Journal of Hydrology 424–425 (2012) 24–36

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Journal of Hydrology

journal homepage: www.elsevier .com/locate / jhydrol

An international standard conceptual model for sharing referencesto hydrologic features

Robert Atkinson a,⇑, Irina Dornblut b, Darren Smith c

a Commonwealth Scientific and Industrial Research Organisation (CSIRO) Land and Water, GPO Box 1666, Canberra ACT 2601, Australiab Global Runoff Data Centre, The Federal Institute of Hydrology (BfG), Am Mainzer Tor 1, 56068 Koblenz, Germanyc Australian Government Bureau of Meteorology, GPO Box 1289, Melbourne VIC 3001, Australia

a r t i c l e i n f o

Article history:Received 5 September 2011Received in revised form 30 November 2011Accepted 1 December 2011Available online 4 January 2012This manuscript was handled by GeoffSyme, Editor-in-Chief

Keywords:Conceptual modelDomain modellingHydrologyHydrographyMultiscale hydrological modellingRunoff modelling

0022-1694/$ - see front matter � 2012 Elsevier B.V. Adoi:10.1016/j.jhydrol.2011.12.002

⇑ Corresponding author. Tel.: +61 419202973; fax:E-mail address: rob.atkinson@csiro.au (R. Atkinson

s u m m a r y

Concepts such as catchment, basin, watershed and river are commonly understood in many fields ofdiscourse, but are described differently according to the focus on various aspects of the hydrology phe-nomenon. The effective exchange of hydrologic data containing references to hydrologic features requiresstandardised semantics of the concepts that allow identification of these features. Here, the scope of com-mon approaches to information modelling of hydrologic features is examined, and is compared to therequirements for feature identification in multiple contexts. A conceptual model is presented that recon-ciles the underlying differences in representation of hydrologic features and levels of detail in typicaldatasets. By providing a stable and common referencing system for hydrologic features, the model willassist in the organization of observation and modelling of such features, and in the aggregation of gen-erated data on a global, regional, national or basin scale.

The model encompasses a number of approaches used in different contexts to identify and modelhydrologic features and enforce the semantics of relationships between different levels of detail. Thus,it provides a semantic framework for common feature identifiers to be developed and embedded in indi-vidual data products, while providing the flexibility to model complex hydrological processes at finedetail. The common identifiers can be used in aggregating data generated using high-detail models ofprocesses, and in partitioning large and complex hydrologic feature datasets into local study areas. Dif-ferent local models can be applied according to the dominant hydrologic processes and the amount ofhydrometric monitoring available for each region.

The model is intended to form the basis for standard practices under the auspices of the World Mete-orological Organization Commission for Hydrology. It is presented here to invite further testing, feedbackand engagement in the process of its acceptance and implementation as an international standard forhydrologic feature identification. Based on the accepted International Organization for StandardizationGeneral Feature Model, it will be possible to realise, in a standardised way, the semantics of feature iden-tification in tools for managing metadata documenting hydrologic datasets and products.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

An increasing number of information networks are being estab-lished to improve the sharing of information and the exchange ofdata. Examples include the Global Terrestrial Network for RiverDischarge (GTN-R) used as a service for the Global Terrestrial Net-work – Hydrology (GTN-H), and regional, national and local sys-tems used to integrate data from multiple sources (EC, 2007a;Lehner and Döll, 2004; Vogt et al., 2007a; Wickel et al., 2007).These networks require standards to improve the interoperabilityof systems used for gathering, processing and retrieving relevant

ll rights reserved.

+61 3 9545 2175.).

information and data. Identification of the objects characterisedby such data is based on the common understanding of the com-munity of practice (domain); however, there is no well-known,agreed and accessible set of specifications for the semantics ofthese features in the hydrology domain.

To relate disparate data sets reliably, common identifiers forhydrological features need to be defined that can handle differentways of representing the real world features in information sys-tems, as well as the cultural variability of naming in aninternation-al context.

Hydrologic processes are constrained by and interact with thelandscape and medium in which they occur. The interactions arecomplex and variable through time; thus, a complete conceptualmodel of all interactions at all scales is not feasible. Nevertheless,it is common practice to identify and refer to specific instances

Fig. 1. Package organization of the Canadian National Hydro Network (NHN).

R. Atkinson et al. / Journal of Hydrology 424–425 (2012) 24–36 25

of a hydrologic feature when describing the state of hydrologicprocesses or associated human activities. For example, persistentlandscape features such as catchments, streams and water bodiesare often used to conceptualise hydrology, making it possible tomonitor, manage or evaluate the state of certain hydrologic pro-cesses over time. However, there is no standard conceptual modelfor hydrologic features. Models vary in terms of scale, level of de-tail, processes included and approaches to describing hydrologicfeatures, and thus produce different and mostly incompatible setsof identifiers for hydrologic features.

Generally applicable identifiers for hydrologic features are notreadily defined. National and subnational mapping agencies suchas the United States (US) National Hydrographic Dataset (NHD)and NHDPlus (NHDPlus, 2008) tend to capture the topography ofinland waters and the details of hydrologic connectivity. At conti-nental scale, the focus is on delineating catchments and identifyingmajor river networks; an example is the European Commission’sCatchment Characterisation and Modelling database (Vogt et al.,2007b). On a global scale, a network routing model has been usedto derive a hierarchical river network from a digital elevation mod-el (DEM), to support numerical modelling (Renssen and Knoop,2000). The HydroSHEDS dataset (Wickel et al., 2007) provides asimilar product, with delineated catchments. Other global datasetsprovide cartographic representations of specific hydrologic fea-tures in general topographic datasets such as the digital chart ofthe world (Danko, 1992), and in datasets of water bodies (Lehnerand Döll, 2004) and major river basins (Wolf et al., 1999).

Each of these datasets contains representations of the samehydrologic features, but each representation has a different form(area, network, etc.), and may express a different level of detailabout parameters such as flow, area and connectivity. The differentrepresentations have different semantics and identifiers, eventhough the hydrologic features they represent may be the same.Nevertheless, the different representations may need to be cross-referenced or combined at different scales (e.g. within a processmodel). Spatial processing can be used to approximate valueswhere the catchment definitions do not match (Bergstrom andGraham, 1998), but more direct correlation can be achieved if thesame definition is used. One possible approach is to derive coarserscale data from higher levels of detail. Stanislawski describes a‘‘feature pruning’’ based on ‘‘knowledge of surface water, theNHD model, and associated feature specification standards’’.(Stanislawski, 2009). Key issues include the need for multiplescales of representation, the difficulty of reproducing multiplescales on demand, the need for a semantic model, the concept offeature specifications and the fact that an implicit informationmodel tied to a particular dataset (NHD) is required to articulatethe rules. These rules could apply equally to other datasets, usingother information models but capturing the same semantics asNHD.

In practice, there are two main challenges: multiple representa-tions of hydrologic features at different scales, and inconsistent useof terminology in different contexts. For example, a river may bemapped as a polygon of confining river banks or by a nominalcentreline, and the name of a river may be used to refer to the en-tire contributing system, its catchment or its main channel,depending on the context.

Many attempts have been made to specify the semantics of aninformation model for hydrology, either as an explicit goal or animplicit aspect of an approach (de Jager and Vogt, 2010; EC,2007b; Kumar et al., 2010; Maidment, 2002; Todini, 2007; WMO,1992). A widely accepted model requires both an accepted formal-ism and a governance framework that allows its adoption. TheWorld Meteorological Organization Commission for Hydrology(WMO-CHy faces this challenge, but cannot directly use or recon-cile the many different models available because the governance

and formalism mechanisms lack a sufficiently stable, global andplatform-independent scope. However, WMO-CHy can take noticeof the emerging trends in hydrologic information models.

The European Union (EU) INSPIRE legislation sets out a frame-work for standardising exchange of environmental data amongEU member states (EC, 2007a). The INSPIRE Hydrography thememodel (INSPIRE Thematic Working Group Hydrography, 2009)incorporates definitions used in the EU Water Framework Directive(EU, 2000). The model extends generic model components that arespecific to INSPIRE, such as a network model and a geographic-naming module. Thus, the model is difficult to use outside of theINSPIRE context.

The Canadian GeoBase data product suite provides a formaldata product specification that includes information models (John-son and Singh, 2003). Like INSPIRE Hydrography, the model is sep-arated into distinct packages reflecting different concerns (Fig. 1).As with the INSPIRE model, GeoBase uses the unified modellinglanguage (UML) notation (Rumbaugh et al., 2004), but with a par-ticular idiom unique to the activity.

The ArcGIS Hydro (ArcHydro) is a data model for representinghydrologic features in geographic information systems (GIS)(Maidment, 2002). Its documentation states, ‘‘the goal is to supporta basic cartographic representation of surface water features whileenabling the integration of those features with hydrologic andhydraulic simulation models’’. ArcHydro uses a numerical key, Hy-droID, that is ‘‘used to link related features, and the association offeatures such as monitoring points with time series data’’. TheArcHydro identifier mechanism is designed for internal consis-tency and relationship handling, not for referencing the same ob-ject across disparate datasets. A GIS-centric data model forcoupling process models illustrates the feasibility of a commonsemantic model, but addresses only its application to locallyscoped spatial representations (Kumar et al., 2010).

Verdin and Verdin succinctly describe the relationship betweenhydrologic processes, hydrologic features and observations: ‘‘Sys-tems that implicitly code basins have arisen from the need to de-velop identification numbers for stream gauging stations. Agauging station’s location, of course, corresponds to a unique up-stream basin and its measurements record runoff integrated overthat surface area’’ (Verdin and Verdin, 1999).

A shared referencing system is required that may assist hydro-logic observation and information modelling at the feature level,and a consistent reporting of state and behaviour of hydrologic fea-tures. Likewise, the aggregation of hydrologic data into data prod-ucts on a global or national, regional or basin scale requires stableand generally applicable feature identification.

This paper presents the results of applying a formal domainmodelling methodology to the definitions of hydrologic phenom-ena accepted by the WMO, taking into account the typical ways

26 R. Atkinson et al. / Journal of Hydrology 424–425 (2012) 24–36

of describing hydrologic features used in practice. The scope of theresulting model is carefully constrained to the issues around iden-tification of features. The most important aspects of the design areexplained (WMO, 1992).

2. Requirements of a common information model forhydrologic features

2.1. Thematic context

Hydrology is ‘‘the science that deals with the waters above andbelow the land surfaces of the Earth, their occurrence, circulationand distribution, both in time and space, their biological, chemicaland physical properties, their reaction with their environment,including their relation to living beings’’ (WMO, 1992). By generaldefinition, a feature is an ‘‘abstraction of real world phenomena’’(ISO, 2002). Consequently, a hydrologic feature is the abstractionof a hydrology phenomenon. For example, a catchment – com-monly recognised as the abstract unit where hydrologic processestake place – is considered an abstract hydrologic feature. A basin isconsidered a special type of catchment; a reporting or manage-ment region may be a subtype too.

The term ‘‘basin’’ is used inconsistently in the literature, some-times to denote an entire river system, and sometimes a sub-basinbetween key features (e.g. major confluences in a river system). Interms of commonality, a definition endorsed by the WMO-CHy isapplied. In the following, ‘‘basin’’ refers to the physiographic unitwhose overall runoff is channelled to a common outlet (WMO,1992).

Basins are organized in hierarchies and are related topologicallyin networks. They may be aggregated to complete river basins or tointermediate, hydrologically discrete upper-level systems such asmanagement or reporting units. Thus, river basins are often parti-tioned into discrete sub-basins within a larger basin, with differentapproaches to aggregating up the lower level of hydrological detail.

Fig. 2. Alternative cartographic and node-

For example, Verdin uses the Pfafstetter coding system (Pfafstetter,1989) for ‘‘delineation and codification of Earth’s river basins’’(Verdin and Verdin, 1999). In terms of network connectivity, a ba-sin receives inflow from upstream basins, and flows into a basindownstream.

Depending on application and scale, hydrologic features may berepresented in multiple and different ways in the real world. Forexample, the Strahler stream order, widely used in GIS applica-tions, is an ordering of streams based on the hierarchy of tributar-ies (Strahler, 1957).

A basin may be described geometrically (e.g. as a rivercentreline or watershed) or topologically (e.g. as a graph ofnodes and links). In many cases, either the geometry or thetopology is unknown. Cartographic representations containingno topology can still be partitioned using the catchment bound-ary on the river-system scale, forcing a simple relationship be-tween a trivial topology and the spatial representation of thesecoarse features.

Fig. 2 shows such a cartographic perspective – where the seg-ments of a river network within a catchment have been high-lighted. Some segments of the network have not been selectedbecause networks built from cartographic inputs are typicallyincomplete, and capturing the entire set of hydrologic featuresfor a catchment requires a spatial operation based on the wa-tershed for the catchment. This situation highlights the need forcartographic and catchment information models to be linked, sincesuch spatial operations have a significant overhead in sourcingwatershed boundaries, reconciling them with the applicationneeds for spatial partitioning and execution over very large spatialdatasets.

In some applications, spatial (geometric) representations arenot required; topological relationships are sufficient to aggregatedata reported against, for example, a catchment hierarchy. Bothspatial and topologic representations must be able to be relatedto the same identifiable features, as illustrated in Fig. 2.

link representations of a river system.

R. Atkinson et al. / Journal of Hydrology 424–425 (2012) 24–36 27

The most common representation of a hydrologic feature is thehydrographic network aggregating water bodies through aconnecting system: the channel network.

By definition, a body of water refers to the mass of water (WMO,1992). The occupied landform defines the extent and shape of thewater body. Dropping below or exceeding the normal confines, theextent and shape of a water body may reversibly deform, andthe nature of flow may change temporarily. The water body con-fines may be represented in different ways (e.g. a notional centre-line or separate river banks and bed).

The confining landforms determine the system used to poten-tially connect water bodies, but they do not define whether andhow a body of water interacts with the water bodies upstreamand downstream. For example, normally stagnant waters may ormay not be connected to streams during flood events, or streamsmay or may not form pools of stagnant water in periods of drought.The connecting system of confines exists whether water flows ornot.

A body of water may be a river confined by a channel, withbanks, a bed and a notional centreline. The banks do not have acharacteristic of, for example, volume of flow for a given period –that is a characteristic of the water body itself. Likewise, the waterbody does not have the material characteristics of the mediumforming the river bank. Thewater body and the landforms confin-ing it need to be kept separate, to simplify the semantics of theircharacteristics.

Basins and representing water bodies are named and form part ofcommon discourse. They may be considered as capable of storingand yielding water, and may have characteristics that are consideredgenerally applicable in spatial, temporal or classification contexts.

Different representations of a hydrologic feature may use alter-native aspects of the confining landform. The model must supportthis situation, but at the same time must unambiguously define thehydrologic feature represented, in terms of its connectivity withother features.

2.2. General applicability

To be generally applicable to all cases where identification ofhydrologic features needs to be maintained or shared, the featurecharacterisations given in different systems must be supported,without specifying what characteristics may be assigned in a spe-cific context. Rather than a comprehensive set of all (or commonlyused) characteristics, a reusable core model that supports applica-tion-specific specialisations is required. For example, for an appli-cation concerned with rainfall runoff, the model could be extendedto include soil moisture and land cover parameters that are notneeded for definition of the unit of study itself. A representationsuch as a remote-sensed grid field can be added for any parameter,as can a simple attribute for a characteristic value. Numerical mod-el starting conditions and parameters may be provided based oncharacterisation of, for example, basin features; model outputsmay be reported at basin scale or combined into a single resultbased on the hydrologic connectivity of basins.

A fundamental issue in cross-border applications and globalinformation frameworks is the naming of a feature within differentcultures and languages. The name of a single feature may vary,depending on locale and usage; also, a name may apply to partof a feature only.

Names of features are a socio-political phenomenon (McDavid,1958). Provided that practitioners in the field of hydrology can suc-cessfully share meaning and feature identifiers, a full conceptualmodel of toponymy (the study of place names) is not required.Nevertheless, the cultural, political and historical variability andthe relationships between alternative names need to be handled.

2.3. Stability of identifiers

Stability of identifiers means that factors that change the repre-sentation of a feature in an information system should not changethe identifier of the feature itself. Such factors may be the im-proved resolution or accuracy of representation; minor changesto physical characteristics of the feature that occur over time;and changes of technology platform, implementation or custodian.

A core requirement of general applicability of a model is that itshould support stability of identifiers across different representa-tions, so that multiple systems can use or map to the identifiers.With regard to hydrology, identifiers must be able to stably reflectthe hydrologic significance of a feature regarding both its contrib-uting catchment and its topological connectivity to features up-stream and downstream. Features that can be given stableidentifiers in this context must be distinguished from those thatare defined within the context of a specific representation. Forexample, a DEM-derived drainage network will have many pre-dicted flow lines, but these may be simply a function of the resolu-tion of the DEM rather than a reflection of physical reality.Nevertheless, key features such as major confluences may be rep-resented; such features are often due to drainage enforcementfrom vector representations, but are nevertheless identifiable.

Fig. 3 shows an example of this phenomenon. Although the con-fluences in the DEM streamline correspond to confluences in thehuman-interpreted cartographic product, the networks vary intopology. Thus, any stream-tracing identification system wouldneed to derive different identifiers for the elements in each network.

2.4. Scale-independence

Hydrologic features must be identified at any scale, from a con-tinental-scale river basin to the catchment of any point on adetailed river network. Whether observing, modelling or reporting,the choice of scale depends on the purpose of the study. Somescales appear to be general because they are widely used, but whenchosen for a specific purpose (e.g. for mapping or comparativereporting), they are still distinct.

Observations are the responsibility of national or subnationalagencies, whereas recorded data are processed and managed on re-gional or even global scales, in various databases held by nationalagencies and international data centres. At a local scale, hydrologyprocesses and features may form complex structures and interac-tions; these can safely be ignored at a broader scale of analysissince, regardless of internal behaviours, the net effect can be re-duced to a single flow.

Scaling up or down leads to multiple representations of thesame hydrologic feature. A common model must support simplifi-cations at small scales and detail at large scales, allowing hydro-logic feature complexes to be encapsulated within simplerfeatures at a less detailed scale. The model must also support thecoexistence of multiple hierarchical aggregations of features intoalternative networks.

The business requirements for identifier stability imply that thesame features must be identifiable where present in differentscales of mapping. Reporting on a coarse scale needs to be sup-ported; it must also be possible to aggregate features at fine levelsof detail in a consistent fashion to generate information at coarsescales.

2.5. Governance

The WMO is the official United Nations’ authoritative voice onweather, climate and water, and it provides the framework forinternational cooperation in the domain of hydrology. WMO-CHy

CartographicCartographic

Fig. 3. Alternative DEM-derived and cartographic representation of streamlines.

28 R. Atkinson et al. / Journal of Hydrology 424–425 (2012) 24–36

shapes water-related activities and provides guidance to WMOmember countries (WMO).

To support stable feature identification, a model must be ac-cepted within the community of use. For this to occur, the modelneeds to be published, promoted and managed by an acceptedauthority. Thus, the hydrologic feature model needs to be acceptedby the WMO-CHy. To this end, the model must conform, whereapplicable, to the definitions endorsed by the WMO-CHy; theseare contained in the IGH (WMO, 1992). Also, the model must notdepend on aspects not recognised by the WMO within its stan-dardisation agenda.

2.6. Implementation neutrality

The model should not specify a specific implementation but itshould be capable of being realised (in part) using existing toolsfor managing hydrologic feature representations. Also, the modelmust not directly reference any more narrowly defined compo-nents, such as national data standards or technologies. Use of theISO 19103 Conceptual Schema Language and Application Schemainformation modelling idiom (ISO, 1999) conforms to thisrequirement.

2.7. Summary of requirements

The proposed model must:

� support both simplified and detailed topologies used in differ-ent scales of representation;� support concepts of contributing catchment area and connectiv-

ity of flow, and the relationships between these;� be generally applicable to different aspects of hydrology,

through providing an extensible core model;

� support stability of identifiers by allowing a distinctionbetween reference points that may be commonly observedand artefacts of scale-dependent representation;� be consistent with the existing governance of definitions

through the WMO CHy;� be formalised in a technology-independent fashion.

2.8. Information modelling methodology

The conceptual model described here is expressed as a set ofApplication Schema containing Feature Type definitions in the Inter-national Organization for Standardization (ISO) TC211 GeographicISO 19103 Information Conceptual Schema Language (ISO, 1999)and ISO 19109 General Feature Model (ISO, 2005). This approachis based on a broad industry standard – the UML (OMG, 2004) –which has been used by other domains to establish internationallyscoped standard information models (Sen and Duffy, 2005; Woolfet al., 2005). Fig. 4 shows the key elements of UML diagrams usedin these standards.

The core elements of this language are:

� the Application Schema, modelled as a UML Package, with ‘‘pack-age import’’ dependencies describing inheritance relationshipsbetween these packages, as shown in Fig. 5;� the FeatureType, modelled as a UML Class.

The model is developed in a multistep process that recon-ciles the requirements for hydrologic referencing with typicaldataset designs and semantics endorsed by WMO-CHy.Thesteps are:1. Analysis of broad commonalities and differences in existing

hydrologic datasets.

class Notation

«Application Schema»DomainModel

«Application Schema»ImportedModel

«featureType»Lion

+ maneLength :Real

«featureType»Cat

denotes that DomainModel extends ImportedModel

Lion is a type of Cat

«featureType»Park

target role of association is theproperty foundIn of the class Lion with datatype Park with 0..* (zero to many) values allowed

attribute maneLengthis a property of the feature type Lion.

«import»

+foundIn 0..*

Fig. 4. Key elements of UML notation used in ISO 19103.

pkg HY_Features package organisation

«Application Schema»HY_Utilities

«Application Schema»HY_HydrometricNetwork

«Application Schema»HY_SurfaceHydroFeature

«Application Schema»HY_AtmosphericHydroFeature

«Application Schema»HY_SubsurfaceHydroFeature

«Application Schema»HY_HydroFeature

«import»

«import»

«import»

«import»

«import»

«import» «import»

Fig. 5. Package organization of HY_Features model.

R. Atkinson et al. / Journal of Hydrology 424–425 (2012) 24–36 29

2. Analysis of implications of business requirements for hydro-logic feature referencing.

3. Development of a draft model of key concepts required to meetbusiness objectives.

4. Analysis of interdependencies and separation of the model intodiscrete modules.

5. Identification of, and reconciliation with, the semantics of termspublished by the WMO.

30 R. Atkinson et al. / Journal of Hydrology 424–425 (2012) 24–36

6. Conformance checking of the model against ISO 19109 rules.7. Community engagement through consultation with experts and

the Open Geospatial Consortium (OGC) Hydrology DomainWorking Group.

8. Design of data products conforming to the model.

These steps are broadly in line with the INSPIRE model develop-ment methodology (EC, 2007b), as is appropriate for developmentof a common standard model in a multinational context.

Step 4 aims to simplify the scope of each part of the model, toimprove its accessibility and provide scope for testing. The goalis for each implemented data product to consider only those partsof the common model that are implicated by its scope.

Steps 4–7 are repeated in an iterative process to ensure thatmodules are cleanly separated and meet the needs of any furthermodelling representations of hydrologic features. The key aspectsof the model presented below represent the results of the first iter-ation of these steps.

Differences in terminology may be explored by reconciling theaccepted definitions for features in common use – as given in theInternational Glossary of Hydrology (IGH) (WMO, 1992) – withtheir representation in various datasets. Our approach is to modelthe constructs found in existing data and apply the appropriateterm from the IGH to the classes identified. This has the effect ofaugmenting the accepted definitions with explicit semantics forthe relationships with other terminology.

3. The HY_Features conceptual model of hydrologic features

3.1. Overview

The HY_Features model is a set of interrelated modules contain-ing definitions for key aspects of hydrologic systems. The separa-tion into modules simplifies each package; for example,HY_HydrometricNetwork, which describes the relative placementof stations in a hydrometric network, is not necessarily requiredto describe a cartographic dataset or catchment-based reportingunits, and is thus an extension that imports the core concepts.Fig. 5 shows these modules and their interdependencies.

class Names and identifiers

++++

«FeatureType»HY_HydroFeature

+ identifier :EXT_IdentificationCode [0..*]

+ name+ trans+ usag

+name

0..*

Fig. 6. HY_Features’ support for m

The HY_prefix follows the ISO naming conventions for UML ele-ments. There is no requirement that these names be used in imple-menting systems for the same semantic elements. Also, the way inwhich mappings between abstract element names and implemen-tations should be recorded is not specified; however, there is anexpectation that interoperability will be facilitated in future bymaking such mappings available as a component of datasetdocumentation.

The HY_HydroFeature model captures the requirements identi-fied above, modelling a subset of abstract hydrologic features thatmay be provided with a stable identifier in multiple representa-tions. The HY_SurfaceHydroFeature module illustrates how the fea-tures defined by HY_HydroFeature may be related to typical spatialrepresentations of surface water features.

3.2. Named feature

HY_NamedFeature provides an abstract pattern shared by allhydrologic features where names are given to a feature eitherthrough common usage or through assignment of an identifier.The abstract HY_HydroFeature class, used for an identifiable hydro-logic feature, has a property name with cardinality 0. . .�; thus, fea-tures may have no name, one name or multiple names (Fig. 6).Likewise, the property identifier is a multi-valued EXT_Identifier-Code (which is a qualified identifier).The model takes the generalconcept of a language-specific name (EXT_Localised Name), andspecialises it with the additional attributes namesPart, preferredBy,source, and variantSpelling.

This pattern for handling names of features is not specific tohydrology, but is necessary for identifying the source of names forhydrologic features. Semantics and rules for partial naming and pre-ferred usage are needed for completeness, even though they are notdirectly supported by the ISO meta-model. These utility data typesare defined in the HY_Utilities package for now, but they may bereplaced with a generalised set of concepts provided within a futureversion of an ISO standard, preserving the semantics of the model.

The INSPIRE Hydrography model shows a similar pattern, alsousing an external utility package to define common data typesexternal to the hydrology domain model. The equivalent HydroOb-

«FeatureType»HY_HydroFeatureName

namesPart :Boolean [0..1] = false preferredBy :CI_ResponsibleParty [0..1] source :CI_Citation [0..1] variantSpelling :Boolean [0..1] = false

«FeatureType»HY_Utilities::EXT_LocalisedName

:ScopedNameliteration :EXT_TransliterationStandardCode [0..1]e :EXT_UsageType [0..1]

ultiple names for a feature.

class Hydro - base: spatial object types

«featureType»HydroObject

«voidable»+ geographicalName: GeographicalName [0..*]+ hydroId: HydroIdentifier [0..*]

+relatedHydroObject0..*

Fig. 7. INSPIRE implementation of named hydrologic feature.

R. Atkinson et al. / Journal of Hydrology 424–425 (2012) 24–36 31

ject class has similar attributes – hydroId holding external identifiers,and geographicalName – as shown in Fig. 7. The HY_Features model iscapable of representing more complex requirements, such as thenaming of rivers whose successive parts have individual names.For example, the succeeding portions of the conventionally namedNile River have individual names such as Victoria Nile, Albert Nile, Bahrel-Jebel, White Nile, and Nile, each having equivalent names in differ-ent cultural contexts.

3.3. Multiple representations

HY_Catchment introduces the basic concept of catchment as anabstract feature represented multiple times in the real world (e.g.geometrically by boundary and area, or topologically by any net-work). This concept is applicable to any designated catchment,and is inherited by any subtype (e.g. HY_Basin). The concept doesnot dictate how the hydrology within such a basin is best repre-sented, if known at all.

In practice, different aspects of the hydrology within a basinmay be known at different levels of detail. This is handled throughthe abstract HY_CatchmentRepresentation class, as shown in Fig. 8.The cardinality and direction of the association between aHY_Catchment and a HY_CatchmentRepresentation are significant.There may be no representation of the catchment (zero cardinal-ity); in such cases, the HY_Features model reduces to the typicaltree-structured node-link network case commonly used in numer-ical models of hydrology.

The direction of the association representedCatchment makes ex-plicit the fact that each representation must have a clear relationship

class Context Diagram : HY_CatchmentRepresentation

«FeatureType»HY_CatchmentArea

+ area: Area [0..1]+ shape: GM_Surface [0..1

«FeatureType»HY_CatchmentBoundary

+ boundary: GM_SurfaceBoundary

«FeatureType»HY_CatchmentRepresentati

Fig. 8. HY_CatchmentRepresentation with specialised imple

to the set of stable identifiers. This is exemplified by the HY_Basinmodel, which shows typical specialised representation typesrequired to model basins. These types are defined by the HY_Hydro-Feature core model and, in the case of simple geometric representa-tions, have appropriately typed attributes. Typical cartographic orother alternative representations of stream networks are handledby adding appropriate attributes to the HY_SurfaceHydrology model,which imports and specialises HY_Network. Using additional spe-cialisations of HY_CatchmentRepresentation, it is possible to linknumerical models of flood responses, surface–groundwater interac-tions, soil saturation fields or any other relevant information to thesame set of basins described by the HY_Features model. The otherkey implication of this pattern is that HY_Features requires thatmodels such as cartographic or watershed models be partitionedacross basins, and each basin representation be treated separately.

3.4. Catchments and basins

The catchment is commonly recognised as the abstract unit ofstudy and reporting in hydrology; a basin as the physiographic unitwhere hydrologic processes take place (Maidment, 2002;Pfafstetter, 1989; Todini, 2007; Verdin and Verdin, 1999; Vogtet al., 2007a). Considering the basin as the physiographic unit whoseoverall runoff is channelled to a common outlet, this common outletdetermines the identifying outfall of the basin. For waters flowingdue to gravitational forces the outfall coincides with the lowestpoint on the summit line bounding the drainage basin (WMO, 1992).

Basins are organized in hierarchies and related topologically innetworks; they may be aggregated to complete river basins or tointermediate hydrologically discrete upper-level systems like man-agement units. For example, generally recognised river basins areoften partitioned in discrete sub-basins within a larger basin, withdifferent approaches to aggregating up the lower level of hydrolog-ical detail. In terms of network connectivity, a basin gets inflowfrom contributing, upstream, basins and flows out into a receivingbasin downstream.

Basins have overall characteristics determined from interac-tions with other basins assigned in spatial, temporal or classifica-tion contexts, for example a range in a classification, anexpression of permanence, or a shared space.

Conceptually, the outfall is a logic point on the land surface. Itdoes not have an explicit location (consider the area of mixing oftwo large rivers), but it is typically given a general location; thatis, it references a point fixed by coordinates. This point may be afixed landmark, a station or cross-section, a point projected ontothe surface, or a logical point created from collapsing other points.

«FeatureType»HY_Network

]

onHY_HydroFeature

«FeatureType»HY_Catchment0..*

+representedCatchment

1

mentations for watershed, area and stream network.

class Context Diagram : HY_Basin

«FeatureType»HY_Basin

+ code: RS_Identifier [0..1]

HY_NetworkHY_BasinHierarchy

«FeatureType»HY_Outfall

HY_HydroFeature

«FeatureType»HY_Catchment

0..* +containingBasin 0..1

1..*

+containingSystem

0..1+contributingBasin

1..*

+pointOfOutflow

1

+pointOfInflow

0..1

+receivingBasin

0..*

Fig. 9. Basin topology model showing HY_Basin, linked by HY_Outfall nodes, as a component of a basin hierarchy.

32 R. Atkinson et al. / Journal of Hydrology 424–425 (2012) 24–36

Topologically, each point on the land surface may be considered asthe outfall of a corresponding basin. In this way, each phenomenonto which a position can be assigned can be related to a correspond-ing basin.

Considering a basin as the unit where hydrologic processes takeplace, the outfall marks the point on the land surface where theseprocesses interact with the processes in the downstream basin.Hence, basins identified by their outfalls are a semantic constructapplicable to linear networks, drainage areas and cartographicrepresentations.

Fig. 9 shows HY_Basin as a specialised HY_Catchment whosepointOfOutflow is potentially the pointOfInflow of one or morereceivingBasins.

HY_Basin may be nested into hierarchies at different scales,through the containingBasin relationship. Each HY_Basin may bedefined within a specified HY_BasinHierarchy (i.e. within the sys-tem of basin definition used to specify the set of basins in any givendataset). Because the HY_BasinHierachy is an explicit element ofthe model, multiple different delineations may coexist in a singledataset. For example a coarse or historical set of assumed water-sheds and basins may exist in the same database as a DEM-derivedset of alternative basin representations.

class Context Diagram : HY_ReferencePoint

«FeatuHY_Refe

+ refPoint: GM_Po+ refPointType: HY

«FeatureType»HY_Outfall

+networkLocation

0..1

Fig. 10. Linking a spatial referen

The HY_Outfall class describes the connection between basins.This may be located at an identifiable point in the network, or itmay be a more abstract concept, such as the connection betweena river with a lake or sea through a delta with multiple distributar-ies. Although the model does not force these to use the sameHY_Outfall nodes, it does permit it; thus, overlapping basin hierar-chies can be cross-referenced if applicable.

The HY_Outfall does not have an explicit geometry – it is a log-ical construct that applies to diffuse outlets, distributary systemsand node-link networks with no explicit spatial geometry. Fig. 10shows how HY_Outfall is modelled, with examples of classificationsof reference point types defined in the IGH (WMO, 1992) .Thismodel does not dictate that a particular reference point shouldprovide the authoritative location for an outfall; rather, it requireseach dataset to define its representation of the location, and thesemay vary.

The HY_Features model can thus support both unconnected andintegrated sets of alternative basin designations. It needs to be ableto do this, given the alternative unconnected hydrology represen-tations that are currently in use and are needed to support the pos-sibility of a fully cross-referenced suite of alternativerepresentations.

«CodeList»HY_RefPointType

+ barrage+ bifurcation+ confluence+ dam+ diversionOfWater+ fork+ hydrometricStation+ inlet+ intake+ outlet+ ponor+ rapids+ referenceClimatologicalStation+ riverMouth+ sinkhole+ source+ spring+ waterfall+ weir

reType»rencePoint

int [0..1]_RefPointType [0..1]

ce point to a logical outfall.

class Segmentation

HY_Network

«FeatureType»HY_HydrographicNetwork::HY_HydrographicNetwork

+ drainagePattern :HY_DrainagePattern [0..1]

«FeatureType»HY_SurfaceWaterBodyConfines::

HY_Shoreline

«FeatureType»HY_SurfaceWaterBodyConfines::

HY_Channel

«FeatureType»HY_SurfaceWaterBodyConfines::

HY_Bank

«FeatureType»HY_SurfaceWaterBodyConfines::

HY_CrossSection

+ area :Area [0..1]+ shape :ScopedName [0..1]

«FeatureType»HY_SurfaceWaterBodyConfines::

HY_RiverBed

«FeatureType»HY_SurfaceWaterBodyConfines::

HY_Reach

+ reachGeometry :GM_Curve

main stem and tributaries or river segments may have different names and characteristics

«FeatureType»HY_HydrographicNetwork::HY_HydrographicFeatureSegment

+ length :Length [0..1]+ surfaceArea :Area [0..1]+ waterBodySegmentName :HY_HydroFeatureName [0..*]

«FeatureType»HY_HydrographicNetwork::HY_HydrographicFeature

+ overallArea :Area [0..1]+ totalLength :Length [0..1]+ waterBodyName :HY_HydroFeatureName [0..*]

«FeatureType»HY_Catchment::HY_ReferencePoint

+ refPoint :GM_Point [0..1]+ refPointType :HY_RefPointType [0..1]

+shoreLine 0..1+channel 0..*

+bank 0..2

+crossSection 0..*

+bed 0..1

0..*

+confinedWaterBody

0..1

+upstreamSegment 0..* +downstreamSegment 0..*

1..*

+hydrographicFeature 1 1..*

+hydrographicNetwork 1

0..*

+crossSectionPoint

0..*

+downstreamReferencePoint

1..*

+upstreamReferencePoint

0..*

Fig. 11. Describing confines geometrically in the context of logical network segments.

R. Atkinson et al. / Journal of Hydrology 424–425 (2012) 24–36 33

A reporting region could be seen as a special type of catchment.However, its boundaries are not necessarily determined by hydro-logic processes and landscape constraints. In the context ofenvironmental reporting, the extent of the reporting region is de-fined implicitly by the boundaries of the evoking institutional orjurisdictional framework. A reporting region may be a river basin,as required by the EU Water Framework Directive (EU, 2000), butalso a country or an administrative or management unit. Like ba-sins, reporting regions may be organized in a simple hierarchicaltree structure by hydrological connectivity, or in a hierarchy ofscales. As with the outfall of a basin, a logical point on the bound-ary line needs to be determined as a reference point, since thisidentifies the hydrologic interaction with downstream reportingunits.

3.5. Geometry and topology: containing landforms and water bodies

Geometry or representation of the confining landform of awater body may give a more complete physical description of itsbehaviour under different flow regimes. However, in practice, thegeometry designated for hydrologic features tends to be a functionof limited observations (e.g. interpretation of an aerial photographcapturing a single snapshot of flow conditions). The inferred exis-tence of the hydrographic feature and how that feature interactswith others is the role of the HY_HydrographicNetwork model.Defining the geometry of the feature is a function of defining theconfining landform, in all likelihood within some nominal set offlow conditions. Fig. 11 shows how confines such as HY_Reachare defined in a separate package, HY_WaterBodyConfines, andoptionally reference (via the confinedWaterBody relationship) acorresponding named hydrographic feature belonging to a network(in this case HY_WaterBodySegment).

The implications of this are that:

� it is possible to define a network of features without specifyingtheir geometry (as in existing catchment products or node-linknetworks);� it is possible to define the geometry of hydrologic features with-

out specifying their identity (as in existing topographic maps);� multiple representations of confines may be defined for a given

hydrologic feature (as in multiple maps at different scales);� there does not need to be a direct correspondence between

geometry and network segmentation – a confine may describemultiple segments.

This concept does not provide a simplified and prescriptive wayof cross-referencing geometric and network perspectives, but itdoes provide a way to describe typical datasets and the semanticsof relationships between these, whenever such cross-referencingshould be undertaken.

4. Discussion

The ad hoc approaches typically used in documenting hydrologicand other datasets does not readily support the matching of asemantic definition of a concept with its implementation in, forexample, a database column, because the semantics may be ex-pressed within external documentation or may not be expressedexplicitly at all. Many implementation platforms provideapproaches to linking semantic definitions with data structures(‘‘data dictionaries’’), but they are often used inconsistently andtend to be platform specific.

The conceptual model introduced in this paper represents areconciliation of the typical alternative semantics of hydrologic

34 R. Atkinson et al. / Journal of Hydrology 424–425 (2012) 24–36

feature definitions found in national, global and local-scale catch-ment, cartographic and network representations of river systems.The model has been designed to support consistent identificationof hydrologic features across different representations and scales,over time, and in a way that is independent of system technology.

At the core of the conceptual model is the separation of con-cerns between the connectivity of abstract hydrologic featuresand the real-world representation of the processes associated withthese features. The connectivity viewpoint may be used to refineaccepted definitions of common hydrologic features. Adoption bythe WMO-CHy will create a standard way to make explicit andname the relationships that exist between these concepts. Thus,it will be possible to design and document the implementation ofthese relationships in data in a standardised context, and interpretthem reliably by reference to the standard conceptual model. Sim-ple hierarchical models of catchment connectivity can be readilysupported, as can highly detailed models of localised hydrologicalprocesses.

4.1. Validation

The validity of this conceptual model hinges on whether it cansuccessfully represent the disparate forms of existing and emerg-ing hydrologic feature representations. To formally test the model,it would be necessary to establish a methodology for such compar-isons. The utility of the model is being tested in the development ofan integrated suite of hydrologic feature representations in theAustralian Hydrologic Geofabric (AHGF) (McDonald, 2009). A sig-nificant validation of the model will be achieved by a successfulrealisation of the key relationships defined with the HY_Featuresin data products that in turn fulfil the business requirements ofhydrologic feature identification. These products are in an ad-vanced stage of development, and their conformance and relation-ship to the conceptual model described here will be presented indue course.

The utility and acceptance in an international, multilingual con-text will be tested by the Global Runoff Data Centre of WMO(GRDC) in the course of developing the Global Terrestrial Networkfor River Discharge (GTN-R) as a service for the Global TerrestrialNetwork – Hydrology (GTN-H). Further validation outside of theseactivities is required, given the potential scope of the model.

4.2. Scope of application

The model presented here may be used in many different as-pects of hydrologic information, but its main goal is to allow stableidentifiers to be created for natural hydrologic features that can becross-referenced to alternative representations. This will make rec-onciling data from multiple sources easier, and will eventually sup-port dynamic linking of online data among information systems.Consistent reporting using stable identifiers will make analysisand reporting of trends easier.

Broader applicability of the proposed feature concept withinnumerical models of hydrodynamic processes is untested. It mightbe useful for defining units of modelling and of disaggregation, asis required for more detailed analysis of hydrologic processes. Theapplicability of the conceptual model may be limited to this situa-tion, but it may also be useful when combining the definitions ofthe feature model with those of the process model being applied.

We have not investigated the applicability of this model todynamic flooding situations that may drastically alter the effectivetopology of the system. However, the model is not expected to dif-fer from existing hydrologic feature definition practices in this re-gard, since it reflects these practices in a more complete andcoherent form. The adoption of a common model may, however,make it more practical to realise multiscale process modelling

and consistent reporting. In turn, this will allow more accuratemodelling of such processes where they are likely to affect theoverall system behaviour.

Applications in the hydrogeology and groundwater domain mayrequire generalisation of the notions of network connectivity to in-clude vertical topology and processes happening at differenttimescales.

4.3. Linear referencing: HY_RiverPositioningSystem

A key requirement in hydrologic systems is to identify a posi-tion of some location within the context of the hydrologic system;for example, a water quality sample may be taken at some arbi-trary location on a river that may be referenced to a fixed land-mark. In hydrologic applications, complex flow patterns aretypically reduced to simplified linear referencing systems for thepositioning of features, often expressed as the distance to a refer-ence point determined by coordinates.

A linear reference system is proposed; this system is intendedto define a feature position that depends on a reference point de-clared by the application. HY_RiverPositioningSystem reflects thesimplified basin hierarchy; it also defines the relative position ofreference points that tie topological representations to the physicallandscape (and is thus part of the HY_HydroFeature base model).Once a model for a simple network of reference points has beenestablished, it can be used to define positions on a notional mainchannel relative to the reference points in the model.

The HY_RiverPositioningSystem module is not complete at timeof writing; it is being analysed to investigate how it relates to gen-eralised linear coordinate reference systems and to ISO abstractmodels.

4.4. Related domains

4.4.1. HydrogeologyThe conceptual model has been discussed in the context of sur-

face water hydrology. Work is underway to reconcile models ofhydrogeologic processes with the need for feature identification.

The model allows for representations of a basin to include anyform of model or cross-domain interaction. Hence, surface–groundwater interactions could be defined for each basin, and dif-ferent scales of basin chosen to match the appropriate level ofdetail.

Network connectivity of aquifers involves processes operatingin media and timescales that differ from those of surface waterrunoff. Further work is planned to explore whether it is feasibleand useful to incorporate these aspects into the common model,or whether they should be left within specific representationmodels.

4.4.2. Estuary and marine environmentsThe behaviour of hydrologic processes in estuarine environ-

ments is complicated by the influence of tides and storms, andthe inherent low relief of such systems. The catchments of estuarysystems may be regarded in toto, or modelled in more detail ascontributions to internal estuarine processes.

Likewise, outflows to marine environments represent a specialcase that can be addressed through the use of a logical outfall node.Areas and minor streams contributing to a diffuse outflow to amarine, estuarine or internal waterbody environment may be char-acterised as basins flowing to a logical node. Multiple streams (e.g.distributaries in a delta) may terminate at a single logical node, oran indicative geographic location may be established for area run-off. The authors are currently developing and testing data productsthat use this approach, and intend to publish a detailed accountseparately.

R. Atkinson et al. / Journal of Hydrology 424–425 (2012) 24–36 35

These cases reflect the basic concept of the basin, with multiple,related, high-detail representations of internal hydrologic pro-cesses. However, identification of estuarine systems at a conceptuallevel may involve additional considerations and stakeholders. Mul-tiple names for a HY_HydroFeature, coupled with usage attribution,allow multiple stakeholders to register identifiers for the estuarysystem, provided the semantics in use are functionally equivalent.Further investigation is required to determine whether the assumedor explicit semantics in use for estuary identification in differentdomains are sufficiently similar to use this simple approach.

4.4.3. Water distribution systemsThe conceptual model presented assumes the basic hydrologic

connections driven by landform and downhill flow. This is consis-tent with many systems and datasets, but a common model needsto provide the means to handle anthropogenic influences. Watermay be diverted and returned to a system. Modelling the effectsof this is beyond the scope of HY_Features; however, the abilityto stably reference hydrologic features allows more complex inter-actions to be modelled.

The ability to handle multiple representations of a system al-lows a specific basin to be represented in an extended model thatincludes water distribution effects. Water transfers between basinsmay be modelled using an extended information model that im-ports the HY_Features model, and hence allows the reuse of the sta-ble identifiers for basins.

4.5. Comparison with existing implementations

A formal and systematic comparison of this conceptual modelwith the extensive set of datasets of hydrologic features main-tained by national and subnational agencies, and the many processmodels in use, is not a realistic goal. Nevertheless, it should be pos-sible to create a mapping between information models defined bysuch datasets and the numerical models, and the concepts definedin the conceptual model.

Mapping existing data to a common interoperability model is aprocess used by EU member states to map their datasets to the IN-SPIRE implementing rules (EC, 2007a). Work is underway withinthe AHGF activity (Atkinson et al., 2008) to develop an integratedsuite of national data products based on this conceptual model.The model itself defines the relationships between separate dataproducts for cartographic streams and water bodies, DEM-derivedflow lines and delineated watershed boundaries, and simplifiedhierarchies of catchments as both polygon and network topologyrepresentations.

Hydrological datasets will map to the conceptual model at dif-ferent levels of detail, depending on whether they have an internalorganization that defines a basin hierarchy. Techniques for describ-ing and testing this at different levels of detail in the basin hierar-chy are yet to be determined.

4.6. Pfafstetter coding

The Pfafstetter coding system (Pfafstetter, 1989) implies afeature model for hydrologic networks, and seeks to assign identi-fiers based on the contributing catchment area. Further work isrequired to determine whether this may be used reliably in con-junction with this model to automate feature identification. Theunderlying networks that Pfafstetter employs can be describedusing the common model. Thus, it would seem likely that Pfafstet-ter coding could be implemented as an extension to the model,simply providing an additional attribute for the identifiers as-signed by the Pfafstetter system. The conceptual model does notrestrict the level of detail each basin may be subdivided into. Itis equally valid to represent the cartographic view of an entire riv-

er system as a single basin, or to break the basin into very smallunits. Pfafstetter has concepts of even distribution of inferred run-off contribution of subareas within a basin, which may be realisedusing the conceptual model. Hence, Pfafstetter could be used to as-sign identifiers in these cases, but would not apply unless suchsubareas were defined in a stable fashion. The sensitivity of Pfaf-stetter encoding to redefinition of subareas would require furtheranalysis to determine whether it meets the criteria for identifierstability.

5. Conclusions

The paper describes various approaches used in different con-texts for modelling information and identifying hydrologic fea-tures. It presents key aspects of a conceptual model designed todefine hydrologic features in a consistent fashion across multiplealternative forms of representation and levels of detail. The modelis expressed in a standard, platform-independent form, and may beused as a basis for referencing the types of hydrologic features thatmay have persistent identity across multiple data systems. Thedefinitions captured in the model are based on semantics definedwithin WMO standards and official literature. The formal modelprovides greater detail by specifying standard ways of describingthe explicit relationships that exist between hydrologic features.The model defines how features in different representations ofhydrologic networks may be related. It does this by defining therelationships that may occur between features that hold persistentidentifiers matching common usage expectations and the specificrepresentation of those features on a case-by-case basis. Thesemantics of these persistent features are well defined in the con-text of broad-scale hydrologic connectivity without imposing asimplified model of hydrologic processes onto specific local-scalehydrologic models. Under the auspices of the joint WMO-CHyand OGC Hydrology Domain Working Group, the model describedhas been developed as a candidate international interoperabilitystandard to enable reuse of catchment design, and reliable regionaland global aggregation over long temporal scales.

This model improves on the available textual definitions pub-lished by WMO, by further describing and formalising the natureand semantics of relationships between the described features.The domain-specific description of features of interest comple-ments the overall description of datasets (ISO, 2003). Future workwill involve using the feature model proposed here to combinemetadata documenting the hydrologic dataset with the descriptionof the represented domain feature of interest.

The model may be used to assist in the design of datasets, datasystems and data interchange formats, and in reconciling datafrom different sources. Using the defined relationships, observersmay link sampling features to the water body intended to be sam-pled, and practitioners and modellers may describe the unit ofstudy or reporting they share. Eventually, the practising hydrolo-gist will see elements of this model as documentation of thesemantics of hydrologic features they encounter in datasets, onlinereferences to hydrologic features, or data characterising some as-pect of the hydrologic cycle. For this to happen, datasets need tobe described in relation to the model, and the model publishedin accessible (e.g. online) forms compatible with citation ofindividual definitions.

Tests are underway on using the model to link alternativerepresentations in multiple datasets in data-product design andonline data delivery contexts. The model presented could also haveapplications in many other situations where there is a need to citehydrologic features.

36 R. Atkinson et al. / Journal of Hydrology 424–425 (2012) 24–36

Role of funding source

This work is part of the water information research and devel-opment alliance between CSIRO’s Water for a Healthy CountryFlagship and the Australian Government Bureau of Meteorology.

Acknowledgements

The authors would like to thank the representatives of theWMO-CHy for their endorsement to develop this model as a contri-bution to sharing hydrological information and exchanging data ina standardised way, and improving the interoperability of water-related information systems on a global scale.

The authors would also like to thank the AHGF Project Team forongoing support of the conceptual modelling aspect of the project;in particular, Elizabeth McDonald from the Australian GovernmentBureau of Meteorology, and Paul Box and David Lemon from CSIRO.Thanks also to the many participants in the requirements-gather-ing workshops for the AHGF and the OGC/WMO Hydrology DomainWorking Groups.

Thanks also to Eric Boisvert and Boyan Broderic of Natural Re-sources Canada for their insights into the modelling process andits potential application into a future extension of the model intothe groundwater domain.

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