Open Geospatial Consortium

Submission Date: 2017-08-17

Approval Date:   2017-10-16

Publication Date:   2018-01-08

External identifier of this OGC® document:

Internal reference number of this OGC® document:    14-111r6

Version: 1.0

Category: OGC® Implementation Standard

Editor:  David Blodgett, Irina Dornblut

OGC® WaterML 2: Part 3 - Surface Hydrology Features (HY_Features) - Conceptual Model

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Document type:    OGC® Standard

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Document stage:    Approved

Document language:  English

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Table of Contents

i. Abstract

The OGC Surface Hydrology Features (HY_Features) standard defines a common conceptual information model for identification of specific hydrologic features independent of their geometric representation and scale. The model describes types of surface hydrologic features by defining fundamental relationships among various components of the hydrosphere. This includes relationships such as hierarchies of catchments, segmentation of rivers and lakes, and the hydrologically determined topological connectivity of features such as catchments and waterbodies. The standard also defines normative requirements for HY_Features implementation schemas and mappings to meet in order to be conformant with the conceptual model.

The HY_Features model is based on an abstract catchment feature type that can have multiple alternate hydrology-specific realizations and geometric representations. It supports referencing information about a hydrologic feature across disparate information systems or products to help improve data integration within and among organizations. The model can be applied to cataloging of observations, model results, or other study information involving hydrologic features. The ability to represent the same catchment, river, or other hydrologic feature in several ways is critical for aggregation of cross-referenced or related features into integrated data sets and data products on global, regional, or basin scales.

ii. Keywords

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

ogcdoc, OGC document, hydrology, feature, identification, conceptual model, UML, implementation standard.

iii. Preface

This standard defines the HY_Features common hydrologic feature model for use in identification of features as specific kinds of hydrologic features. It is intended to be used to document and share information about features that are the subject of hydrologic studies and reporting for a range of applications. HY_Features was originally commissioned to link hydrologic information across the scientific and technical programs of the World Meteorological Organization (WMO), and to assist WMO Members in discovering, accessing, and utilizing hydrologic data from different sources.

This standard is also intended to support the need for governance and guidance by national and international authorities. Aspects of the standard that support this goal are 1) canonical form, 2) implementation neutrality, 3) conformity to internationally recognized geographic information standards, and use of semantics inferred from terminology endorsed by the WMO and the UN Educational, Scientific and Cultural Organization (UNESCO).

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):

  • U.S. Geological Survey (USGS), USA

  • Federal Institute of Hydrology (BfG), Germany

  • CSIRO Land and Water, Australia

  • Bureau of Meteorology, Australia

  • Metalinkage, Australia

  • French Geological Survey (BRGM), France

  • University of Texas at Austin, USA

  • Eurecat, Spain

  • INCLAM S. A., Spain

  • 52°North, Germany

v. Submitters

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



David Blodgett

U.S. Geological Survey

Irina Dornblut

Federal Institute of Hydrology (BfG)

Robert A. Atkinson


Joshua Liebermann

Harvard University

Darren Smith

Australian Bureau of Meteorology

Bruce Simons

CSIRO Land and Water

David Arctur

University of Texas at Austin

Mickael Beaufils

French Geological Survey (BRGM)

Sylvain Grellet

French Geological Survey (BRGM)

Aitor Corchero


vi. Relationship to the OGC® Abstract Specification

The HY_Features conceptual model, described in this standard, references concepts defined in the OGC Abstract Specification (OGC AS). Conformance to the OGC AS may be achieved via specialization of the relevant concept. An example of this kind of conformance is a geographic feature specialization that carries specific hydrologic feature attributes and associations. Conformance may also be achieved by asserting a correspondence or analogy between HY_Features concepts and OGC AS concepts. For example, the river reference system for referencing positions along a nominal flowpath corresponds to linear referencing along any linear feature element. This standard requires no changes to the current version of the OpenGIS Abstract Specification.

1. Scope

This OGC Implementation Standard defines a common conceptual feature model for use in identification of features as typical features of the hydrology domain using established models and patterns in use in the Hydrology domain and endorsed by WMO and UNESCO such as those documented in the "International Glossary of Hydrology".

This standard, along with expected future implementations of this conceptual model, will form the third part of an intended 'OGC WaterML 2' suite of standards that groups water-related OGC standards. Following Part 1: Timeseries, Part 2: Ratings, Gaugings and Sections, this standard is titled Part 3: Surface Hydrology Features Conceptual Model.

This document introduces the HY_Features conceptual model only. The normative model is a machine-readable UML artifact published by the OGC in conjunction with this document at: Clause 7 of this document presents requirements classes (ways of conforming to the conceptual model) and requirements for model implementations to meet in order to be considered conformant with the conceptual model. Conformance classes indicating how to demonstrate conformance to requirements are presented in Annex A.

Future standards documents in this series are expected to specify particular implementations of the conceptual model. For example, an OWL/RDF implementation will define machine-processable entities and relationships in the OWL schema language that implement the HY_Features conceptual model elements. HY_Features implementations are expected to support documentation and discovery of data as well as aid data transformation efforts. For example, a discovery service such as a catchment catalog might use HY_Features concepts to index services that provide differing catchment representations as well as related water quality and quantity observations. A catalog client could then use the index for automated analysis and data product generation.

The initial scope has been defined by the WMO Commission for Hydrology (WMO-CHy): to facilitate data sharing within the hydrologic community of the WMO Member countries and to improve the quality of data products based on these data by defining hydrologic features to convey their identification through the data processing chain "from measurement to hydrological information" [8].

To enable semantic interoperability of hydrologic data and services, it is necessary to agree on common concepts and methods. The HY_Features model was developed in order to formalize concepts and relationships of hydrologic and hydrographic features using the WMO/UNESCO "International Glossary of Hydrology" as a starting point. One of HY_Features main objectives is to provide a standard terminology for description of relationships between hydrologic features that can be used to implement data transfer formats. However, this conceptual model should be seen as a building block toward a hydrographic data interchange standard, but such a standard would require additional efforts.

HY_Features is meant to support linking data products across differing applications and jurisdictions. To enable this, the concept of a holistic catchment is defined and "hydrology-specific realizations" of the catchment concept are modeled as typical hydrologic features. This allows a particular catchment to be represented in different feature data products for different purposes while still retaining its identity.

The HY_Features model provides a basis for common and stable references that identify hydrologic features in a wide variety of applications:

  • to link hydrologic observations to their feature-of-interest, e.g. link a streamflow observation to the river or catchment being observed,

  • to allow aggregation of cross-referenced features into integrated data sets and data products on global, regional, or basin scales,

  • to enable systems to unambiguously link data between systems and domains,

  • to enable cross-domain or multi-discipline services to communicate through reference to standard concepts.

While HY_Features is intended to support a wide variety of applications, it is narrowly focused on surface water hydrographic features. As such, nuances of waterbodies and storages such as glaciers, snowfields, and wetlands are left to future work. Additionally, temporal aspects of hydrologic features are not addressed in this standard and would be expected to be handled by implementations using HY_Features classes for feature identification. Finally, as this standard precedes expected encoding standards, implementation details, such as the structure and handling of feature identifiers are out of scope.

This standard defines a conceptual model (normative) containing feature type definitions that conform to the OGC General Feature Model (GFM) (ISO 19101:2002 and ISO 19109:2006), expressed in the Geographic Information Conceptual Schema Language (ISO 19103:2005) using the Unified Modeling Language (UML). The GFM is a meta-model developed as a general framework for features and their properties in the context of geographic information. Feature types specific to an application domain are defined as instances of the general feature metaclass with their own sets of typical characteristics (property types and constraints) such as attributes, associations, or operations.

As shown in Figure 1, the HY_Features conceptual model defines instances of the general feature metaclass specific to the hydrology domain. A general HY_HydroFeature type is defined to carry properties such as identifier and name that all hydrologic features should have. Specific feature types are then derived from HY_HydroFeature to reflect different aspects of hydrology and carry properties specific to each aspect. Figure 1 also illustrates that HY_Features features can fill the role of features-of-interest or sampling features for hydrologic observation data conforming to the Observation and Measurement (O&M) model (ISO 19156:2011) or to hydro-specific O&M profiles such as in the OGC WaterML 2' suite of water-related standards.

Figure 1. HY_Features in the context of the OGC Abstract Specifications

2. Conformance

This standard defines a conceptual model for hydrologic features and their fundamental relationships. Requirements for conformance with the conceptual model are defined for two types of implementation targets: a) implementation schemas for representation of hydrologic data or b) implementation mappings between terms or concepts from two or more other existing hydro data schemas based on common intermediary HY_Features concepts. Any conformant implementations are required to satisfy the requirements defined in Clause 7 of this document for the requirements class(es) with which they assert conformance. This is true whether the implementation is explicit in the case of a schema (such as an OWL/RDF ontology or XML schema) or implicit in the case of common concepts for a schema mapping (such as an XSLT rule). The accuracy or detail with which an external term is mapped to a HY_Features common concept is beyond the scope of this standard, as is specification of a particular concept mapping methodology.

Annexes C-G of this document provide, nevertheless, informative mappings to and from HY_Features for the following hydro data models:

  • Australian Hydrological Geospatial Fabric (AHGF) [11],

  • USGS National Hydrography Dataset Plus (NHD Plus) [10],

  • INSPIRE Hydrography theme [2],

  • French National Service for Water Data and Reference-dataset Management (SANDRE) [12],

  • Canadian National Hydrography Network (NHN) [13]

These mappings are intended to provide a general understanding of the correspondence between HY_Features concepts and existing conceptual / physical models. They are informative with respect to any future mapping implementations.

Conformance of implementation targets with this standard shall be verified 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 met to claim conformance are specified in the OGC Compliance Testing Policies and Procedures and the OGC Compliance Testing web site.

All requirements-classes and conformance-classes described in this document are owned by the HY_Features standard.

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 Abstract Specification Topic 1, Feature Geometry
(aka ISO 19107:2003, Geographic Information — Spatial schema), 2003

OGC: OGC Abstract Specification Topic 2, Spatial referencing by coordinates
(aka ISO 19111:2007, Geographic Information — Referencing by coordinates), 2007

OGC: OGC Abstract Specification Topic 5, Features
(aka ISO 19101:2002, Geographic Information — Reference Model), 2002

OGC: OGC Abstract Specification Topic 11, Metadata (aka ISO 19115-1:2014, Geographic Information — Metadata — Fundamentals), 2014

OGC: OGC Abstract Specification Topic 19, Geographic information - Linear referencing (aka ISO 19148:2012, Geographic Information — Linear referencing), 2012

OGC: OGC Abstract Specification Topic 20, Observations and Measurements (aka ISO 19156:2011, Geographic Information — Observations and Measurements), 2011

ISO: ISO/TS 19103:2005, Geographic Information — Conceptual schema language, 2005

ISO: ISO 19108:2006, Geographic Information — Temporal schema, 2006

ISO: ISO 19109:2006, Geographic Information — Rules for application schemas, 2006

ISO: ISO 19133:2005, Geographic Information — Location-based services — Tracking and navigation, 2005

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. application schema

Conceptual schema for data required by one or more applications [ISO 19101]

4.2. boundary (line)

Geometric representation of a (catchment) boundary, usually a geometric composite curve.

This definition references the definition of a divide described as summit or boundary line [9].

4.3. catchment

A physiographic unit where hydrologic processes take place. This class denotes a physiographic unit, which is defined by a hydrologically determined outlet to which all waters flow. While a catchment exists, it may or may not be clearly identified for repeated study.


This approach considers the catchment concept to be the basic unit of study in hydrology, water resources management, and environmental reporting. The approach is meant to be holistic, referring to the continuous interaction of surface and subsurface waters within a catchment, even if a particular representation of the catchment refers to only surface or subsurface aspects of the catchment. Special subtypes such as drainage basin and groundwater basin may be defined with a particular application.

The synonymous use of the terms catchment and catchment area in the WMO/UNESCO International Glossary of Hydrology (which is the key reference for the definitions in the HY_Features model) does not clearly distinguish between the catchment concept and its geometric representations such as catchment area, nor between catchment and its possible specializations like drainage basin and groundwater basin.

To satisfy the need for a both a holistic definition of catchment and various refinements of it, the catchment feature type is refined by defining feature types that reflect distinct perspectives on the holistic catchment concept. In this standard, this type of refinement is referred to as hydrology-specific catchment realization. Also see clause 5.4 of this standard on the use of WMO terminology.

4.4. catchment area

Two-dimensional (areal) hydrology-specific realization of the holistic catchment. Topologically, the catchment area can be understood as a face bounded by catchment divide and flowpath edges. The concept of a face bounded by edges is described in detail in the ISO topology model [ISO19107]. The catchment area is usually represented as a geometric surface, and the measure of the catchment area may be denoted as surface area.

4.5. catchment divide

One-dimensional (linear) feature that is a hydrology-specific realization of the holistic catchment. Topologically, catchment divide can be understood as an edge bounded by inflow and outflow nodes. The concept of an edge bounded by nodes is described in detail in the ISO topology model [ISO19107]. The catchment divide is usually represented as a geometric curve, or as a polygon ring feature.

4.6. catchment network (topology)

Edge-node topology pattern of a set of catchments connected by their hydro nexuses. The topology pattern is derived from flowpaths, but ultimately reflects the inferred hydrologic connectivity between catchments and their hydro nexuses whether or not corroborated by geometric representations.

4.7. (internal) catchment topology

Topology pattern between hydrology-specific realization of a particular catchment. Example: Catchment area associating its catchment divide and the draining flowpath. This could include subsurface and atmospheric volumes, surficial catchment area, network of drainage channels and/or waterbodies, a linear flowpath, and an outlet (and possible inlet) node.

4.8. channel

Natural or artificial waterway, clearly distinguished, which periodically or continuously contains moving water, or which forms a connecting link between two bodies of water [9].

4.9. channel network

Connected set of depressions and channels that continuously or periodically contain water.

4.10. cross section (of a stream)

Section of a stream at right angles to the main (average) direction of flow [9].

4.11. cross section (of a stream bed)

transversal section of a stream bed in a vertical plane. This definition references the definition of a bed profile describing the shape of a stream bed in a vertical plane [9].

4.12. data

Documented value of some characteristics of a real-world phenomenon.

4.13. data set (also dataset)

Data compiled and arranged into a set. Used in its concatenated form to indicate a specific dataset, or as data set, for non-specific sets of data.

4.14. data product

Data set compiled for a specific purpose, e.g. for global dissemination using Web services.

4.15. dendritic catchment

Catchment in which all waters flow to a single common outlet. A dendritic catchment is permanently connected to others in a dendritic (tree) network, the most common drainage pattern of streams ultimately flowing into the ocean after joining together at confluences into larger and larger streams.

4.16. depression

Landform lower than the surrounding land and partially or completely closed that is able to, but does not necessarily, contain water.

4.17. domain feature

Feature of a type defined within a particular application domain. [ISO 19156].

4.18. endorheic (drainage)

Draining [ultimately] into interior catchments [9].

4.19. exorheic (drainage)

Draining [ultimately] into the ocean.

4.20. feature

Abstraction of real-world phenomena [ISO 19101].

4.21. flowpath (also flow path)

One-dimensional (linear) feature that is a hydrology-specific realization of the holistic catchment. Topologically, flowpath can be understood to be an edge bounded by inflow and outflow nodes, and associated with left-bank and right-bank sub-catchment faces. The concept of an edge bounded by nodes is described in detail in the ISO topology model [ISO19107]. The flowpath is usually represented as a geometric curve.


A flowpath feature may form the "main stem" of the stream network flowing to the catchment outflow node from its inlet node(s).

With respect to the river referencing system described in this standard, the flowpath corresponds to the linear element 'that serves as the axis along which linear referencing is performed' as described in the OGC Abstract Specification Topic 19, Linear referencing [aka ISO 19148].

NOTE 3: Hydrologically, the flowpath references the path line __ described by a moving particle of water [9].

4.22. flow line (also flowline)

Geometric property of a flowpath, usually a geometric curve.

This definition references the definition of a path-line [9] through using the synonym flow line to express the geometric property of a flowpath.

4.23. hydrographic network

Aggregate of rivers and other permanent or temporary watercourses, and also lakes and reservoirs [WMO, 2016].

Not to be confused with the network of hydrological stations and observing posts.

4.24. hydrologic complex

Collection of distinct hydrologic features forming a hydrologically connected system where the union of one or more catchments and a common hydro nexus is realized by multiple complexes of hydrology-specific topological elements. For example, a single catchment may be hydrologically realized as a face-edge complex of subcatchment areas and divides, or an edge-node network of flowpaths and hydro nexus nodes, and also as a dendritic edge-node network of either waterbodies or containing channels.

4.25. hydrologic feature

Feature of a type defined in the hydrology domain, whose identity can be maintained and tracked through a processing chain from measurement to distribution of hydrologic information.

4.26. hydro(-logic) location

Any location of hydrologic significance located "on" a hydrologic network that is a hydrology-specific realization of a hydrologic nexus. In a given dataset, hydro locations may or may not have an associated hydrologic nexus and associated catchment features. In such cases, hydro locations would typically be linearly-referenced to a defined set of catchments' flowpaths. Topologically, a hydro-location can be understood as an inlet or outlet node located at the end of a flowpath edge. The hydrologic location is usually represented as a geometric point.

With respect to the river referencing system described in this standard, a hydro-location feature can correspond to either the referent that specifies a 'known, already referenced location on the linear element' as described in the OGC Abstract Specification Topic 19, Linear referencing [aka ISO 19148] or the reference location whose distance from a referent can be measured along a linear element.

4.27. hydro(-logic) nexus

Conceptual outlet for water contained by a catchment. The hydro nexus concept represents the place where a catchment interacts with another catchment. Every catchment flows to a hydro nexus, conversely every location in a hydrologic system can be thought of as a hydro nexus that drains some catchment. Similar to catchments, hydro nexuses can be realized in several hydrology-specific ways.

If a given hydro nexus does not have a known hydrology-specific realization or is undetermined, it is termed 'nillable' in this standard. For example, a hydro nexus exists in the form of flow to the subsurface or atmosphere but may be undetermined and unrepresented within implementations focused on surface water hydrology and would not be included or referenced.

4.28. hydrologic realization

A hydrologic feature type that reflects a distinct hydrology-specific perspective of the catchment or hydro nexus feature types. Shares identity and catchment-nexus relationships with the catchment or nexus it realizes but has hydrologically determined topological properties that express unique ways of perceiving catchments and hydrologic nexuses. Distinct from representation in that it is a refinement of the holistic catchment, allowing for multiple geometric representations of each hydrologic realization.

4.29. hydrology

Science that deals with the waters above and below the land surfaces of the Earth, their occurrence, circulation and distribution, both in time and space, their biological, chemical and physical properties, their reaction with their environment, including their relation to living beings. [9]

4.30. hydrometric feature

Feature of a type which denotes a physical structure intended to observe properties of a hydrologic feature.

This definition references the definition of a hydrometric station at which data on water in rivers, lakes or reservoirs are obtained on physical and chemical properties of water [9]. A hydrometric feature may be a composite station configured by arranging several monitoring components. Used to sample a hydrologic feature, a hydrometric feature may be considered a sampling feature as defined in the ISO Observation model. A sampling feature is described in general in ISO 19156: Observation and Measurement, the special monitoring point of hydrologic observation is described in the OGC WaterML 2.0: Part 1- Timeseries standard [4].

4.31. hydrometric network

Aggregate of hydrologically connected monitoring stations situated on and used for hydrologic observation of a feature such as a catchment or hydrographic network. This definition references the definition of a synonymous hydrological network consisting of hydrological stations and observing posts situated within a catchment in such a way as to provide the means of studying its hydrological regime [9].

4.32. hydrometry

Science of the measurement and analysis of water including methods, techniques and instrumentation used in hydrology [9].

4.33. indirect position

Position expressing the location of a feature relative to the known location of another feature. Indirect position requires a logical axis, bounded by the feature being placed and the feature being used as a reference, along which the position can be determined. Common examples in hydrology are mileposts along a river referencing the river source and/or mouth, and the placement of monitoring stations referencing already located stations.

4.34. interior catchment

Catchment in which all waters are collected and drainage is endorheic; an interior catchment does not drain to other catchments. This definition is rooted in the blind drainage pattern of water collecting in sinks or lakes not connected to streams [9].

4.35. longitudinal section (of a stream)

Vertical section of a stream in longitudinal direction. This definition is rooted in the definition of a longitudinal section along a channel at its center line [9], but generalized for all types of vertical section along a line.

4.36. longitudinal section (of a stream bed)

Longitudinal section of a stream bed in a vertical plane. This definition references the definition of a bed profile describing the shape of a stream bed in a vertical plane [9].

4.37. main stem (also mainstem)

Main course along which water flows in a catchment excluding tributaries. For any identified catchment, the flowpath connecting inflow and outflow locations would typically correspond to or follow the main stem, but the main stem is conceptually broader than the catchment flowpath concept of HY_Features.

4.38. mapping

Establishing a semantic relationship between concepts in different information models using a formalism that specifies how elements from a source information model may be transformed into elements of a target model. Every pair of N models generally require a separate (2-way) mapping for each source concept (a total of N!/[2(N-2)!] mappings). Mappings that in contrast involve transformation by way of a common concept can be more efficiently expanded to more than two models as each additional model only requires mapping once into the set of common concepts (N mappings).

4.39. named feature

Feature identified by a name. Hydrologic features may have multiple names depending on the cultural, political or historical context.

4.40. nillable

Nillable is meant in this standard to signify that a feature property logically exists but may not be determined in a given implementation.

4.41. referent

Feature with a known location being used as a reference to locate another feature on the logical axis that stretches between the two. The (indirect) position of a new location is expressed as the distance along that linear element from the known referent to the feature being placed.

4.42. representation

Any process-able data, data set, or data product which characterizes a given feature concept.

4.43. river referencing

Referencing along a river applied to place a feature on a (linear) waterbody feature. The feature location is specified as an indirect position expressed as distance along the watercourse on which the feature is to be placed.

A (hydrologic) feature of interest which is located along the locating one-dimensional flowpath between inflow and outflow nodes, corresponds to the linear referencing described in the OGC Abstract Specification Topic 19, Linear referencing 'specifying a location relative to a linear element along that element' [ISO 19148].

4.44. storage (of water)

Impounding of water in surface or underground reservoirs, for future use. [9]

NOTE: Storage refers to a body of water from the perspective of a usable water resource. The management of a reservoir itself, as a human action with the objective of efficient and sustainable use of the resource, is not in the scope of the conceptual model.

4.45. stream

Water, generally flowing in a natural surface channel, or in an open or closed conduit, a jet of water issuing from an orifice, or a body of flowing groundwater [9].

4.46. waterbody (also water body)

Mass of water distinct from other masses of water [9].

4.47. watercourse

Natural or man-made channel through or along which water may flow [9], including large interstices in the ground, such as cave, cavern or a group of these in karst terrain.

5. Conventions

This section provides details and examples for any conventions used in the document. Examples of conventions are symbols, abbreviations, use of XML schema, or special notes regarding how to read the document.

5.1. Identifiers

The normative provisions in this specification are denoted by the URI

All requirements and conformance tests that appear in this document are denoted by partial URIs which are relative to this base.

5.2. Symbols (and abbreviated terms)


WMO Commission for Hydrology


Geography Markup Language


Global Runoff Data Centre (GRDC)


GroundwaterML 2


OGC Hydrology Domain Working Group


International Organization for Standardization


Open Geospatial Consortium


Web Ontology Language


Unified Modeling Language

WaterML 2

working title for the indented suite of water-related OGC standards


WMO Information System


WMO Integrated Global Observing System


World Meteorological Organization


eXtensible Markup Language

5.3. UML notation

Most diagrams that appear in this specification are presented using the Unified Modeling Language (UML) static structure diagram, as described in Sub-clause 5.2 of the OGC Web Services Common Implementation Specification (OGC 04-016r2). UML classes are named in UpperCamelCase and property names in lowerCamelCase.

5.4. WMO Terminology

The HY_Features model uses—​as far as possible—​terminology recommended by the WMO Commission for Hydrology for use in the WMO Member countries. The key reference is the "International Glossary of Hydrology" [9] a joint publication of the WMO and the UNESCO (hereinafter referred to as WMO/UNESCO glossary of hydrology). Wherever appropriate, terms from this glossary are applied to the feature concepts in this standard to capture meaning and contextual relationships. The synonym approach widely used in this glossary is interpreted as signifying that synonymous terms are not necessarily synonymous in every respect. Differences in terminology were explored through reconciling the explicit definitions documented in the glossary with usage reflected in various data sets and products. The accepted terms were augmented with the relationships inferred from other terminology in order to define complex terms that do not clearly distinguish between a specific logical concept and its geometric representation or between a general term and its specific conceptual meaning. The definitions used in the conceptual model described in this standard may be understood as a conceptual refining and/or narrowing of the definitions given in the WMO/UNESCO Glossary of Hydrology.

Some words have been avoided because of their disparate uses in various implementation schemes. The words "reach" and "mainstem" are examples that are used to refer to such a variety of collections of features, that they have been avoided. Similarly, the term "watershed" is used and defined differently depending on its context and has been avoided. Unfortunately, it is inevitable that the terminology in HY_Features will conflict with one or more schemes or implementations of hydrologic features. In these cases, great care will need to be taken and the specific feature type names from HY_Features should be used when necessary.

Though rooted in the definitions given in the WMO/UNESCO Glossary of Hydrology, many of the hydrologic features in this standard have been defined more specifically for relevance to the surface water domain. Some requirements defined in this standard refer to the Scoped Name concept of ISO 19103: Conceptual Schema and any such Scoped Name implementations should reflect if possible a name endorsed by the WMO.

5.5. Naming convention

The HY-prefix used in the UML model follows the ISO naming conventions for UML elements. There is no explicit requirement to use this same name in a corresponding implemented term but conformance will require that each such term be clearly connected back to its corresponding conceptual model term.

6. Clauses not Containing Normative Material

6.1. The abstract idea of the hydrology phenomenon

Processes that continuously deplete and replenish water resources cause or result in a wide range of phenomena that are the subject of monitoring, modeling and reporting in hydrology and related sciences. This standard conceptualizes these distinctly named or otherwise uniquely identified real-world hydrologic phenomena as hydrologic features.