standard describes a conceptual and logical model for the exchange of groundwater data, as well as a GML/XML encoding with examples.

II.  Keywords

The following are keywords to be used by search engines and document catalogues.

ogcdoc, OGC document, groundwater, hydrogeology, aquifer, water well, observation, well construction, groundwater flow, groundwater monitoring, UML, GML, GroundwaterML, GWML2

III.  Preface

III.A.  Motivation

A significant portion of the global water supply can be attributed to groundwater resources. Effective management of such resources requires the collection, management and delivery of related data, but these are impeded by issues related to data availability, distribution, fragmentation, and heterogeneity: collected data are not all readily available and accessible, available data is distributed across many agencies in different sectors, often thematically fragmented, and similar types of data are diversely structured by the various data providers. This situation holds both within and between political entities, such as countries or states, impairing groundwater management across all jurisdictions. Groundwater data networks are an emerging solution to this problem as they couple data providers through a unified data delivery vehicle, thus reducing or eliminating distribution, fragmentation, and heterogeneity through the incorporation of standards for data access and data content. The relative maturity of OGC data access standards, such as the Web Feature Service (WFS) and Sensor Observation Service (SOS), combined with the rise of water data networks, have created a need for GroundWaterML2 (GWML2), a common groundwater data standard.

III.B.  Historical background

Several activities have influenced the development of GWML2.

  • GWML1: a GML application schema for groundwater data developed at Natural Resources Canada and used to exchange groundwater data within Canada, between Canada and the USA, and in some other international efforts (Boisvert & Brodaric, 2012).

  • GWIE1: an interoperability experiment within the OGC HDWG, in which groundwater data was shared across the USA-Canada border (Brodaric & Booth, 2011).

  • GW2IE: a second interoperability experiment within the OGC HDWG, that designed and tested a precursor of GroundWaterML2 (GWML2, version 2.1): a conceptual, logical, and encoding specification for the representation of core groundwater data (OGC, 2016).

  • INSPIRE Data Specification on Geology — hydrogeology package: a conceptual model and GML application schema for hydrogeology (INSPIRE, 2013), with regulatory force in the European Union and for which GWML2 is expected to be an encoding candidate.

  • BDLISA: the French Water Information System information models for water wells and hydrogeological features (BDLISA, 2013).

The primary goal of this standard is to capture the semantics, schema, and encoding syntax of key groundwater data, to enable information systems to interoperate with such data.

IV.  Security Considerations

No security considerations have been made for this standard.

V.  Submitting Organizations

The following organizations submitted this Document to the Open Geospatial Consortium (OGC):

VI.  Submitters

All questions regarding this submission should be directed to the editor or the submitters:

Table 1

Name Affiliation
Boyan Brodaric GSC
Eric Boisvert GSC
Francois Létourneau GSC
Jessica Lucido USGS
Bruce Simons CSIRO
Peter Dahlhaus FedUni
Mickaël Beaufils BRGM
Sylvain Grellet BRGM
Laurence Chery BRGM
Alexander Kmoch U Salzburg

OGC WaterML 2: Part 4 - GroundWaterML 2 (GWML2)

1.  Scope

This document is an OGC® conceptual, logical and encoding standard for GWML2, which represents key groundwater data. GWML2 is implemented as an application schema of the Geography Markup Language (GML) version 3.2.1, and re-uses entities from other GML application schema, most notably the OGC Observations & Measurements standard and the OGC/IUGS GeoSciML 4.0 (16-008) standard. GWML2 version 2.2 (this document) updates version 2.1, which was developed by the GW2IE (OGC, 2016), by importing GeoSciML 4.0 instead of GeoSciML 3.2.0, and by using TimeseriesML (15-042r3) instead of OGC WaterML2.0 part 1 — Timeseries.

GWML2 is designed to enable a variety of data exchange scenarios. These scenarios are captured by its five motivating use cases, including:

  1. a commercial use-case focused on drilling water wells with knowledge of aquifers,

  2. a policy use case concerned with the management of groundwater resources,

  3. an environmental use-case that considers the role of groundwater in natural eco-systems,

  4. a scientific use-case concerned with modeling groundwater systems, and

  5. a technologic use-case concerned with interoperability between diverse information systems and associated data formats.

GWML2 is designed in three stages, each consisting of a schema that builds on the previous stages. The three schemas include:

  1. Conceptual (UML): a technology-neutral schema denoting the semantics of the domain,

  2. Logical (UML): a GML-specific schema that incorporates the OGC suite of standards,

  3. XML schema (XSD): a GML syntactical encoding of the logical schema.

In addition, this standard describes general and XML-specific encoding requirements, general and XML-specific conformance tests, and XML encoding examples. The standard is designed for future extension into other non-XML encoding syntaxes, which would require each such encoding to describe the related schema, requirements and conformance classes, as well as provide examples.

The GWML2 Logical and XML schemas are organized into 6 modular packages:

  1. GWML2-Main: core elements such as aquifers, their pores, and fluid bodies,

  2. GWML2-Constituent: the biologic, chemical, and material constituents of a fluid body,

  3. GWML2-Flow: groundwater flow within and between containers,

  4. GWML2-Well: water wells, springs, and monitoring sites,

  5. GWML2-WellConstruction: the components used to construct a well,

  6. GWML2-AquiferTest: the elements comprising an aquifer test (e.g., a pumping test).

Altogether, the schemas and packages represent a machine-readable description of the key features associated with the groundwater domain, as well as their properties and relationships. This provides a semantics and syntax for the correct machine interpretation of the data, which promotes proper use of the data in further analysis. Existing systems can use GWML2 to ‘bridge’ between existing schema or systems, allowing consistency of the data to be maintained and enabling interoperability.

2.  Conformance

This standard has been written to be compliant with the OGC Specification Model — A Standard for Modular Specification (08-131r3). Extensions of this standard shall themselves be conformant to the OGC Specification Model.

2.1.  XML implementation

The XML implementation (encoding) of the conceptual and logical groundwater schemas is described using the XML Schema language and Schematron.

Requirements for one standardization target type are considered:

  • data instances.

i.e., XML documents that encode groundwater data. As data producing applications should generate conformant data instances, the requirements and tests described in this standard effectively also apply to that target.

Conformance with this standard shall be checked using all the relevant tests specified in Annex A (normative) of this document. The framework, concepts, and methodology for testing, and the criteria to be achieved to claim conformance are specified in ISO 19105: Geographic information — Conformance and Testing. In order to conform to this OGC encoding standard, a standardization target shall implement the core conformance class, and choose to implement any one of the other conformance classes (i.e., extensions).

All requirements-classes and conformance-classes described in this document are owned by the standard(s) identified.

2.2.  Use of vocabularies

Controlled vocabularies, also known as code-lists, are used in data exchange to identify particular concepts or terms, and sometimes relationships between them. For example, an organization may define a controlled vocabulary for all observed phenomena, such as water quality parameters, that are to be exchanged between parties. Some of these definitions may be related by hierarchical relationships, such as specialization, or through other relationships such as equivalence.

GroundWaterML2.0 does not define a set of vocabularies for groundwater data exchange in this version. It is envisaged that specific communities will develop local vocabularies for data exchange within the community. Future work within the Hydrology Domain Working Group could address standardized controlled vocabularies for the groundwater domain. Such vocabularies require a governance structure that allows changes to be made as definitions evolve, possibly using the OGC definition namespace (, which is governed by the OGC Naming Authority (OGC-NA). The OGC-NA is responsible for processing requests to change or add new definitions to this namespace. The procedures for the OGC-NA are outlined in OGC document 09-046 (OGC-NA – Procedures) and the structure of URIs is outlined in OGC 09-048 (OGC-NA – Name type specification – definitions). Any URIs for vocabulary items (e.g. identifiers for various property values, properties, roles or other fixed labels) in this specification are included as examples only, for illustration purposes, and will not resolve, because GWML2 vocabularies are not defined. However, some such URIs in various example encodings may resolve if data providers have defined and implemented vocabularies for particular services.

The following convention has been used throughout the document to identify attributes requiring controlled vocabularies:

  • In the conceptual model, such attributes are typed with a name ending by Type (ex: PorosityType); and

  • In the logical model this suffix becomes TypeTerm (ex: PorosityTypeTerm).

2.3.  Groundwater data

Groundwater data conforming to this standard are encoded in GML-conformant XML documents, for this version of GWML2. It is anticipated that future versions or extensions will develop additional encodings such as JSON or RDF. The standard MIME-type and sub-type for GML data should be used to indicate the encoding choice as specified in MIME Media Types for GML, namely: application/gml+xml.

Conformance with this standard shall be checked using all the relevant tests specified in Annex A (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 site1.

In order to conform to this OGC™ interface standard, a software implementation shall choose to implement:

  1. Any one of the conformance levels specified in Annex A (normative).

All requirements-classes and conformance-classes described in this document are owned by the standard(s) identified.

3.  Normative references

The following documents are referred to in the text in such a way that some or all of their content constitutes requirements of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

James Tomkins , Dominic Lowe : OGC 15-043r3, Timeseries Profile of Observations and Measurements . Open Geospatial Consortium (2016).

Policy SWG: OGC 08-131r3, The Specification Model — Standard for Modular specifications. Open Geospatial Consortium (2009).

Peter Taylor: OGC 10-126r4, OGC® WaterML 2.0: Part 1- Timeseries. Open Geospatial Consortium (2014).

James Tomkins and Dominic Lowe: OGC 15-042r3, TimeseriesML 1.0 – XML Encoding of the Timeseries Profile of Observations and Measurements. Open Geospatial Consortium (2016).

Boyan Brodaric: OGC 15-082, OGC GroundWaterML 2 – GW2IE FINAL REPORT. Open Geospatial Consortium (2016).

GeoSciML Modeling Team: OGC 16-008, OGC Geoscience Markup Language 4.1 (GeoSciML). Open Geospatial Consortium (2017).

Arliss Whiteside Jim Greenwood : OGC 06-121r9, OGC Web Service Common Implementation Specification. Open Geospatial Consortium (2010).

ISO: ISO/TS 19103:2005, Geographic information — Conceptual schema language. International Organization for Standardization, Geneva (2005).

ISO: ISO 19123:2005, Geographic information — Schema for coverage geometry and functions. International Organization for Standardization, Geneva (2005).

ISO: ISO/TS 19139:2007, Geographic information — Metadata — XML schema implementation. International Organization for Standardization, Geneva (2007).

ISO: ISO 19156:2011, Geographic information — Observations and measurements. International Organization for Standardization, Geneva (2011).

ISO: ISO 8601:2004, Data elements and interchange formats — Information interchange — Representation of dates and times. International Organization for Standardization, Geneva (2004).

Simon Cox: OGC 10-004r3, Topic 20: Observations and Measurements. Open Geospatial Consortium (2010).

Roger Lott: OGC 08-015r2, Topic 2 — Spatial referencing by coordinates. Open Geospatial Consortium (2010).

OGC: OGC 07-011, Topic 6 — Schema for coverage geometry and functions. Open Geospatial Consortium (2007).

ISO: OGC 01-111, Topic 11 — Metadata. Open Geospatial Consortium (2001).

Clemens Portele: OGC 07-036r1, OpenGIS Geography Markup Language (GML) Encoding Standard — with corrigendum. Open Geospatial Consortium (2018).

ISO: ISO 19136:2007, Geographic information — Geography Markup Language (GML). International Organization for Standardization, Geneva (2007).

Simon Cox: OGC 10-025r1, Observations and Measurements — XML Implementation. Open Geospatial Consortium (2011).

Alexandre Robin: OGC 08-094r1, OGC® SWE Common Data Model Encoding Standard. Open Geospatial Consortium (2011).

Simon Cox: OGC 06-188r1, GML Encoding of Discrete Coverages (interleaved pattern). Open Geospatial Consortium (2007).

ISO/IEC: ISO/IEC 19757-3:2006, Information technology — Document Schema Definition Languages (DSDL) — Part 3: Rule-based validation — Schematron. International Organization for Standardization and International Electrotechnical Commission, Geneva (2006).

Mike Botts, Alexandre Robin, Eric Hirschorn: OGC 12-000r2, OGC SensorML: Model and XML Encoding Standard. Open Geospatial Consortium (2020).

Schadow, G and McDonald, C.: Unified Code for Units of Measure (UCUM) — Version 1.8 (2009)

OMG UML 2.3, Unified Modeling Language. (2010).

Extensible Markup Language (XML) — Version 1.0 (Fourth Edition) (2006)

XML Schema — Version 1.0 (Second Edition) (2004)

4.  Terms and definitions

For the purposes of this document, the following terms and definitions apply.

This document uses the terms defined in 06-121r9, Clause 5.3, 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.

This document also uses terms defined in the OGC Standard for Modular specifications (OGC 08-131r3), also known as the ‘ModSpec’. The definitions of terms such as standard, specification, requirement, and conformance test are provided in the ModSpec.

For the purposes of this document, the following additional terms and definitions apply.



Feature that acts as a function to return values from its range for any direct position within its spatial, temporal or spatiotemporal domain.

[SOURCE: ISO 19123:2005, Clause 4.17]


domain feature

Feature of a type defined within a particular application domain.

Note 1 to entry: This may be contrasted with observations and sampling features, which are features of types defined for cross-domain purposes.

[SOURCE: ISO 19156:2011, Clause 4.4]


element <XML>

Basic information item of an XML document containing child elements, attributes and character data.

Note 1 to entry: From the XML Information Set ― each XML document contains one or more elements, the boundaries of which are either delimited by start-tags and end-tags, or, for empty elements, by an empty-element tag. Each element has a type, identified by name, sometimes called its “generic identifier” (GI), and may have a set of attribute specifications. Each attribute specification has a name and a value.

[SOURCE: ISO 19136:2007]



Abstraction of a real-world phenomena.

[SOURCE: ISO 19101-1:2014, Clause 4.11]


GML application schema

Application schema written in XML Schema in accordance with the rules specified in ISO 19136:2007.

[SOURCE: ISO 19136:2007]


GML document

XML document with a root element that is one of the elements AbstractFeature, Dictionary or TopoComplex, specified in the GML schema or any element of a substitution group of any of these elements.

[SOURCE: ISO 19136:2007]


GML schema

Schema components in the XML namespace as specified in ISO 19136:2007.

[SOURCE: ISO 19136:2007]



Set of operations having the objective of determining the value of a quantity.

[SOURCE: ISO 19101-2, Clause 4.20]



Act of observing a property.

Note 1 to entry: The goal of an observation may be to measure or otherwise determine the value of a property.

[SOURCE: ISO 19156:2011, Clause 4.10]


observation procedure

Method, algorithm or instrument, or system which may be used in making an observation.

[SOURCE: ISO 19156:2011, Clause 4.11]


observation result

Estimate of the value of a property determined through a known procedure.

[SOURCE: ISO 19156:2011]


property <General Feature Model>

Facet or attribute of an object referenced by a name.


Abby’s car has the colour red, where “colour red” is a property of the car instance.


sampled feature

The real-world domain feature of interest, such as a groundwater body, aquifer, river, lake, or sea, which is observed.

[SOURCE: ISO 19156:2011]


sampling feature

Feature, such as a station, transect, section or specimen, which is involved in making observations of a domain feature.

Note 1 to entry: A sampling feature is purely an artefact of the observational strategy, and has no significance independent of the observational campaign.

[SOURCE: ISO 19156:2011, Clause 4.16]


schema <XML Schema>

XML document containing a collection of schema component definitions and declarations within the same target namespace.

Example Schema components of W3C XML Schema are types, elements, attributes, groups, etc.

Note 1 to entry: The W3C XML Schema provides an XML interchange format for schema information. A single schema document provides descriptions of components associated with a single XML namespace, but several documents may describe components in the same schema, i.e., the same target namespace.

[SOURCE: ISO 19136:2007]



Type of observation procedure that provides the estimated value of an observed property at its output.

Note 1 to entry: A sensor uses a combination of physical, chemical or biological means in order to estimate the underlying observed property. At the end of the measuring chain electronic devices often produce signals to be processed.

5.  Contributing organizations

The organizations that submitted this standard are listed in Section iv.

The following organizations contributed to the initiation or development of this standard:

6.  Conventions

6.1.  Requirements class

Each normative statement (requirement or recommendation) in this standard is a member of a requirements class. Each requirements class is described in a discrete clause or sub-clause, and summarized using the following template:

Table 2

Requirements Class: [label]
Target type[artefact or technology type]
Dependency[identifier for another requirements class]
Requirement /Recommendation[repeat as necessary]

All requirements in a class must be satisfied. Hence, the requirements class is the unit of re-use and dependency, and the value of a dependency requirement is another requirements class. All requirements in a dependency must also be satisfied by a conforming implementation. A requirements class may consist only of dependencies and introduce no new requirements.

6.2.  Requirement

All requirements are normative, and each is presented with the following template:

Table 3

Requirement [serial number]:
[Normative statement]

where /req/[classM]/[reqN] identifies the requirement or recommendation. The use of this layout convention allows the normative provisions of this standard to be easily located by implementers.

6.3.  Conformance class

Conformance to this standard is possible at a number of levels, specified by conformance classes (Annex A). Each conformance class is summarized using the following template:

Table 4

Conformance class /conf/{classM}
Dependency [identifier for another conformance class]
Requirements /req/{classA}
Tests [reference to clause(s) containing tests]

All tests in a class must be passed. Each conformance class tests conformance to a set of requirements packaged in a requirements class.

W3C Schema (XSD) and ISO Schematron (SCH) files are considered as part of this standard, although available online only, due to concerns about document size. Many requirements are expressed in a single XSD or SCH file although tests are listed individually in the conformance annex (one test for XSD and one test for SCH).

Schematron files explicitly specify which requirements are being tested in the title of the schematron pattern.

<pattern id="origin_elevation">

<title>Test requirement: /req/well-xsd/origin-elevation</title>

<rule context="gwml2w:GW_Well">

<assert test="count(gwml2w:gwWellReferenceElevation/gwml2w:Elevation[gwml2w:elevationType/@xlink:href='']) = 1">A GW_Well needs at least one origin Elevation</assert>



6.4.  Identifiers

Each requirements class, requirement and recommendation is identified by a URI. The identifier supports cross-referencing of class membership, dependencies, and links from each conformance test to the requirements tested. In this standard, identifiers are expressed as partial URIs or paths, which can be appended to a base URI that identifies the specification as a whole in order to construct a complete URI for identification in an external context.

The URI for each requirements class has the form:[classM].

The URI for each requirement or recommendation has the form:[classM]/[reqN].

The URI for each conformance class has the form:[classM].

The URI for each conformance test has the form:[classM]/[testN].

6.5.  External package abbreviations

Concepts from schemas defined in some other International Standards are designated with names that start with alpha codes as follow:

Table 5

GFISO 19109:2005 General Feature Model
GFIISO 19156:2011 General Feature Model Instances
TMISO 19108:2002 Temporal Schema, Temporal Objects
MDISO 19115 Metadata
CVISO 19123:2005 Schema for Coverage Geometry and Functions
OMISO 19156:2011 Observations and Measurements
DQISO 19157:201X Data Quality
WML2OGC® WaterML 2.0: Part 1- Timeseries
GWGroundwaterML 2.0

6.6.  Abbreviated terms

In this document the following abbreviations and acronyms are used or introduced:

Table 6

APIApplication Program Interface
GeoSciML 3.2GeoScience Mark-up Language version 3.2
GeoSciML 4.0GeoScience Mark-up Language version 4.0
GMLOGC Geography Mark-up Language
GWML1Groundater Markup Language version 1.0 (Natural Resources Canada)
GWML2Groundwater Markup Language version 2.0 (this standard)
GWML2-MainUML Logical Model of the primary GroundWaterML2 elements (namespace
GWML2-FlowUML Logical Model of the elements required to capture groundwater flow (namespace
GWML2-ConstituentUML Logical Model of the groundwater fluid body constituents and their relationships (namespace
GWML2-WellUML Logical Model of the features and properties associated with water well (namespace
GWML2-WellConstructionUML Logical Model of the well drilling and construction details (namespace
GWML2-AquiferTestUML Logical Model of the features and properties associated with aquifer test (namespace
INSPIREInfrastructure for Spatial Information in the European Community (Directive 2007/2/EC)
ISOInternational Organization for Standardization
IUGSInternational Union of Geological Sciences
NACSNNorth American Commission on Stratigraphic Nomenclature
NADMNorth American geological Data Model
OGCOpen Geospatial Consortium
O&MOGC Observations and Measurements Conceptual Model
OMXMLObservations and Measurements XML Implementation
SensorMLSensor Model Language
SOSSensor Observation Service
SWESensor Web Enablement
UMLUnified Modeling Language
UTCCoordinated Universal Time
URIUniversal Resource Identifier
URLUniversal Resource Locator
WML2WaterML 2.0 — Part 1
XMLExtensible Markup Language
XSDW3C XML Schema Definition Language

6.7.  UML notation

The diagrams that appear in this standard, including the GWML2 Conceptual and Logical schemas, are presented using the Unified Modeling Language (UML), in compliance with ISO/IEC 19505-2.

Note: Within the GWML2 conceptual and logical diagrams, the following color scheme is used to identify packages in some cases. This is just for information purposes.

Amber: GWML2 defined within this standard

Green and Purple: from GeoSciML 4.0

Blue: from O&M

6.8.  Finding requirements and recommendations

This standard is identified as For clarity, each normative statement in this standard is in one and only one place, and defined within a requirements class table and identified with a URI, whose root is the standard URI. In this standard, all requirements are associated to tests in the abstract test suite in Annex A. using the URL of the requirement as the reference identifier. Recommendations are not tested but are assigned URLs and are identified using the ‘Recommendation’ label in the associated requirements table.

Requirements classes are separated into their own clauses, named, and specified according to inheritance (direct dependencies). The Conformance test classes in the test suite are similarly named to establish an explicit and mnemonic link between requirements classes and conformance test classes.

7.  Background

7.1.  Technical Basis

This standard builds on a number of standards for encoding XML data, including:

This standard also builds on existing schema, primarily Observations & Measurements (OMXML) and GeoSciML 4.0 (16-008). It accomplishes this by (a) extending these schemas with groundwater specializations, (b) referring to a class in these schema in order to type a named property, or © using a class from the schemas as one of the two participants in a binary relationship.

7.2.  Overview of Observations & Measurements

ISO 19156:2011 — Observations and Measurements is a generic GML schema for observations. As shown in Figure 1, it defines an observation as “…​an act associated with a discrete time instant or period through which a number, term or other symbol is assigned to a phenomenon. It involves application of a specified procedure, such as a sensor, instrument, algorithm or process chain. The procedure may be applied in-situ, remotely, or ex-situ with respect to the sampling location. The result of an observation is an estimate of the value of a property of some feature.

7.2.1.  Sampling features

Sampling features in O&M are defined as a “feature, such as a station, transect, section or specimen, which is involved in making observations concerning a domain feature.” Sampling features in the groundwater domain are features along which, or upon, observations are made. The most relevant are water wells and boreholes, which effectively host observations along staged intervals; a collection of these intervals and their observations constitutes a log.