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

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

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

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.

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.

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.

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.

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.

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].

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

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

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].

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

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.

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

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.

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

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

Draining [ultimately] into interior catchments [9].

Draining [ultimately] into the ocean.

Abstraction of real-world phenomena [ISO 19101].

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.

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

Geometric property of a flowpath, usually a geometric curve.

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

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.

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.

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.

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.

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.

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]

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

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].

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

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.

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].

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.

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].

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.

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).

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

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

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.

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

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.

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

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].

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

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.

The normative provisions in this specification are denoted by the URI

http://www.opengis.net/spec//hy-features/1.0

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

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.

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.

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.

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.