GIS for water resources

Hydrologist use many data  sources to assess water quality, determine water supply, prevent flooding, understand environmental issues, and manage water resources. During the 1990s, GIS emerged as s significant support tool for hydrologic modeling. In particular, GIS provided a consistent method for watershed and stream network delineation using digital elevation models (DEMs) of land-surface terrain. Standardized GIS data sets for land cover, soil properties, gaging station locations, and climatic variables were developed, and many of these data sets are published on the internet. GIS data processors were developed to prepare input data for water flow and water-quality models. GIS is now accepted as a useful tool for assembling water resources information, and the community of water resources and GIS specialists who are familiar with these tools and data sets is growing.

While much progress has been made with the application of GIS in water resources, and the creation of national, regional, and local data sets, many challenges remain:

• It is essential for hydrologist to depict the flow of water through the landscape in space and time. How can time series data on water flow and water quality be integrated with geospatial data describing the water environment? This is perhaps the most critical challenge.
• Water management agencies are building GIS data sets to support their operations, but often each particular project or group develops its own data. How can an agency produce a common geospatial data infrastructure to support a range of water resources projects?
• In water resources study using hydrologic models, each model usually operates independently with its own GIS database. How can a framework be developed to support several linked hydrologic model.
• As digital elevation data sets for land-surface terrain become more refined, their cell size decrease and the number of cell increases to the point where studying a large region with a single digital elevation model data set is cumbersome and impractical. How can terrain information be subdivided into subareas for analysis, and the results merged to form a watershed data set for the whole region?
• Triangulated irregular network (TIN) data is used to describe river channels and floodplains, but their degree of detail also becomes overwhelming as the length of river being studied increases. How can a vector description of channel shape be built from a TIN surface for each river segment, and then the results combined to form a continuous description of channel shape for the whole length of river?
• Ecologists and geomorphologists conduct detailed surveys of river reaches to identify substrate and organism types. How can the information gathered in these surveys be stored in a structured way so that it is linked to the geometrical information about the stream channel shape?  
• Standardized hydrology data sets are being produced to store the “blue line” describing streams, rivers, lakes, and coastal water bodies. How can stream hydrography be linked with independently delineated watershed area defining the land areas draining to these streams?
• A large number of point features are associated with river networks, including gaging stations, hydraulic structures, intakes for water supply systems, discharge points for wastewater treatment plants, bridges, and locations where rivers cross administrative or aquifer boundaries. How can this information be addressed along river network so that it is easy to tell what is upstream and downstream of particular location?
• Urban hydrologic systems pose many challenges not seen in rural systems. In particular, the drainage areas of storm sewer systems may be quite different than the drainage areas of surface streams in the same region. How can this discrepancy be accounted for?

The common theme of these challenges is the need for integration: integration of different type of GIS and water resources data, integration of GIS data for modeling, integration across spatial scales. The need is clear, but not so the mean for accomplishing it. Some requirements for an integrated water resources data system are:

·         All data must be held in a common geospatial coordinate system.

·         The primary structure used for representation of a large region must be vector data (points, lines and areas), supported by raster and TIN surface data where necessary.

·         Relationship among geographic features in different data layer are needed to trace water movement from features to features through the landscape.

·         It is critical to be able to link geospatial information describing the water environment with time series information about water measurement to form a complete information system for water resources.

[ David R. Maidment, 2002. Arc Hydro, GIS for Water Resources]