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.

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 (http://www.opengis.net/def/gwml/2.2), 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.

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

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

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

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='http://www.opengis.net/req/well/origin_elevation']) = 1">A GW_Well needs at least one origin Elevation</assert>

</rule>

</pattern>

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:

http://www.opengis.net/spec/groundwaterml/2.2/req/[classM].

The URI for each requirement or recommendation has the form:

http://www.opengis.net/spec/groundwaterml/2.2/req/[classM]/[reqN].

The URI for each conformance class has the form:

http://www.opengis.net/spec/groundwaterml/2.2/conf/[classM].

The URI for each conformance test has the form:

http://www.opengis.net/spec/groundwaterml/2.2/conf/[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

6.6.  Abbreviated terms

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

Table 6

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 http://www.opengis.net/spec/groundwaterml/2.2. 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.

7.1.  Technical Basis

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

• OMXML (10-025r1)

• sweCommon (08-094r1)

• GML ISO 19136:2007 (07-036r1)

• ISO/TS 19139:2007 (Metadata)

• W3C XSD

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.

Figure 1 — Observation in O&M (from ISO 19156:2011)

7.3.  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.

8.1.  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.

8.2.  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.

8.3.  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.

8.4.  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.

Figure 2 — Example flow system with two subsystems (after Freeze & Cherry, 1978, p. 204)

8.5.  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.

8.6.  Conceptual Model Specification

Figure 3 — GWML2 CM — Hydrogeological Unit

Figure 4 — GWML2 CM — Groundwater Properties

Figure 5 — GWML2 CM — Fluid Body

Figure 6 — GWML2 CM — Groundwater Flow

Figure 7 — GWML2 CM — Wells

8.7.  Requirements

Target logical models that are compliant with the conceptual model shall implement components of the conceptual model respecting their semantics, i.e., their definition and intent. In other words, the logical model must be highly semantically similar to components of the conceptual model and must not specify any requirements that would contradict or result in non-conformance to the conceptual model. Semantic similarity can be tested in multiple ways, including but not limited to: (i) direct comparison of UML components, (ii) comparison after mapping components to a common expressive knowledge representation language, such as first order logic or common logic, or (iii) comparison after mapping components to a reference ontology. The target can reuse and adapt existing logical models.

9.1.  Logical Model Specification

Figure 8 — GWML2 LM — Package Dependencies (Internal).

Figure 9 — GWML2 LM — Package Dependencies (External — indirect dependencies not shown).

Figure 10 — GWML2 LM — Hydrogeological Unit.

Figure 11 — GWML2 LM — Groundwater Properties.

Figure 12 — GWML2 LM — Fluid Body.