# arc hydro groundwater data model - .1 arc hydro groundwater data model gil strassberg and david r

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ARC HYDRO GROUNDWATER DATA MODEL

Gil Strassberg and David R. Maidment

ABSTRACT: The Arc Hydro data model is a synthesis of geospatial and temporal information that characterize a hydrologic information system which supports surface water hydrologic analysis and modeling in the ArcGIS environment. The wide use of groundwater information and attempts to integrate groundwater and surface water models necessitate the extension of the current model to describe the groundwater environment. Integration of the groundwater data model into Arc Hydro will enable the representation of surface water and groundwater features simultaneously within ArcGIS and will support the connectivity between models, thus providing a better understanding of the hydrological cycle. Representing the 3-dimensional characteristics of groundwater systems introduces an additional level of complication to Arc Hydro. Features such as geological formations, wells and the representation of flow in aquifers require 3-dimensional representation and visualization capabilities. The groundwater data model is designed to be scalable and will support the management of regional data as well as site scale information. It will enable the connectivity and the ability to support modeling. The GIS representation of the groundwater systems is utilized to develop interfaces between the ArcGIS environment and groundwater models (e.g. MODFLOW). Examples illustrating these concepts include large scale simulations of water availability in Texas, site scale contamination investigations and a regional scale simulation model in Florida. KEY TERMS: Groundwater; Data model; Geographic Information Systems; Arc Hydro; ArcGIS.

Arc Hydro is a geographical data model that describes hydrological systems. A data model is a set of concepts expressed in a data structure; the data model describes a simplification of reality using tables and relationships within a database. Geographic data models use database structures to describe the world or part of it using Geographic Information Systems (GIS) technology. The Arc Hydro data model is a conceptualization of surface water systems and describes features such as river networks, watersheds and channels . The data model can be the basis for a hydrologic information system which is a synthesis of geospatial and temporal data supporting hydrologic analysis and modeling (Maidment, 2002). Arc Hydro integrates geospatial and temporal information into a defined structure. Based on this structure analysis and modeling tools can be applied. The data model provides a common characterization and understanding of the hydrological system and this description can be utilized by multiple models, analysis tools and decision support systems all referring to the same common structure. Figure 1 shows the main components of the model.

Flow

Time

Time Series

Hydrography

Hydro Network

Channel System

Drainage System Flow

Time

Time Series

Flow

Time

Time Series

HydrographyHydrography

Hydro NetworkHydro Network

Channel SystemChannel System

Drainage SystemDrainage System

Figure 1. Components of the Arc Hydro data model describing surface water

The majority of water resource applications approach groundwater and surface water as separate elements. By

incorporating a groundwater component into the existing Arc Hydro data model, users will be able to represent both groundwater and surface water characteristics simultaneously. This will provide a more comprehensive description of the hydrological cycle including groundwater and surface water within the data model and support a wider array of applications for Arc Hydro users. When designing a data model one has to define the main components which define the system modeled. Thus the first step in the design process is the conceptualization of the system and the definition of the objects required to characterize it. Aquifers, wells, hydrogeologic units and boreholes are some of the main objects commonly used in

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groundwater studies. Once the components of the data model are defined, the attributes of the objects and the relationships between them are designed to capture the behavior of the system. Figure 2 shows a conceptual view of the Arc Hydro data model with groundwater components. Arc Hydro will include a groundwater description, the components of the groundwater and surface water will be related, and both will be associated with temporal information. It is important to keep in mind that the design of the groundwater section is still in design phase and the components shown in the figure are not final.

HydrographyHydrography

Hydro NetworkHydro Network

Channel SystemChannel System

Drainage SystemDrainage System

Time

Time Series

Time

Time Series

Time

Time SeriesSurface water

Temporal

Hydrogeologic unit Borehole

Groundwater

WellAquifer

Figure 2. Components of the Arc Hydro data model including groundwater objects (components for the groundwater

section are still in the design phase) Representing the 3-dimensional nature of the subsurface and describing the hydrogeologic architecture of

groundwater systems in a GIS framework is an important aspect of this study. The 3-dimenasional nature of groundwater systems introduces a new level of complexity into the Arc Hydro design. Traditionally, GIS applications focus on 2-dimensional problems and many models used 2 or 2.5 dimensions to represent the subsurface and characterize groundwater systems. Turner (1989, 2003) emphasizes the importance of 3-dimensional characterizations in subsurface studies. He stresses the need for accurate 3-dimensional data to describe depositional systems and aquifer heterogeneity in order to accurately simulate hydrodynamic flow. This need has lead to the development of a unique branch of GIS specialized for 3-dimensional modeling for geoscience applications called Geoscientific Information Systems (GSIS). The GSIS is mainly differentiated from GIS by its 3-dimensional spatial representation, either by using volume elements or surface representations (Setijadji, 2003). Turner (2003) and Fisher (1993) provide more detailed descriptions of volume and surface representations used in subsurface studies. The ability to describe the 3-dimensioanl properties of the subsurface within Arc Hydro is dependent on the available objects in ArcGIS. 2-dimensional objects such as points, lines and polygons can be used to create a 2-2.5 dimensional representation of regional groundwater systems. These conceptualizations are reasonable because in large scale cases the horizontal dimensions of the geological systems are usually several orders of magnitude larger than the vertical dimension. Such characterizations may not be adequate for site specific studies such as hazardous waste site investigations and contaminant modeling. In these cases a detailed 3-dimensional representation of the site is needed. Figure 3 shows an example of 2-dimensional representations of groundwater systems, the Edwards and Wilcox aquifers in Texas, and a 3-dimensional representation of a nuclear waste site, the Savannah River Site in South Carolina.

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Figure 3. 2-dimensional regional representation of the Edwards and Wilcox aquifers (left) and a 3-dimensional

representation of the Savannah River nuclear waste site (Right)

Three-dimensional objects currently exist in the form of 3D points, 3D lines and 3D polygons. Figure 4 shows the available features within ArcGIS, these features can be used to represent groundwater objects such as wells, aquifers and boreholes in 2 and 3 dimensions. These features can be stored within one geodatabase and related to each other and to time series through relationships in the database.

2D features: Points, lines and polygons

Points - Wells

Lines- River network

Polygons - Aquifers

3D features: 3D points, 3D lines and 3D polygons

3D points 3D lines

3D polygons

Tables with information

Time series

Geodatabase

Figure 4. ArcGIS 2-dimensional and 3-dimensioanl features

The above features can provide 3-dimensional views of data but there is still a lack in the representation of solids

and tools that operate in 3 dimensions (e.g. 3-dimensional intersections and queries). A 3-dimensional characterization is still possible within Arc GIS using the above features and relationships within the geodatabase, but it will lack many of the 2-dimensional geographic relationships and analysis tools available in 2-dimensions. Other GIS capabilities inherent within GIS software can contribute to describe groundwater systems . Bonham-Carter (2000) outlined the activities applicable for geosciences: these include data organization, visualization, spatial querying, data integration, data analysis and supporting decision making. These activities can be supported by a data model which provides a structure for describing groundwater objects.

Once the data model is completed it provides a common conceptualization of the groundwater system and can be the basis for modeling and decision support systems. Information can be extracted from the data model for specific tasks and results can be written back to the data model. There are two common ways to interface with data models: The first is to build custom tools which use the data model structure. This option is applicable when the application is designed with the data model in mind. Many preexisting models can not directly interface with the data model, for example MODFLOW inputs are text files and the model can not directly read inputs from the geodatabase. In these cases a data model in