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.

Water is moving from the atmosphere to the Earth and back to the atmosphere due to the processes forming the Water Cycle (Figure 2). Water from precipitation reaching the land surface is accumulated in waterbodies occupying empty space on the land surface or in water bearing formations of soil and rock. Excess water overflows these formations and is driven downhill by gravity. Water flowing over soil or rock causes erosion to occur. This erosion tends to concentrate flowing water into waterbodies that flow downhill using a connecting system of channels intersecting other waterbodies along their way to a common outlet, conceptualized as a single (potentially complex or ambiguous) hydrologic location which may be described as an outlet, confluence, or nexus.

Looking back upstream from the outlet location, the corresponding catchment feature draining to it can be thought to have several distinct hydrology-specific forms: a flowpath feature, a catchment area feature, a catchment divide feature, a network feature of waterbodies, a network feature of channels (or waterbody containers), or a network feature of hydrometric (or observation) stations. All these are examples of what are referred to in HY_Features as realizations of the holistic catchment concept.

The most general conceptualization of hydrology phenomena is the catchment. A catchment is a recognized unit of study where hydrologic processes form physiographic features that are described in various data products. It serves as the commonly recognized unit of water resources assessment and management that can cross administrative jurisdictions. In multi-stakeholder collaboration and cross-domain research projects, catchments may be recognized as shared monitoring and reporting units. Most typically, river monitoring stations are assigned to a recognized catchment. Catchment inflow and outflows connect information between recognized catchments. Regardless of the way catchments are realized and represented, their basic identification can serve as the critical link between disciplines and jurisdictions. Examples of defined organizational catchments include the "Hydrologic Unit Code" (HUC) catchments defined by the U.S. Geological Survey for research and regulatory data systems to use as a reference [7] and "River Basin Districts" of the European Water Framework Directive [1]. In contrast to these, monitoring and research stations and hydrologic locations like bridges and confluences often imply or hydrologically determine identified catchments.

In this specification, a catchment feature provides a means to have an identified grouping of hydrologic study activities and data artifacts. A catchment may be realized by catchment-realization features whose connectivity may be described using one or more topological properties and geographic characteristics represented by geometric properties. The term realization is used here to imply that a holistic feature type like catchment can have more specific hydrologic feature types that share identity with but express a different aspect of it. The term realization is not meant to be interpreted as a Unified Modeling Language realization relationship.

The three terms, identification, realization, and representation, are very important to understanding the HY_Features conceptual model. Identification of a hydrologic feature implies that a feature has a hydrologically significant identity and would normally have a name in common usage. Realization implies a particular hydrologic feature type which has specific hydrologically determined topological properties and relationships. Representation implies that the particular realization of a feature has been defined by a particular geometric shape. For example, the Amazon River is identified in that we have a name for it and a vague notion of the Amazon River catchment. We can realize the Amazon River catchment, thinking of it in a number of ways: as a boundary ridge line, a surface covering the drained area, a linear flowpath from source to mouth, etc. Each of those hydrologic realizations may have different representations: the boundary may have surface and groundwater contributing variants, the area might have elevation and land cover variants, the flowpath might have a one dimensional simple and three-dimensional complex variant. Using HY_Features, each of these representations could be identified as being a particular representation of a particular hydrologic realization of the Amazon River catchment.

Depending on application and (spatial) scale, the same catchment may be realized in a number of different ways. The following are examples of the multiple realization pattern:

Figure 3 illustrates some possible alternative catchment realizations.

The core concept of the HY_Features model is that studies of hydrology phenomena have a need to reference common conceptual entities of the hydrosphere—​such as catchment, waterbody, channel, or monitoring position—​through the use of feature types defined in a specific model (as per ISO 19109 General Feature Model). Depending on the field of study or application, complex hydrology phenomena may be hydrologically realized as different feature types. Recognizing the catchment as the holistic unit of study allows multiple realizations and the information they carry to be connected to the same identified conceptual entity. Figure 4 is a graphic meant to represent the basic catchment concept visually without highlighting any particular realizations.

Fundamental to the catchment concept is that each catchment is defined by and paired hydrologically with a hydro nexus which forms the outlet to which all catchment waters drain and potentially the inlet that corresponds with the outlet(s) of upstream catchment(s). A hydro nexus is always defined as the feature that provides connectivity between catchments and can have a hydrology-specific realization that is a hydrologic location. Hydrologic locations stand alone as features such as confluences, stream gages, pour points, or bridges and can be the real-world feature that we think of as the location of a nexus between two or more catchments. It’s important to highlight that hydrologic locations do not have to realize a hydro nexus. In many cases, they will be network-linked features without associated catchments, while in other cases, they are realizations of hydro nexus features.

Hydro nexuses, like catchments, are features that may have one or more hydrology-specific realizations that are single locations, complex locations, or even diffuse or indistinct geographies. In the absence of such realizations, systems of catchments and hydro nexuses can form a topological complex based on their hydrologic connectivity even if their topological elements are not or cannot be represented geometrically. Building on concepts of hydrologically determined catchment topology and multiple catchment realizations, a collection of hydrologic features that realize a larger catchment can be said to form a hydro complex that is comprised of the collection of catchment and hydro nexus features and any other feature(s) that realize the catchment and/or the hydro nexus. A hydro complex may include multiple realized topologies such as flowpath, catchment area, stream or channel networks.

Catchment area, catchment divide, and linear flowpath are the most common topological realizations of the catchment concept, and are widely used to create cartographic representations of a catchment. Map layers that provide visualizations of the hydrographic network of waterbodies (typically as blue lines for small rivers and blue polygons for larger rivers and lakes) or of channels (typically lines displaying a drainage pattern indicating the path that flow may follow), or a network of hydrometric monitoring stations, are usually combined to represent the catchment as a whole. Figure 5 adds graphical representations of such realizations to the idealized catchment of Figure 4.

An unlimited number of overlapping catchments can potentially be defined in a given region since every point in that region corresponds to some catchment that drains to it. However, catchments are normally connected in drainage networks and built around significant features of the terrain such as confluences. Catchment networks, that might form a continuous coverage of the landscape, provide continuity between catchments, the ability to aggregate catchments, and to trace flow up- or down-stream.

In a network of catchments, morphological detail may be specified in many ways. Inflows and outflows are often complex where water flows out of one catchment and into another. As shown in Figure 6, catchments may connect through simple confluences (Figure 6a), waterbodies or wetlands (Figure 6c), intermittent or subsurface flows (Figure 6d), complex braided streams (Figure 6e), or distributary systems like deltas (Figure 6f). In some situations, diffuse (multiple) inflows can be conceptually joined in a 'conjoint' catchment (Figure 6b) and spread (multiple) outflows may be joined in a catchment flowing out at a single, conceptual outflow (Figure 6e).

Although these cases require different geographic representations, they can be represented using the same pattern of the catchment and hydro nexus. Since all these cases can be specified referencing a simple internal edge-node catchment topology, no special treatment is required to handle the variation of flow processes. While Figure 6 illustrates a simple dendritic junction, and a method to handle complexity through encapsulation, it’s important to note that HY_Features can support non-dendritic network topology where a given hydro nexus may be distributary and contribute flow (be an inflow hydro nexus) to multiple catchments.

Any catchment may also be nested in a larger containing catchment or split into multiple sub units to define catchment hierarchies. Two types of catchment hierarchical relations are supported in HY_Features: basic nesting and dendritic aggregation. Basic nesting allows any catchment to reference a containing catchment (Figure 7) without defining any particular interconnections between the two. Dendritic aggregations (Figure 8) consist of specialized dendritic catchments that contribute exorheic flow to an outlet and support simple topological relationships that allow determination of flow from upstream to downstream catchments. Specialized interior catchments add support for endorheic flow or interior drainage within dendritic aggregations.

As discussed above, catchments may have a number of hydrology-specific realizations. A network of catchments that interact at hydro nexuses can be realized as a network of catchment realizations. For example, a network of catchments, each realized as a flowpath, can be realized as the network of linear flowpath edges connected by hydro location nodes. Figure 9, Figure 10, and Figure 11 illustrate how a single catchment C1 is realized as a catchment area (light grey catchment area A) and also as a flowpath (red line F). Each catchment is potentially connected to other catchments at its outflow nexus n1 (Figure 10) and/or inflow nexus n2 (Figure 11). The flowpath geometry may trace the main flowpath through the catchment or it could be a purely schematic straight-line representation of a topological edge. Figure 9, Figure 10, and Figure 11, include UML diagrams that describe the hydrologic features highlighted in the figures.

In a network of dendritic catchments, one or more catchments may contribute outflow to a given hydro nexus, but only one catchment can receive inflow from that nexus. This hydrology-specific topological relationship is maintained regardless of the geometric representation of the hydro location which realizes the nexus. The association role names inflow and outflow are used to unambiguously describe the flow direction at a hydro nexus with respect to a dendritic catchment.

Catchment networks that appear non-dendritic, such as broad river bottoms with complex braided channels and two or more primary inflows, can be modeled with HY_Features. While a catchment contributes flow to a single outflow hydro nexus, there is no restriction on the number of catchments contributing to a hydro nexus. Figure 12a illustrates a case where hydro nexus nodes n2 and n3, both have two contributing catchments and have one and two receiving catchments respectively. Such situations are quite common in hydrologic systems. Examples include prairie pothole or ponded wetland landscapes, networked urban drainage systems, waterbodies with multiple outflows, and non-surface-contributing regions with ambiguous or complex outflows. While HY_Features does not attempt to model all these types of features, it does support non-dendritic catchments, if an application supports them, and it provides mechanisms to encapsulate such real-world complexity.

By introducing conjoint catchments that encapsulate non-dendritic parts, such complex situations can also be modeled as dendritic networks of catchments. Figure 12 shows a non-dendritic stream network, where it is not possible to determine to what extent flow from catchment F contributes to catchments E or C (Figure 12a) without additional information. Joining the catchments E, B, and C (Figure 12b) and collapsing their inflow (/outflow) nexuses n2 and n3 into a single virtual inflow hydro nexus, accumulates all the flow from catchments D and F in the resulting catchment X. Some implementations could go even further, eliminating node n2 and lumping catchment D into X as well. While not required, this encapsulation approach can be used to model a complex catchment topology as a simple dendritic network.

The topological pattern of a catchment network is shared with all of its hydrology-specific realizations, although the topological "level" (solid, face, edge, node) of a realization may vary. A given scheme topologically realizes catchments as a solid bounded by inflow/outflow faces, a face bounded by inflow/outflow edges, or an edge bounded by inflow/outflow nodes. A single catchment may also be realized as a topological complex consisting of all of the flowpath edges, and hydro location nodes forming the surface drainage of that catchment. The topological role that each hydrology-specific realization feature plays may or may not correspond directly to its geometric representation. For example, a waterbody plays the role of a flowpath edge between its inlet and outlet, but may be represented geometrically as a polygon ring.

Maps displaying a representation of a catchment are very common in hydrology research and engineering. Depending on the scientific concern and application, as well as the spatial scale of interest, different aspects and details of hydrology phenomena may be represented using application-specific map symbols. HY_Features accommodates this diversity with multiple alternate hydrology-specific realizations of the catchment and hydro nexus concepts which in turn may be represented alternately as geometric points, lines, polygons, or surfaces, or as aggregates of these geometric types. This standard is based on a hydrologically determined topology model of directed hydro nexuses acting as inflow or outflow locations for catchments which in turn connect them. This topological catchment network pattern can be realized in context-specific networks of features which each realizes a single catchment or hydro nexus, especially in hydrographic networks of waterbodies or networks of surface depressions and channels that may contain waterbodies. These network features may in turn have a variety of scale- and application-specific geometric representations. For example, a fixed landmark point on a waterbody, or a cross-section line separating a watercourse can each represent a hydro location node that realizes a nexus within the hydrology-specific topological hydro complex.

It is common practice in hydrology to reference hydrologic locations (typically observation stations, but also designated reaches, or flood plain zones) to an existing dataset, expressing the locations as a distance along a particular linear flowpath waterbody. Given that the flowpath has an established hydro nexus realization, whether located as a network feature or inferred through a dataset convention such as digitization direction, any hydrologic location can be referenced to the network through association with an existing outflow (or inflow) location and distance along the flowpath. Such a network location is often determined on the fly through topological and/or geometric means when a search is executed or as part of some automated data processing routine. This standard does not preclude such techniques, but does not attempt to address the permanence or method of establishing such network locations. Instead, HY_Features provides a linear referencing data model to facilitate relating hydrologic locations across data sets and scales.

To allow expression of cross-dataset links, this standard specifies a system for referencing "along a river" by defining the one-dimensional flowpath which realizes a particular catchment as a linear element and a hydro location which realizes the hydro nexus of that catchment as the reference location. A hydrologic location can be thought of as a tie point between different hydrologic datasets. That is, the hydro nexuses in one dataset can be realized as hydro locations and referenced in another dataset. Similarly, hydro locations may actually be monitoring locations, without identified hydro nexuses - which they theoretically may realize. Whether used to link hydro nexuses and associated catchments or other types of information, an identified hydro location provides a way to link specific network locations between data sources.

The HY_Features conceptual model defines an indirect position which has three properties: 1) a hydro nexus realized by a hydrologically significant "reference location"; 2) a "linear element" defined by the flowpath which starts or ends at the reference location; and 3) a "distance from referent" measure providing an absolute or relative (percentage) value. The relationship between the identified hydro nexus (hydro location) and catchment (flowpath) is either inlet and downstream catchment or outlet and upstream catchment, and can be used to provide upstream or downstream directionality of the distance.

With a specified inflow or outflow hydro nexus as reference location and a specific realization of its flowpath, each catchment can support its own river referencing. Realized as a hydro location, a located hydro nexus is the potential reference location at the end of a flowpath that could split the original reference flowpath at the newly located hydro location. For example, if two bridges are thought of as the inflow and outflow locations of a catchment, the downstream bridge hydro location, which realizes an outflow hydro nexus, may form the reference location for stream gages upstream from the bridge and the next upstream bridge. Similarly, the upstream bridge hydro location, which realizes the inflow hydro nexus, may be the reference location for stream gages downstream from the bridge and the next downstream bridge.

Note that in these cases, the upstream or downstream orientation of the measurement is always declared in relation to the catchment inflow or outflow. Since this varies in application, the orientation is specified with an inflow or outflow association between the flowpath linear element (catchment realization) and the reference point hydro location (hydro nexus).

Applying the indirect position model, a feature of interest can be related to a hydro nexus and the associated catchment by linking it to an existing hydro location, or as a new hydro location linked to a flowpath linear element and hydro nexus referent. It’s worth mentioning that a special case of hydro location that is coincident with a hydro nexus referent (at distance '0') could be considered a realization of the hydro nexus.

This standard defines the HY_Features conceptual model as a standard for the identification and description of hydrologic features reflecting both hydrologic significance and topological connectivity of hydrologic features. HY_Features formalizes the fundamental relationships between components of the hydrosphere describing the hydrosphere as a hierarchical network of hydrologically connected catchments, the various realizations a catchment may have and the organization of catchments, waterbodies or channels in networks.

The single concept that governs HY_Features is the hydrologically determined union of a catchment and its hydro nexus: any place on the land surface can be considered the hydro nexus of a corresponding catchment. Catchments and hydro nexuses together form the basis of networks of connected, usually named, hydrologic features. Other core concepts of HY_Features are: 1) an abstract idea of 'catchment' which has many possible geometric and/or topological feature realizations, 2) catchments realized as networks of waterbody features, and 3) linear referencing of river positions using a nominal main flowpath.

The conceptual model elements are grouped into three modules. A model implementation may include any or all feature concepts included in any module, include or exclude feature properties, and allow changing cardinality of one or more associations to 'nillable,' expressing that these logically exist but are not realized in a particular implementation. Table 3 lists the modules, the leaf packages included in each, and the concepts reflected therein.

The conceptual model is expressed in the Geographic Information Conceptual Schema Language (ISO 19103:2005) based on the Unified Modeling Language (UML). The organization of the HY_Features hydrology model classes into modules, packages, and dependencies is shown in Figure 20.

The HY_Features model specified in this standard is a 'conceptual model,' and not intended to be directly implementable for data exchange or persistence. The conformance target of the HY_Features model is therefore either an implementation schema that implements concepts defined in the HY_Features model, or a schema mapping that maps between disparate existing feature types by means of common HY_Features concepts.

The feature types in the HY_Features conceptual model are defined below according to the modules and leaf packages described above into which their UML classes have been grouped.

The HydroFeature application schema provides the core concepts of a named hydrologic feature in the Named Feature (shown in Yellow) of a hydrologic complex. For each complex, the union of catchment and its common outlet is realized by a collection of hydrologic features (shown in Blue) and a river referencing system, which allows placement of an arbitrary feature 'along a river' using linear referencing (shown in Green) along a one-dimensional topological flowpath realization.

If required, an implementation should use terms from the code lists in Annex B of this standard, defined specifically to conform to a terminology common in the hydrology domain. Note that alternative code lists may be used but should be related to the terms in Annex B.4 using an appropriate formalism.

The HY_HydroFeature feature type is defined as hydrology-specific instance of the General Feature metaclass (as defined in the OGC General Feature Model, GFM), whose identity needs to be maintained and tracked through a processing chain from measurement to distribution of hydrologic information.

As an instance of the General Feature metaclass a feature type is identified by a unique identifier and typical properties. Typically, a hydrologic feature is additionally identified through names in common usage and through hydrologically significant characteristics.

The HY_HydroFeature feature type is specialized into more specific feature types, which specify additional properties and represent particular hydrologic phenomena (Figure 23).

The definitions of HydroFeature feature types are rooted in the definitions documented in the WMO/UNESCO Glossary of Hydrology. They are applied regardless of their application context in respect to the Earth’s surface. For the purpose of testing the applicability of the HY_Features conceptual model in the context of surface water hydrology, the definitions in this standard refer to surface water hydrology. A conceptual model capturing the specifics of features associated with the groundwater domain is developed with reference to the OGC WaterML 2: Part 4 - GroundwaterML2 standard [6].

Providing a standard terminology for the typical relationships between hydrologic features allows the hydrosphere to be expressed in a consistent way across multiple data products, regardless of various spatial or temporal representations.

The Surface Hydro Feature application schema provides common concepts of hydrologic features occurring on the land surface and specifies the core concepts defined in the abstract Hydro Feature application schema. This will enable contextually linked information models to build relationships between multiple realizations of the same catchments. Typical realizations of the catchment concept and hydro nexuses can be described in a consistent way using standard terminology for the relationships between surface water features defined in this application schema.

The Surface Hydro Feature model (Figure 37) conceptualizes the accumulation of water on the land surface in waterbodies (shown in Blue), each made unique by its origin, size, or movement. With respect to the management and storage of water resources, a concept of water storage is provided and allows any waterbody type to be considered a managed reservoir (shown in Purple).

Relying on a conceptual separation of waterbody and container, the Surface Hydro Feature schema defines a network of potentially connected depressions and channels on the land surface which periodically or continuously contain water (shown in Orange). Separate from the hydrographic network of permanent or temporary waterbodies, the channel network can be used as the connecting system.

The definitions in this schema are rooted in the definitions given in the WMO/UNESCO Glossary of Hydrology which defines a network of watercourse regardless of the location in respect to the Earth’s surface. The conceptual model defined here has been vetted in the context of surface water hydrology. In other words, in this standard, 'channel network' and 'hydrographic network' refer to surface channels or other containers for surface-waterbodies.

The Surface Hydro Feature application schema contains the leaf packages: Channel Network, Hydrographic Network, Water Body Types and Storage. Figure 38 shows the external and internal dependencies.

The Hydrometric Network application schema (Figure 44 and Figure 45) define a logical model to take into account a network of hydrometric stations as a specific realization of the catchment in the perspective of hydrologic observation, without the detail of an observation strategy.

The Hydrometric Network model specifies the concepts defined in the Hydro Feature core model. The general concept is that of a network of logically connected hydrometric stations realizing as a whole the catchment. This enables contextually related information models to relate monitoring stations and observing posts to hydrologic features, as is often required for environmental reporting or when interpreting, analyzing and processing observation results.

The hydrometric network model introduces the concept of a 'position on river' which allows a hydrologic station, which may not have a precisely known location, to be the located on a river. This supports the establishment of upstream-downstream relationships between hydrometric features, assignment of a position relative to a 'fixed' hydro nexus, or to place other features of interest relative to the hydrometric station.

These test suites verify the compliance of the conformance targets for the HY_Features conceptual model with the HY_Features specification. Each instance of hydrologic feature data is encoded according to a specific implementation schema, so conformance of a schema or a mapping to/from such a schema with the abstract specification is defined as a clear derivation of schema or mapping entities from the concepts, definitions, and constraints of HY_Features.