WaterML2 Part 3: Surface Hydrology Features (HY_Features)

David L. Blodgett
06-2017

A conceptual feature model for surface hydrology.

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What is HY_Features?

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HY_Features is a conceptual model describing hydrology features and their relationships

  • The model describes hydrology specific feature types, e.g. catchments, drainage networks, rivers, lakes and waterbodies.
  • Defined in the Unified Modeling Language (UML).
  • All features specialize the General Feature of the ISO / OGC General Feature Model.
  • Most features “specialize” a general HY_HydroFeature providing multilingual naming.
  • HY_Features models hydrologic units and surface waterbodies.
  • Focus is on surface water features and the networks they form.

What is HY_Features for?

HY_Features provides a common conceptual basis for identification of hydrologic features.

This should help provide:

  1. shared feature types defined for use in the hydrology domain,
  2. a common language for documenting datasets,
  3. a standard conceptual model for data and software,
  4. a means to unify hydrologic feature identities across data products.
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What kinds of features are in HY_Features?

HY_Features includes: hydrologic units, the junction features that join them, and the waterbody features that drain them. The core feature concepts are:

  • catchment: a holistic and abstract hydrologic unit concept, that links inflow and outflow.
  • hydro nexus: a holistic and abstract junction concept, that connects interacting catchments.
  • realization: a hydrology-specific way of interpreting a holistic feature.
  • waterbody: an identified body of water, that accumulates runoff in a catchment.
  • container: a depression or channel that may hold a waterbody, that drains a catchment.
  • hydro network: a network of catchments that forms a larger, containing catchment.

Note: Catchment and nexus are not modeled as abstract classes in UML. Abstract is used to describe that these are meant to represent the abstract notion of a hydrologic unit or junction.

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What is a catchment?

Catchment is a holistic abstract notion of a hydrologic unit (a feature type).

It is a physiographic unit defined by a common outlet (and potentially an upstream inlet.)

In other words a catchment is…

  • the basic unit of study in hydrology and water resources management
  • a hydrologic system that drains to a defined outlet
  • an area in which hydrologic processes take place
  • the link from one inlet hydro nexus to one outlet hydro nexus
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What is a Hydro Nexus?

A hydro nexus is a place where two or more catchments interact.

Theoretically, there is a hydro nexus anywhere on the landscape.

However, we usually identify significant places to break up our catchments.

A hydro nexus…

  • can act as a pour point at the boundary of a catchment
  • can act as an inlet to or an outlet from a catchment
  • can receive flow from and give flow to multiple catchments

Note: Typically, a hydro nexus is a location at the end of a flow path, but other realizations, such as areas of infiltration, could be introduced in future work.

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What is hydrologic realization?

Catchment and hydro nexus are meant to be holistic and have several hydrology-specific interpretations.

HY_Features deals with this by saying they can be realized in a number of ways.

Note that “realization” and “realized” are UML class association names between a holistic feature (the realized feature) and a specific feature (the realization feature).

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What is hydrologic realization?

As shown in this figure, a catchment can be realized as a divide (A.), an area (B.), a main flowpath (C.), a network of catchments (D.), a network of waterbodies (E.), a dendritic topologic network (F.), a complex feature network (G.), or a collection of monitoring sites (H.).

Note that these realizations are meant to be conceptual. The graphics are only meant to illustrate the concept.

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How do waterbodies work in HY_Features?

  • First, consider a main flowpath waterbody of a catchment.
  • Zoom out to a network of sub-catchments and their flowpaths.
  • Now we have a network of waterbodies, each of which is a realization of a sub-catchment.
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How do waterbodies work in HY_Features?

  • A main flowpath or network of waterbodies realizes a catchment.

  • A main flowpath or network delivers a catchment's flow to a hydro nexus.

  • A hydro nexus is realized by a hydro location on the waterbody network.

img

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Waterbody – Hydro Location and Catchment – Hydro Nexus

The relationship between these four types of features, is best illustrated as a sequence of steps.

  1. Given a waterbody feature, lets say it's a line.

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Waterbody – Hydro Location and Catchment – Hydro Nexus

The relationship between these four types of features, is best illustrated as a sequence of steps.

  1. Given a waterbody feature, lets say it's a line.
  2. Choose two locations on that line, you have two hydrologic locations.

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Waterbody – Hydro Location and Catchment – Hydro Nexus

The relationship between these four types of features, is best illustrated as a sequence of steps.

  1. Given a waterbody feature, lets say it's a line.
  2. Choose two locations on that line, you have two hydrologic locations.
  3. Delineate the divide around the area that drains to the blue line.

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Waterbody – Hydro Location and Catchment – Hydro Nexus

The relationship between these four types of features, is best illustrated as a sequence of steps.

  1. Given a waterbody feature, lets say it's a line.
  2. Choose two locations on that line, you have two hydrologic locations.
  3. Delineate the divide around the area that contributes to the blue line between the two hydro locations.

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Waterbody – Hydro Location and Catchment – Hydro Nexus

The relationship between these four types of features, is best illustrated as a sequence of steps.

  1. Given a waterbody feature, lets say it's a line.
  2. Choose two locations on that line, you have two hydrologic locations.
  3. Delineate the divide around the area that drains to the blue line.
  4. Give the delineated hydrologic unit a name and ID. This is your catchment.

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Waterbody – Hydro Location and Catchment – Hydro Nexus

The relationship between these four types of features, is best illustrated as a sequence of steps.

  1. Given a waterbody feature, lets say it's a line.
  2. Choose two locations on that line, you have two hydrologic locations.
  3. Delineate the divide around the area that drains to the blue line.
  4. Give the delineated hydrologic unit a name and ID. This is your catchment.
  5. Now that you have identified your catchment, you can consider your hydrologic locations to be realizations of hydrologic nexuses of your catchment.

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Waterbody – Hydro Location and Catchment – Hydro Nexus

Now we have catchment 12 with inflow hydro nexus 34 and outflow hydro nexus 56.

Catchment 12 is realized as a linear divide polygon, a planar area and a main flowpath line.

Hydro nexuses 34, and 56 are realized as hydro locations.

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Waterbody – Hydro Location and Catchment – Hydro Nexus

In many cases, several alternate geometric representations for the catchment defined between Nexus 34 and 56 will exist:

  • for mapping at different scales
  • from differing data capture or processing methods (e.g raster, vector)
  • originating from different organizations
  • for representing different processes, for example surface water or groundwater contributing catchments

As long as alternate data products represent the same hydrology-specific catchment realization and identify the same realized catchment, they can be associated directly along with any related data and reports.

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Networks of Waterbodies or Channels and Catchment Interactions

A catchment can be realized as a network of waterbodies, shown here as a simple network of blue lines meant to represent waterbodies.

A catchment can also be realized as the network of its channels, shown here as a network of orange lines that may contain the waterbodies.

Also illustrated here, two catchments and their associated waterbodies/channels interact at a hydrologic location that is referenced to the waterbody/channel network that realizes the whole catchment.

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Networks of Waterbodies or Channels and Catchment Interactions

If we create hydro locations at junctions in the waterbody network and associate them with hydro nexuses and catchments, it becomes clear that a waterbody or catchment (boundary) network is also be a network of many individual catchment realizations.

If we connect water bodies with hydro locations at their inlets and outlets, we get a network of catchments interacting at their hydro nexuses. By using consistent identifiers for the hydro nexuses and the associated catchments, datasets representing waterbodies/channels that are built at different scales can be tied together.

The ability to both tie datasets together accross scales and to support linking various data sets that represent the same real-world features, is the primary goal of HY_Features conceptual model. Future work will be able to build on this conceptual basis, adding formal encodings for linking data together and encoding hydrographic data directly.

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Ways of Conforming to HY_Features – Conformance Classes

1) Data Model Mapping
HY_Features can be useful in describing the characteristics of an existing data model. In this case, conformance is accomplished by recording how a given dataset “maps” onto HY_Features. Note that the actual level of coorespondence can be low. As long as the mapping itself considers all mandatory elements of HY_Features, it would be considered conformant.

2) Implementation of an Encoding of HY_Features
In this case, all mandatory properties, associations, and default values are required in the implementation for one or more HY_Features feature type. Implementations would necessarily extend and profile the HY_Features conceptual model, adding specific geometric representations and specifying which HY_Features featuretypes are to be included.