Publication Date: 2023-03-06
Approval Date: 2022-10-06
Submission Date: 2022-09-13
Reference number of this document: OGC 22-040
Reference URL for this document: http://www.opengis.net/doc/PER/Hydrofabric-er
Category: OGC Public Engineering Report
Editor: David Blodgett, J. Michael Johnson
Title: Hydrologic Modeling and River Corridor Applications of HY_Features Concepts
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- 1. Subject
- 2. Executive Summary
- 3. References
- 4. Terms and definitions
- 5. Overview
- 6. Use Cases
- 7. Use Case Information Requirements
- 7.1. Use Case Information Summaries
- 7.2. Information Summary
- 8. Logical Model
- 9. HY_Features Change Requests
- Appendix A: PlantUML Source for Logical Model
- Appendix B: Revision History
- Appendix C: Bibliography
Hydrologic geospatial data products contain geometries that represent features such as river segments and incremental catchments. The combination of these provides a 2D (XY) geospatial fabric (hydrofabic) that discretizes the landscape and flow network into hydrologically relevant features at a defined level of scale, resolution, or organization. Hydrofabrics have been created at the national and continental scale in many parts of the world. This engineering report presents progress on formalizing a hydrofabric for drainage basins that adheres to HY_Features concepts with a focus on the use of the concepts in modeling hydrologic processes. Furthermore, this report documents efforts to integrate river corridor data with the traditionally 2D hydrofabric representations. River corridors include the channel and adjacent land required to maintain or restore a dynamic geomorphic equilibrium.
2. Executive Summary
The WaterML2: Part 3 - Surface Hydrology Features (HY_Features) Conceptual Model was published by OGC in 2018. This report documents the use of HY_Features concepts in support of two key tasks: (1) local to continental hydrologic modeling; and (2) referencing river corridor data to hydrographic networks. The presented use cases are applicable in hydroscience research and assessments, water resources engineering practices, and drought and flood responses.
Before the HY_Features conceptual model there was no internationally recognized standard for the design of software and data for the hydroscience and engineering community. This report presents progress towards a logical data model that interprets the abstract HY_Features concepts for use in geospatial workflows, modeling applications, and web data systems that integrate hydrologic data.
The use cases addressed include: (1) hydrologic model control volume definition; (2) hydrologic network connectivity; (3) characterization of catchments with landscape and atmospheric data; (4) river corridor characterization; (5) hydrologic location; and (6) flow network location. Each use case is described briefly along with an analysis of the information requirements. This report presents a summary of the logical model designed to satisfy the needs of these use cases and a summary of updates and changes proposed for HY_Features.
Changes for consideration by the HY_Features Standards Working Group include the following.
Provide more clarity on the inherited properties and associations of features that "realize" the catchment and nexus concepts from HY_Features.
Add nexus realization feature types to represent the outlet of catchments that are "frontal" (terminate to the ocean or a large waterbody) or "inland sinks."
Add a "HY_Flowline" feature as a superclass of HY_Flowpath providing linear referencing on waterbodies that are not catchment realizations.
Add an association or interface to support connection between surface catchments and hydrogeologic units.
2.1. Document Contributor Contact Points
All questions regarding this document should be directed to the editor or the contributors.
U.S. Geological Survey
J. Michael Johnson
Lynker - NOAA Affiliate
U.S. Geological Survey
U.S. Geological Survey
Contractor to the U.S. Geological Survey
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. The Open Geospatial Consortium shall not be held responsible for identifying any or all such patent rights.
Recipients of this document are requested to submit, with their comments, notification of any relevant patent claims or other intellectual property rights of which they may be aware that might be infringed by any implementation of the standard set forth in this document, and to provide supporting documentation.``
The following normative documents are referenced in this document.
4. Terms and definitions
For the purposes of this report, the definitions specified in Clause 4 of the HY_Features conceptual model standard OGC 14-111r6 shall apply with the addition of the following terms and definitions.
Like the catchment feature type, a drainage basin is a holistic feature defined as the total upstream area draining to an outlet. It is comparable to a catchment with no inflows and a single outlet. Drainage basins can be thought of as the total accumulated or total upstream catchment and can be described with a pair of locations defining: 1) the headwater area with no discernible flowpaths where flow initiates; and 2) the outlet where flow enters a larger river, waterbody (including oceans), or inland sink. A single mainstem flowpath connects a drainage basin’s headwater to its outlet. See  for additional discussion.
A flowline is a one-dimensional (linear) feature that represents a flowing body of water and is functionally similar to a flowpath but does not realize the catchment concept and as such does not have flow from or to a hydrologic nexus. A flowline should be thought of as a hydrographic connector with an inlet and an outlet that does not receive lateral flow from a hydrologic unit.
A headwater is scale-dependent and represents the most upstream location where water can exist in a drainage basin or catchment where flow initiates. Such a point is typically along a drainage basin boundary. Given that the definition of a flowpath does not necessitate the existence of water, headwater can be imagined as a point where an extended flowpath touches a drainage basin boundary. See  for additional discussion.
Hydrologic Geospatial Fabric | Hydrofabric
A hydrologic geospatial fabric (hydrofabric) discretizes a landscape according to hydrologic processes and the hydrologic network that conveys water and its constituents downstream. This integration of spatially extensive catchments and an expansive network of flowpaths enables integration of landscape and river data for hydroscience applications. Conceptually, a hydrofabric includes hydrogeologic features and their association to surface hydrologic features but these associations are not typically represented in most current hydrofabrics.
The mainstem concept extends and constrains the concept of a flowpath by designating a single path from headwater to outlet through a drainage basin. In other words, a mainstem is a linear realization or backbone of a drainage basin. See  for additional discussion.
A river corridor is part of a fluvial system that includes the landscape of streams, floodplains, and wetlands that are connected by surface water and/or groundwater and function as a continuum longitudinally and laterally with exchanges of mineral and organic components as water flows downstream to a lower elevation under the force of gravity. See  for additional discussion.
Section 6: Use Cases introduces the use cases considered for this engineering report. They involve hydrofabric definition for hydrologic modeling and river corridor data integration.
Section 7: Use Case Information Requirements provides important details for understanding the use cases.
Section 8: Logical Model outlines the logical data model used in experiments and ongoing development activities.
Section 9: HY_Features Change Requests summarizes the findings in relation to the HY_Features baseline concepts.
6. Use Cases
This report concerns two top level use cases:
geospatial frameworks for hydrologic modeling; and
river corridor data integration.
These use cases have considerable overlap but serve very different goals. The Hydrologic Model Control Volume Definition, Hydrologic Network Connectivity, and Characterization of Catchments with Landscape and Atmospheric data, and River Corridor Characterization use cases align directly with the top-level summary use cases. The Hydrologic Location and Flow Network Location are applicable to both. Locating information assets along hydrologic flowpaths is the primary area of overlap. Together, these use cases are representative of a complete "hydrofabric," which encompasses both the hydrology and the river corridor specific use cases.
The following sections describe a collection of use cases that illustrate the two summary use cases. Each begins with a relatively specific example followed by more general discussion of the use case.
6.1. Hydrologic Model Control Volume Definition
Hydrologic modelers need to define spatial features (catchments) to be used as control volumes for water budget calculations. Each unit may produce excess runoff that must be "routed" through downstream catchments.
In hydrologic analysis, the concept of a control volume is a common abstraction used to track the fluxes and state variables of a physical system. The catchment is a hydrology-specific control volume with specific characteristics: a realized flowpath connecting inflow (or headwater) to outflow; and an area bounded by a divide that connects landscape processes to a flow network.
The factors defining a control volume go beyond the scope of this report. However, the characteristics of the spatial features that can be altered to meet provided objectives include catchment area and flowpath length. A catchment can be made larger by encapsulating (aggregating) more tributary networks or smaller by splitting the catchment and realized flowpath into one or more smaller catchment features.
6.2. Hydrologic Network Connectivity
A hydrologist will often need to find information such as monitoring data upstream or downstream of an unmonitored study location in order to predict hydrologic behavior. The network upstream of a location may also be used to accumulate landscape characteristics to further aid in predicting water quantity or quality at an unmonitored location.
When represented as a collection of areas, catchments form a continuous feature coverage that partitions the landscape into hydrologic units. This coverage can have a connected graph of realized flowpaths that define the hydrologic network. This hydrologic network can be used to define control volumes for models of flowing water and as a reference system for hydrologic locations such as streamgages, water intakes, bridges, and other application relevant on-network features. The hydrologic network can be thought to exist whether a channel contains water at all times or not. With this in mind, methods for determining connectedness are beyond the scope of this report.
A hydrologic network enables four common operations (sub-use cases).
Network navigation, where network connections are followed to find hydrologically connected features, or to subset a drainage basin for a known outlet.
Accumulation, where the network is used to infer accumulation of attributes or flow constituents along the network. This application is similar to activities like [EPA’s StreamCat program](https://www.epa.gov/national-aquatic-resource-surveys/streamcat-dataset) .
River identification, where the network connectivity is used to infer collections of flowpaths that make up the main path of a river within a drainage basin.
Hydrologic addressing, where locations of interest can be associated with the hydrologic network to query control volumes, inject structural information about the network, or initialize river identification or network navigation.
6.3. Characterization of Catchments with Landscape and Atmospheric data
A water quality analyst may need to know the percent coverage and spatial distribution of landscape characteristics (e.g., agricultural practices, impervious area, or shallow groundwater) in the catchments of a hydrologic geospatial fabric so they can make estimates of water quality in a downstream location.
Catchment characterization is a key use case for hydrologic modeling and related applications. For this use case, a catchment divide polygon is used as a spatial unit to subset static or time-varying spatial data (e.g., landscape, geology, and atmospheric data). In hydrologic modeling, catchment characteristics are used as model parameter estimates, meteorological forcing inputs, or baselines to inform calibration. In other applications, catchment characteristics can be used directly in an analysis or decision process (determining presence or absence of certain conditions within a drainage basin, for example).
6.4. River Corridor Characterization
A geomorphologist needs to estimate in-stream and overbank river corridor characteristics to inform sediment transport patterns, plant species distribution, and floodplain elevation zones to better understand and improve floodplain ecosystem functions. This information must be broken up into different ecological, geomorphic, and hydrogeologic zones depending on the hydraulic, ecological, and groundwater setting of the river corridor.
River corridor characterization is important for estimating hydraulic or hydrologic routing models as well as geomorphic, ecosystem, groundwater, and other applications. Unlike catchments, which have an explicit 2D spatial expression, river corridors can be 1-dimensional (e.g., river center line or bank line), 2-dimensional (e.g a floodplain delineation), or 3-dimensional (e.g., a channel geometry model). Given that a river corridor can have these different representations, the characteristics that describe them are quite diverse.
6.5. Hydrologic Location
An emergency response coordinator needs to know what access locations and sensitive infrastructures lie downstream of a contaminant spill or flood event in order to dispatch monitoring teams and warn infrastructure operators, such as water treatment facilities. Hydrologic locations (river addresses) are essential information in this task.
Hydrologic locations link the network to any data that can be idealized as a point location. Any location that can be located along a flowpath is conceptually a hydrologic location. As a use case, there are two ways to define hydrologic location for modeling and analysis: (1) determining the specific flowpath a location is associated with (hydrologic addressing) and (2) using known locations.
To determine a hydrologic location, river identification (a sub-use case of hydrologic network connectivity) is key, and additional information is often required to determine if a location is on a main river or a nearby tributary. The integration of existing hydrologic locations is a major component of any modeling or analysis that supports assimilating information from sources such as streamgages or that reports information along the network at these locations.
6.6. Flow Network Location
A civil engineer responsible for maintaining a navigation channel along a major river must maintain detailed records of cross sections and bathymetries in channels in and among islands in the middle of a major river. The channels are artifacts of sediment transport and dredging and do not have well-defined catchments but are part of a large river system.
In contrast with the hydrologic location use case, flow network locations are not necessarily associated with a catchment or hydrologic network. Rather, they are associated with a flowline feature that forms a hydraulic connection between waterbodies. Determining which flowline a flow network location belongs to is similar to hydrologic location, but accumulated hydrologic attributes such as total drainage area can not be used to disambiguate which flowline the hydrologic location belongs to.
Functionally, flow network locations are more general, and normally a more resolved, version of hydrologic location. The need for this is to locate features such as cross sections or sediment samples along certain branches (flowlines) of rivers rather than locating features such as streamgages on a main stem that monitor flow from the entire drainage basin.
7. Use Case Information Requirements
All above use cases require common information to describe networks of flowing water. Hydrologic phenomena are landscape-wide processes that coalesce into paths of preferential flow (flowpaths and flowlines). Fluvial phenomena are an expression of hydrologic phenomena, yet are confined to river corridors where flowing water interacts with sediment and the ecosystem to form complex flowing systems. Hydrologic phenomena are considerably more abstract and can be studied across the widest range of spatial scales — continental to sub-field.
With this context, the following information types are required for all use cases.
7.1. Use Case Information Summaries
7.1.1. Hydrologic Model Control Volume Definition
As shown in Figure 1, a hydrologic modeler seeks to understand control volumes at various hydrologic locations. Therefore, a clear and concise knowledge of modeling objectives driven by the spatial domain, functional needs, model complexity, and other factors must be provided; i.e., a collection of important hydrologic locations and a pre-existing catchment network. In many cases, the pre-existing network is an elevation surface (where grid cells act as catchments) and hydrologic locations are provided "pour points" where catchment outlets are desired.
7.1.2. Hydrologic Network Connectivity
Hydrologic connectivity is typically implicit in hydrographic datasets. That is, the geometric topology of the realized flowpath can be transformed into a feature topology that links realized catchment features. This is true whether the source of geometric topology is a collection of vector geometries or a path of high-flow accumulation derived from elevation data. For this use case, the hydrologic modeler must determine how the hydrologic network should be represented (as an edge list, an edge/node graph, strictly dendritic, etc.) and what hydrographic data are needed to produce the connected network.