Table of Contents
Introduction to Schematic Networks
The schematic network, a product created by the Arc Hydro Toolset, represents connectivity between features in the landscape through a network of nodes and links. A schematic network may be created through the attributes and relationships of Arc Hydro from up to two feature classes. Typically, Watersheds and HydroJunctions are used for this purpose. The JunctionID attribute on a Watershed stores the HydroID of the HydroJunction that serves as the outlet for that Watershed. The NextDownID attribute of a HydroJunction stores the HydroID of the next downstream HydroJunction (Figure 1). With these attributes, connectivity is established between Watersheds and HydroJunctions. From this connectivity, it is possible to create a schematic network representing those features (Figure 2).
Figure 1 - Geospatial Data Representing Watersheds and the Hydro Network
Figure 2 - A Schematic Network. Green links and node represent transfer from land to a river channel. Red links and nodes represent transport through a river channel.
The schematic network is composed of two feature classes: SchemaLink and SchemaNode. SchemaNodes are located at the centroid of the features that they represent. SchemaLinks are straight lines that connect SchemaNodes. The SchemaNodes contain a field called FeatureID, which stores the HydroID of the source feature that is represented by the schematic feature. SchemaLinks contain FromNodeID and ToNodeID, which identify the HydroID of the SchemaNodes connected to a given SchemaLink. FromNodeID identifies the from node, and ToNodeID identifies the to node. The schematic features are further subdivided into two types. Type 1 features are associated with a JunctionID-HydroID relationship from the source features, such as JunctionID on Watershed pointing to the HydroID of that Watershed's outlet HydroJunction. Type 2 features are associated with a NextDownID-HydroID relationship from the source features. The types are specified in the SrcType attribute of SchemaNode, and the LinkType attribute of SchemaLink. Once the schematic network has been created, attributes from the source features may be copied to the schematic features using the FeatureID-HydroID association.
Schematic Network Processing for
Hydrologic Modeling?
Schematic networks provide a summarized view of how mass is transferred through the surface water environment. For example, when rain falls on a watershed, a portion is transferred to the stream system within that watershed. From there, water is transported along the river system to the basin's outlet. This transfer of water through the environment can be represented by a schematic network with two types of links and nodes. Type 1 links and nodes represent the transfer of water from the watershed to the river system. Type 2 links and nodes represent the transfer of water through the river system. Each type of link and node is governed by different equations. Transfer from the watershed to the river can be estimated with a loss rate equation and transfer along a river can be estimated using routing equations.
Once a geodatabase with Schematic Network has been generated and populated with the necessary information, the next step is to process the schematic network features to simulate hydrologic processes. The procedure for processing schematic networks may be summarized into two major components: Data Preparation and the Process Loop. In Data Preparation, the features in the schematic network are sorted from upstream to downstream, and collections are initialized that will store values during the processing of the network. Once the features have been sorted, each feature is processed in the correct order until all features have been processed. For a given feature, a DLL may be called to handle the processing for that feature. Two types of processing may occur. The first is called a Receive Process. This process calculates a total value for a given feature when supplied with values that are received from upstream as well as the incremental value for that feature. The second type of process is called a Pass Process. This process calculates a value that the given feature will pass to the next downstream feature when supplied with a DLL.
Tim Whiteaker
Graduate Research Assistant
Center for Research in Water Resources
Department of Civil Engineering, 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 2003 Center for Research in Water Resources.