by Gil Strassberg and Suzanne Pierce
Table of Contents
Groundwater resources compose approximately 0.6% of the world’s freshwater resources, second only to glacial ice reserves (Fetter, 1994). These important stores of water represent a significant portion of the hydrologic cycle that must be understood and incorporated into water resource assessments and studies for a wide range of uses.
The majority of water resource applications in common use today approach groundwater and surface water as separate elements. By incorporating a groundwater module into the existing Arc Hydro data model, users will be able to represent both groundwater and surface water characteristics simultaneously.
The need for an Arc Hydro Groundwater Data Model
The Arc Hydro data model defines a “hydrologic information system” which is a synthesis of geospatial and temporal data supporting hydrologic analysis and modeling (Maidment, 2002). Since its development the data model has come into widespread use by data producing agencies, as well as private and public user groups.
The incorporation of groundwater elements is necessary in order to provide a wider array of applications to Arc Hydro users and provide a data model that reflects surface and ground water interactions within the hydrologic cycle more accurately.
An Arc Hydro Groundwater Data Model will:
Eventually, provide a suite of groundwater data analysis tools that are readily accessible to users through the Arc GIS program interface.
Goals and Objectives
An initial set of goals has been defined to provide long-term vision and general direction to this project. In addition, conceptual objectives have been developed to identify logical steps needed to achieve the broader goals. A list of the goals and objectives is presented in the following table:
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Arc Hydro Groundwater Data Model Development Project Goals & Objectives |
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Goals
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Objectives
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Groundwater applications range from regional studies, which usually describe the flow in aquifers as 1-dimensional, to site investigations that model the 3-dimensional nature of the flow through the aquifer architecture. Other common applications use the interaction between surface water and groundwater as part of surface water modeling. Recharge of streams into aquifers and discharge from aquifers to the surface are important for surface water studies as well as groundwater studies.
To meet the demands for the wide-ranging types of groundwater and surface water models and allow for an array of applications, two models are envisioned as necessary. Initially these models are designed independently but will ultimately be connected.
Model 1: 2-Dimensional surface-groundwater interaction data model:
This model will be embedded into the current Arc Hydro data model. The 2-D data model will be useful for studying subjects such as aquifer recharge from streams, water balances and contaminant transport between surface and groundwater systems. The primary focus of this data model is upon the points of river networks and groundwater interaction. This is a logical starting point because many key groundwater elements are observed in conjunction with surface water expressions.
Model 2: 3-Dimensional representation of groundwater systems
This model will provide a 3-dimensional representation of the geologic framework, aquifer architecture and boreholes within groundwater systems. This data model will be useful in regional groundwater studies as well as site investigations. Once the data model is completed it will also enable an interface with groundwater modeling software such as Modflow. The following sections describe the development of data models in more detail:
2-Dimensional surface-groundwater interaction data model:
The 2-dimensional model is designed to be embedded in the existing Arc Hydro data model. In a sense, the groundwater data model development is being constructed from the surface water interface downward into the subsurface. Three feature classes: aquifers, wells and edges are the building blocks of the model. The following diagram shows the groundwater features and the relationships between them.
Figure 1 - Groundwater feature classes embedded into the Arc Hydro data model
The three feature classes used in the data model inherit characteristics from the HydroFeature class and the relationships between the feature classes give the logical connections between features.
Aquifers are represented by a polygon feature class. These features can be related to edges of the river network and to wells. Wells are represented as point features and the edges as polylines. The relationships (1 : *) show that each aquifer can be related to a number of wells and a number of edges. The relationships provide a link from the river network to the aquifer and to the wells.
The edge feature class already exists in the Arc Hydro data model (HydroEdge), thus only the aquifer and well feature classes need to be added to the data model. The following diagram shows the relationship between the existing Arc Hydro data model and the newly incorporated groundwater features.
Figure 2 - Arc Hydro framework with groundwater feature classes
As the image above shows the Arc Hydro framework is connected to the aquifer and well feature classes through the HydroEdge feature class.
The relationship between the edges and aquifers is based on spatial intersections between the two features. The aquifer features include an attribute describing if the feature is part of the aquifer's outcrop or downdip, and only the HydroEdge features that intersect the outcrop will be related to the aquifer. The following image illustrates this concept.
Figure 3 - River edges intersecting the aquifers outcrop
The river edge is intersected with the outcrop of the aquifer and is split at the intersecting points. This process creates multiple edges from one edge, and the new edges can be related to the aquifer outcrop. When this process is applied systematically over all edges of the network, a new network can be generated, built from the split edges. This network can then easily be related to aquifer outcrops and subsequently from the aquifer to wells.
An example that illustrates the process of splitting a network and defining its association with an aquifer is presented for the Guadalupe basin river network at its intersection with the Edwards south and Wilcox aquifers. The intersection points are used to split the original network and construct a new network that is related to outcropping sections of the aquifers.
Figure 4 - Guadalupe river basin intersecting the Wilcox and Edwards south aquifers
In addition to the relationship between river edges and aquifers, the connection between well features and aquifers can also be defined. Wells are related to aquifers through an aquifer identifier (AquiferID). The aquifer-well relationship describes in which aquifer the well is screened. Although the well may be drilled through several layers and aquifers the actual connection is through the wells screen. The following image shows the wells features based on their relationship with the aquifers.
Figure 5 - Wells screened in the Wilcox and Edwards south aquifers
Once the relationships between river network edges, aquifers and wells are established, applications such as water balances, recharge/discharge estimations and solute transport between the surface and groundwater systems can be represented.
3-Dimensional representation of groundwater systems
Hydrogeologic units, layers and Hydroelements
The 3-dimensional data model describes the hydrogeologic architecture of groundwater systems, including the extent and thickness of geologic formations, physical properties of the subsurface layers and the description of boreholes. In order to model the continuous architecture of the subsurface, the physical characteristics are grouped into hydrogeologic units and these are subdivided into layers. Layers are assigned physical characteristics (such as lithology, hydraulic conductivity, porosity etc.) and the hydrogeologic unit groups the layers into one meaningful assemblage (such as an aquifer system).
Figure 6 - Hydrogeologic units and layers
Layers can be composed from more than one feature. The simplest case is a homogeneous layer represented by one polygon feature. This type of description is frequently used in regional assessments. This representation is not very useful in detailed studies where the layer properties are not assumed to be homogeneous. To accommodate the need of more detailed information a Hydro Element feature class was created. The Hydro Element feature is a polygon feature class that divides the extent of the layers into subsections. As the following image illustrates, the Hydro Element can be viewed as a vertical slice of the hydrogeologic unit, where the same Hydro Element is related with a number of layers.
Figure 7 - Hydro element related to layers and hydrogeologic unit
A data model design of the Hydro Element features and the hydrogeologic units and layers is presented below. In the data model only the Hydro Elements are feature classes. Thus the only spatial class in the data model is the Hydro Element. These features are assigned relationships with the layers and through the layers with the hydrogeologic unit. The Hydro Elements may also be related to time series, which contain temporal information. The conceptual framework demonstrating key features for the 3-dimensional data model is presented below
Figure 8 - Classes and relationship of the 3-dimensional data model
The following images illustrate this concept for a case study of the Savannah River Site (South Carolina) Old Radioactive Burial Ground. The Fishnet function, which generates an assembly of polygon features in a grid form, was used to create the models HydroFeatures. Attributes such as hydraulic conductivity, layer thickness and transmisivity can be assigned for each feature in the fishnet. In the data model these attributes are stored in the "Layers" table. Zonal statistics, in spatial analyst, can be used to transfer information from grid format into the fishnet by averaging the grid values over each fishnet feature. In this case the zonal statistics were used to transfer elevation information from a DEM to the fishnet.
Figure 9 - Savannah River Site Old Radioactive Burial Ground model grid
The Savannah River Site conceptual model consists of 5 geological layers; 3 layers are water baring formations and 2 are clay layers. Each HydroElement is related to 5 features in the "Layers" table (one for each layer). Properties such as the layer top elevation and the layer thickness enable the presentation of the data in 3-dimensions. The following image shows the architecture of the site using Arc Scene to present the information in 3-dimensions.
Figure 10 - Savannah River Site layers presented in 3-dimensions using Arc Scene
Download an example of the 3-dimensional data model applied on the Savannah River Site
Boreholes
The location of boreholes, their 3-dimensional characteristics and the information related to boreholes are important for groundwater studies. Boreholes provide a “window” into the subsurface and the primary source for information regarding the state of an aquifer at any given time. The majority of data describing aquifer characteristics and temporal groundwater conditions are related to boreholes; therefore the development of a borehole feature is important to accurately describe the groundwater system.
A borehole can be viewed as a point feature with attributes defining measures in the vertical dimension, or as a line feature (vertical line) with events on the line giving the same information. In both cases the well contains information in 3-dimensions. Boreholes can hold two types of information. The first is related to the aquifer architecture (such as lithology), which is usually recorded in borehole logs as the borehole is constructed and can generally be assumed as constant over time. The second data type is temporal information such as the hydraulic head or water quality measurements. Temporal data is usually related to well screens, where the information is collected. The screens can be related to data in time series format. Each borehole may have a number of screens and each screen can have its own time series.
Information in the vertical dimension can be measured from the land surface elevation or from a benchmark, which is a known location with a measured elevation. An example of this type of information is water level data that might be incorporated into a potentiometric surface map. The following image shows these concepts.
Figure 11 - Schematic representation of boreholes
A data model design for the borehole features is shown in the following image. The boreholes are represented by a point feature class, which are related to borehole intervals. The borehole intervals can be associated with time series. The borehole point feature will give the spatial location of the well and the interval table will represent the vertical dimensions by measurements referenced to a known elevation (benchmark elevation or land elevation).
Figure 12 - boreholes data model
3-Dimensional presentation of information
Groundwater systems have many 3-dimensional characteristics. Displaying this information in ArcGIS is important to achieve a better understanding and presentation of the 3-dimensional nature of the subsurface. Data such as geologic layers, flow patterns and well locations can be better understood when displayed in a 3-dimensional manner. Many of the computations needed in groundwater modeling also require the calculation of volumes. The current ArcGIS capabilities enable presentation of data in 3-dimensions but do not yet provide solid objects from which volumes can be calculated.
ArcGIS ArcScene extension facilitates the display of 3-dimensional information by extruding features to a given attribute value. For example, a well point feature can be extruded from the land surface elevation to the depth of the wells screen or a layer can be visualized by extruding the Hydroelement polygon feature from the layers top to its base.
The following examples show the use of extruded features to visualize groundwater systems.
Figure 11 - 3 dimensional presentation of the woodbine aquifer thickness
The above image presents the thickness of the Woodbine aquifer (located in Northeast Texas) as an extruded fishnet. A fishnet is an assembly of polygon features that create a mesh. Each feature in the fishnet can be assigned attributes describing the geology of the system, such as layer thickness or lithology. The features of the fishnet can also store hydrogeologic characteristics such as hydraulic conductivity, porosity etc. Fishnets can be used to create the HydroElement features discussed previously.
Many of the available information describing groundwater systems is currently available in grid format. This data can be averaged over the fishnet features using zonal statistics functions in ArcGIS allowing for the migration of the available data from grid format to vector format (in this case polygons) and then displaying the data in 3-dimensions.
The image below shows a 3-dimensional presentation of the woodbine aquifer with the wells extruded from the surface to their screening depth. The upper layer shows the aquifer top, which is divided into the outcrop and the downdip, and the bottom layer (in black), shows the aquifer base. The aquifer top and base are given in grid format and the outcrop and downdip are stored as polygon features. The base heights of the polygon features are set equal to the aquifer’s top which enables the presentation of the polygon features in 3-dimensions.
Figure 11 - 3 dimensional presentation of wells screened in the Woodbine aquifer
These examples highlight the capabilities of ArcScene to visualize groundwater systems in 3-dimensions. There is still a need for stronger 3-dimensional capabilities, particularly for the creation of solids and volume computations together with 3-dimensional geoprocessing tools, such as intersection of 3-dimensional features.
The construction of the groundwater section of the Arc Hydro data model will be an iterative and phased process. Earlier sections of this report describe the concepts that underlie the data model and development steps completed to date. The 2-dimensional Arc Hydro data model can be used to study interactions between surface and groundwater. Future work will include designing applications that utilize the relationships created in the Arc Hydro data model.
The 3-dimensional model is still in the initial stages of design, and more work is needed to complete the data model and make it compatible with modeling software. Future work will focus on the completion of the data model and the subsequent creation of interface with other modeling software. The initial steps will focus on developing a link between the Arc Hydro data model and MODFLOW groundwater modeling software.
Fetter, C.W., 1994, Applied Hydrogeology, 4th Ed., Prentice Hall College Division, Upper Saddle River, N.J, pp. 691.
Maidment, 2002, Arc Hydro GIS for Water Resources. ESRI Press Redlands California.
Gil Strassberg
Graduate Research Assistant
Center for Research in Water Resources
Department of Civil Engineering, University of Texas at Austin
(512) 471-0073
Suzanne Pierce
Graduate Research Assistant
Center for Research in Water Resources
Department of Geological Sciences, University of Texas at Austin
(512) 471-0073
These materials may be used for study, research, and education, but please credit the authors and the Center for Research in Water Resources, The University of Texas at Austin. All commercial rights reserved. Copyright 2001 Center for Research in Water Resources.