Hydrologic modeling:

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Integrating GIS and hydrologic modeling involves connecting-geospatial data describing the phisical environment with hydrologic process models describing how water moves through the environment. Observed time series of water resources data at monitoring points are needed to verify hydrologic models. Arc Hydro provides a robust environment for integrating geospatial andtime series data for water resources with simulation models for water flow and quality. A variety of interfaces exist for accessing Arc Hydro data, including ArcGIS, Microsoft Access, Microsoft Excel, and direct interface programming with Visual Basic. Dynamic linked libraries provide a secure, efficent way to link process models with Arc Hydro data.

INTEGRATING GIS WITH HYDROLOGIC MODELING.Water is constantly flowing through the landscape in response to weather events-at times a deluge during a storm, at other times a trickle during a drought. Most often, water flows in the regular patterns that we come to expect from rivers and streams. Water flow carries microorganisms, dissolveld chemicals, and sediment that define water quality. We need to understand these flow and qualitypatterns, and how the change under different management plans for water resources. Hydrologic simulation models provide a representation of water flow and quality for rivers, streams, lakes, bays, and estuaries. GIS supports hydrologic analysis and modeling by describing the physical environment through which water flows.

A hydrologic simulation model may be formally coded in a programming languageor it may be developed in a spreadsheet format. Spreadsheet are particularly useful for analysis of observed hydrologic data and for developing prototype simulation models used in a local setting. For a long time, hydrologists have wanted to apply GIS data in a straightforward manner using spreadsheets. Arc Hydro facilitates this goal since the Arc Hydro personal geodatabase is Microsoft Accessfile, and there is link between Microsoft Excel and Microsoft Access allowing Arc Hydro data to be directly viewed and manipulated in Microsoft Excel.

Formally coded hidrologic simulation models describe paricular hydrologic processes and are intented to be applied at any geographic location. Some of the most famous of these models were developed during the 1960s and have been in use for decades,such as the HEC-1 and HEC-2 flood simulation models from the Hydrologic Engineering Center of the U.S. Army Corps of Engineers. As new software engineering approaches have emerged, they have been adopted for hydrologic and hydraulic modeling, such as the HEC-HMS and HER-RAS simulation models. These are written in object-oriented programming languages and are the successor programs to HEC-1 andHEC-2. During the 1990s, special-purpose GIS interfaces were constructed to supply geospatial data for popular hydrologic models such as the HEC-GeoHMS and HEC-GeoRAS systems, which were developed using the avenue language in ArcView 3.

GIS can be used in various ways to supports hydrologic modeling. GIS can:

• Manager data-GIS perfoms basic geospatial data-management tasks (data storage,manipulation, preparation, and extraction) and spatial data processing (overlays and buffering).

• Extract parameters, GIS provides characteristic properties of watersheds and river reaches for hydrologic modeling.

• Provide visualization-GIS displays data, either before the hydrologic analysis is performed to verify the basic information, or after the analysis to evalue the results. Forexample, floodplain mapping in GIS shows the extens of areas damaged by floods.

• Model surfaces-GIS delineates watersheds and represents channel shapes based on digital terrain or elevation models.

• Develop interfaces-Map based interfaces to hydrologic models can be developed using GIS tools.

The EPA basins programs for water-quality simulation is a particularly sophisticated integration of...
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