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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


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.

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.

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.

iv.  Submitting organizations

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

Geological Survey of Canada (GSC), Canada
U.S. Geological Survey (USGS), United States of America
Commonwealth Scientific and Industrial Research Organisation (CSIRO), Australia
Bureau of Meteorology (BOM), Australia
Federation University Australia (FedUni), Australia
Bureau de Recherches Géologiques et Minières (BRGM), France
Salzburg University (U Salzburg), Austria


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

Geological Survey of Canada (GSC), Canada
U.S. Geological Survey (USGS), United States of America
Commonwealth Scientific and Industrial Research Organisation (CSIRO), Australia
Federation University Australia (FedUni), Australia
Bureau of Meteorology (BOM), Australia
European Commission, Directorate General – Joint Research Centre  (JRC), European Union
Polish Association for Spatial Information
Polish Geological Institute (PGI), Poland
Geological Surveys of Germany (GSG), Germany
Salzburg University (U Salzburg), Austria
Bureau de Recherches Géologiques et Minières (BRGM), France
British Geological Survey (BGS), U.K.
International Groundwater Resources Assessment Centre (IGRAC), UNESCO

v.  Submitters

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


Name Affiliation OGC Member?

Boyan Brodaric



Eric Boisvert



Francois Letourneau



Jessica Lucido



Bruce Simons



Peter Dahlhaus



Sylvain Grellet



Laurence Chery



Alexander Kmoch

U Salzburg


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  (OGC 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 (OGC 15-042r2) 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 (OGC 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:

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 OGCencoding 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).

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

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 site[1].

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

a)     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.    References

The following normative documents contain provisions that, through reference 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: OGC 15-043r3, Timeseries Profile of Observations and Measurements (2016)
OGC: OGC 08-131r3, The Specification Model – A Standard for Modular Specification (2009)
OGC: OGC 10-126r4, WaterML2.0 part 1 – Timeseries (2014)
OGC: OGC 15-042r2, TimeseriesML 1.0 – XML Encoding of the Timeseries Profile of Observations and Measurements (2016)
OGC: OGC 15-082, OGC GroundWaterML 2 – GW2IE Final Report (2016)
OGC: OGC 16-008, OGC Geoscience Markup Language 4.0 (GeoSciML) (in publication)
OGC: OGC 06-121r9, OGC Web Services Common Standard (2010)
ISO / TC 211: ISO 19103:2005, Conceptual Schema Language (2005)
ISO: ISO 8601:2004, Data elements and interchange formats – Information interchange – Representation of dates and times (2004)
OGC: OGC 10-004r3, OGC Abstract Specification Topic 20 – Observations and Measurements (aka ISO 19156:2011) (2011)
OGC: OGC 08-015r2, OGC Abstract Specification Topic 2 – Spatial Referencing by Coordinates (aka ISO 19111:2007) (2007)
OGC: OGC 07-011, OGC Abstract Specification Topic 6 – Schema for Coverage geometry and functions (aka ISO 19123:2005) (2005)
OGC: OGC 01-111, OGC Abstract Specification Topic 11 – Geographic information — Metadata (aka ISO 19115:2003) (2003)
OGC: OGC 07-036, Geography Markup Language (aka ISO 19136:2007) (2007)
OGC: OGC 10-004r1, Observations and Measurements v2.0 (also published as ISO/DIS 19156:2010, Geographic information — Observations and Measurements) (2010)
OGC: OGC 10-025r1, Observations and Measurements - XML Implementation v2.0 (2011)
OGC: OGC 08-094r1, SWE Common Data Model Encoding Standard v2.0 (2011)
ISO/IEC: Schematron: ISO/IEC 19757-3:2006, Information technology — Document Schema Definition Languages (DSDL) — Part 3: Rule-based validation — Schematron (2006) (see
OGC: OGC 12-000, SensorML (2014)
Schadow, G and McDonald, C.: Unified Code for Units of Measure (UCUM) – Version 1.8 (2009)
OMG: Unified Modeling Language (UML). Version 2.3 (2010)
W3C: Extensible Markup Language (XML) – Version 1.0 (Fourth Edition) (2006)
W3C: XML Schema – Version 1.0 (Second Edition) (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.

4.1       coverage

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

[ISO 19123:2005, definition 4.17]

4.2 domain feature

Feature of a type defined within a particular application domain.

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

[ISO 19156, definition 4.4]

4.3 element <XML>

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

NOTE: 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.

[ISO 19136:2007]

4.4 feature

Abstraction of a real-world phenomena.

[ISO 19101:2002, definition 4.11]

4.5 GML application schema

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

[ISO 19136:2007]

4.6 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.

[ISO 19136:2007]

4.7 GML schema

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

[ISO 19136:2007]

4.8 measurement

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

[ISO/TS 19101-2:2008, definition 4.20]

4.9 observation

Act of observing a property.

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

[ISO 19156:2011 definition 4.10]

4.10 observation procedure

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

[ISO19156, definition 4.11]

4.11 observation result

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

[ISO 19156:2011]

4.12 property <General Feature Model>

Facet or attribute of an object referenced by a name.

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

4.13 sampled feature

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

[ISO 19156:2011]

4.14 sampling feature

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

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

[ISO 19156:2011, definition 4.16]

4.15 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: 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.

 [ISO 19136:2007]

4.16 sensor

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

Note: 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.

[OGC SWE Common 2.0, definition 4.5.]

5.    Conventions

5.1    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:

Requirements class


Target type

[artefact or technology type]


[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.

5.2    Requirement

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

Requirement /req/[classM]/[reqN]

[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.

5.3    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:

Conformance class /conf/{classM}


[identifier for another conformance class]




[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: 
    <rule context="gwml2w:GW_Well">
            = 1">A GW_Well needs at least one 
            origin Elevation</assert>

5.4    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].

5.5    External package abbreviations

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

GF                   ISO 19109:2005 General Feature Model

GFI                 ISO 19156:2011 General Feature Model Instances

TM                 ISO 19108:2002 Temporal Schema, Temporal Objects

MD                 ISO 19115 Metadata

CV                   ISO 19123:2005 Schema for Coverage Geometry and Functions

OM                 ISO 19156:2011 Observations and Measurements

DQ                  ISO 19157:201X Data Quality

WML2                        OGC® WaterML 2.0: Part 1- Timeseries

GW                 GroundwaterML 2.0

TS                   TimeseriesML

5.6    Abbreviated terms

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

API                                    Application Program Interface

GeoSciML 3.2                   GeoScience Mark-up Language version 3.2

GeoSciML 4.0                   GeoScience Mark-up Language version 4.0

GML                                 OGC Geography Mark-up Language

GWML1                            Groundater Markup Language version 1.0 (Natural Resources Canada)

GWML2                            Groundwater Markup Language version 2.0 (this standard)

GWML2-Main                  UML Logical Model of the primary GroundWaterML2 elements (namespace

GWML2-Flow                  UML Logical Model of the elements required to capture groundwater flow (namespace

GWML2-Constituent       UML Logical Model of the groundwater fluid body constituents and their relationships (namespace

GWML2-Well                   UML Logical Model of the features and properties associated with water well (namespace

GWML2-WellConstruction   UML Logical Model of the well drilling and construction details (namespace

GWML2-AquiferTest       UML Logical Model of the features and properties associated with aquifer test (namespace

INSPIRE                            Infrastructure for Spatial Information in the European Community (Directive 2007/2/EC)

ISO                                    International Organization for Standardization

IUGS                                 International Union of Geological Sciences

NACSN                             North American Commission on Stratigraphic Nomenclature

NADM                              North American geological Data Model

OGC                                  Open Geospatial Consortium

O&M                                 OGC Observations and Measurements Conceptual Model

OMXML                          Observations and Measurements XML Implementation

SensorML                          Sensor Model Language

SOS                                    Sensor Observation Service

SWE                                   Sensor Web Enablement

TSML                                TimeseriesML

UML                                 Unified Modeling Language

UTC                                  Coordinated Universal Time

URI                                    Universal Resource Identifier

URL                                   Universal Resource Locator

WML2                               WaterML 2.0 – Part 1

XML                                 Extensible Markup Language

XSD                                   W3C XML Schema Definition Language

5.7    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

5.8    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.

6.    Background

6.1    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 (OGC 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 (c) using a class from the schemas as one of the two participants in a binary relationship.

6.2    Overview of Observations & Measurements

ISO19156 – 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.”

6.2.1    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.


Observation in O&M (from ISO 19156).
Figure : Observation in O&M (from ISO 19156).

6.3    Overview of GeoSciML 4.0

GeoSciML 4.0 is a GML schema for core geological entities including geological units, structures, and earth materials. It is particularly relevant to GWML2 because bodies of rock serve as containers for subsurface water bodies. Such rock bodies possess variable hydrogeologic properties according to their material composition and topological organization. Thus, geological units and earth materials are the key GeoSciML 4.0 entities required by GWML2.

GeoSciML 4.0 defines a geological unit as “a body of material in the Earth whose complete and precise extent is inferred to exist (NADM GeologicUnit, Stratigraphic unit in sense of NACSN or International Stratigraphic Code), or a classifier used to characterize parts of the Earth (e.g. lithologic map unit like ‘granitic rock’ or ‘alluvial deposit’, surficial units like ‘till’ or ‘old alluvium’).

GeoSciML 4.0 defines an earth material as “naturally occurring substance in the Earth” and intuitively refers to various types of rocks such as sandstone, granite, and gneiss.

7.    Conceptual Model

The GWML2 conceptual model is designed to be technology-neutral, and focused on the semantics of the groundwater domain. It consists of five components, as well as related properties and other entities: hydrogeological units, fluid bodies, voids, fluid flow, and wells. Conceptually, these entities form a simple template for a subsurface water container: the fluid container (a unit or its materials), the fluid itself (fluid body), the spaces in the container occupied by the fluid (void), the flow of fluid within and between containers and their spaces (flow), and the natural and artificial artifacts used to withdraw, inject, or monitor fluid with respect to a container (wells, springs, monitoring sites).

Well construction details are excluded from the conceptual model, but are included in the logical model for two reasons: (1) thematic, inasmuch as well construction was considered on the periphery of groundwater science, but important to resource management as well as important to significant data exchange scenarios, and (2) practical, as it is sufficiently modeled in GWML1 and could thus be directly imported with few changes. This eliminates the need for its re-conceptualization in the GWML2 conceptual model, keeping it tightly focused.

7.1    Hydrogeological Units

These are distinct volumes of earth material that serve as containers for subsurface fluids. The boundaries of a unit are typically discriminated from those of another unit using properties related to the potential or actual ability to contain or move water. The properties can be geological or hydraulic, and typically include influences from the surrounding hydrological environment. More specifically, the conceptual model delineates two types of hydrogeological units, with slightly different orientations: aquifer-related units have boundaries delimited by the hydrogeological properties of the rock body, while groundwater basins have boundaries delimited by distinct flow regimes. Aquifer-related units are subdivided into aquifer systems, which are collections of aquifers, confining beds, and other aquifer systems. Confining beds are units that impede water flow to surrounding units, and supersede notions such as aquitards, aquicludes, and aquifuges, which are not included herein, as it is difficult to differentiate these in practice.

Several significant properties are typically attributed to hydrogeological units, such as porosity, permeability, and conductivity, but these and others are modeled more accurately here as occurring necessarily concurrent with (dependent on) voids or fluid bodies. For example, porosity, in its various forms, requires both the presence of a unit (container) and its voids, as it is typically defined as the proportion of void volume to total unit volume (i.e. volume of solid material plus voids). Likewise, properties such as hydraulic conductivity and yield require the presence of units and fluid bodies, as they are concerned with the rate of movement of a fluid through a unit. Note that permeability and hydraulic conductivity are differentiated here: permeability refers to intrinsic permeability, which measures the ability of a unit to host fluid flow, independent of fluid properties and based solely on the connectivity and size of voids, whereas hydraulic conductivity additionally considers fluid properties.

Likewise, management areas are also relational entities in the sense that they are typically necessarily linked with a unit (or system) and possibly a fluid body. Management areas are earth bodies identified for groundwater management purposes and their boundaries can be delineated by social factors, such as policy or regulation, in addition to physical factors related to hydrogeology or hydrology.  

7.2    Fluid Bodies

These are distinct bodies of fluid (liquid or gas) that fill the voids in hydrogeological units. Fluid bodies are made of biologic (e.g. organisms), chemical (e.g. solutes), or material constituents (e.g. sediment). While it is expected that the major constituent of a fluid body will be water, the conceptual model allows for other types of major constituents such as petroleum. Minor constituents are not necessarily fluids, but can be gases, liquids, or solids (including organisms), and are included in the fluid body in various forms of mixture, such as solution, suspension, emulsion, and precipitates. Fluid bodies can also have other fluid bodies as parts, such as plumes or gas bubbles. Surfaces can be identified on a fluid body, such as a water table, piezometric or potentiometric surface, and some such surfaces can contain divides, which are lines projected to the fluid surface denoting divergence in the direction of flow systems within the fluid.

7.3    Voids

Voids are the spaces inside a unit (e.g. aquifer) or its material (e.g. the sandstone material of an aquifer), and might contain fluid bodies. Voids are differentiated from porosity, in that porosity is a ratio of void volume to total volume of unit plus voids, while voids are the spaces themselves. It is important to conceptually differentiate voids from units and their containers, in order to represent, for example, the volume of fractures, caves, or pores in a particular unit or its portion.

7.4    Flow

Groundwater flow denotes the process by which a fluid enters or exits a container (unit) or its voids, or flows within them. Flow between one container or void and another is named InterFlow, and flow within a container or void is named IntraFlow. Recharge is the flow into a groundwater container or void, and discharge is flow out of a groundwater container or void. The reciprocal source or destination entity can be any appropriate container or void such as a river, lake, pipe, reservoir, canyon, flood plain, ground surface, etc. A flow system is then a collection of flows ordered in a sequence from recharge to discharge, such that the flow segments of the system make up a connected flow path from source to destination. A water budget is a measure of the balance of recharge and discharge valid for a specific time and relative to a specific groundwater feature, such as a basin, aquifer, management area, or well.

Many of these concepts are depicted in Figure 2. Shown is a flow system (A+B) and two subsystems (A, B) that are its parts. Each subsystem is composed of interior flows, indicated by the solid lines with arrows, as well as input and output flows indicated as recharge and discharge, respectively. These flow systems are contained by three distinct hydrogeologic unit bodies, with the middle body oriented at an angle and having a K (hydraulic conductivity) value of 10-5. Intraflow is exemplified by a flow line within the right hydrogeologic unit body, while Interflow is exemplified by the flow from right body (the source container) to middle body (the destination container). The boundary between the bodies serves as the interface through which the flow occurs. While not shown, the three hydrogeologic unit bodies contain a groundwater body (i.e. a fluid body) in their pores (i.e. voids), and it is this groundwater body that is flowing.

Example flow system with two subsystems (after Freeze & Cherry, 1979, p. 204).
Figure : Example flow system with two subsystems (after Freeze & Cherry, 1979, p. 204).

7.5    Wells

Well-related entities include water wells, springs, and monitoring sites. Water wells are man-made constructions for monitoring, withdrawing, or injecting water from/into a hydrogeological unit, while springs are features where water discharges to the surface naturally. Both wells and springs possess important links to the hydrogeological environment, including their host units and materials, as well as the intersecting fluid body. Monitoring sites are locations where devices are placed to measure various properties of significance to hydrogeology, such as water level, flow rate, water temperature, or chemical composition, or to take samples. As such, monitoring sites are roles played by other features, for example, water wells or springs.

7.6    Conceptual Model Specification

GWML2 CM - Hydrogeological Unit.
Figure : GWML2 CM - Hydrogeological Unit.

GWML2 CM - Groundwater Properties.
Figure : GWML2 CM - Groundwater Properties.

GWML2 CM - Fluid Body.
Figure : GWML2 CM - Fluid Body.

GWML2 CM - Groundwater Flow.
Figure : GWML2 CM - Groundwater Flow.