NHD versus NHDPlus

Nishesh Mehta, CRWR

NHDPlus is an enhancement to the 1:100,000-scale medium resolution National Hydrography Dataset designed to improve the utility of the NHD in applications, particularly those that deal with the modeling of water flow and those that need to tie the landscape to the stream network.

NHDPlus Data Organization

Unlike NHD where data can be ordered by choosing the area of interest by marking out an area on the map online, the data in NHDPlus is divided into various hydrologic regions. Furthermore the elevation, flow accumulation and flow direction grids are further organized by sub parts of the hydrologic region. As an example, say one would like to use the NHDPlus dataset for to find the watershed of Corpus Christie Bay in the Texas. Here is how the data may be obtained.

On the NHDPlus website (http://www.horizon-systems.com/nhdplus/data.php), this is the map of various hydrologic regions of the country for which data has been organized. The region of interest here is the Texas gulf region (region 12).

Figure 1 – Subdivisions of mainland US for NHDPlus

If region 12 is selected here is how the data is organized.

Region 12 a-f represent the further subdivisions of Texas gulf.

Figure 2a Subregion - Texas gulf Figure 2b- Description of data available for a sub region

Another important difference to note between NHD and NHDplus is that while NHD plus complied and returned the data in a geodatabase this feature is not available in NHDPlus. The individual components of the dataset can be chosen and obtained separately. This has both advantages and disadvantages.

NHDPlus Schema

Figure 3a – NHDPlus schema (Source: http://www.horizon-systems.com/nhdplus/data/NHDPLUS_UserGuide.pdf) Figure 3b – NHDPlus relationship classes (Source: NHDPlus user guide)

A huge advancement in NHDPlus is the NHDPlus schema. This schema defines how attribute tables of the individual feature classes may be joined and related to the other and also lays out what attribute in the table should be used as the connector. The NHDPlus database tables are stitched together by the relationship classes. This enables quick and easy access to information pertaining to a particular feature such as an individual streamlines catchments etc. The above figure represents diagrammatically, the various relationships that exist in the NHDPlus database. The second part of the diagram explains which attribute may be used as a connector when doing so. The advantages of having these relationships are numerous. Most importantly, it organizes the data within the dataset and enables very easy navigation to the attribute of interest. Here is an example. Once the database is complete and the relationship classes created. This is how easily an attribute table may be accessed.

Figure 4 – Attribute table linked to various other attribute tables through relationship defined by NHDPlus Schema

Another example, suppose we want to find out the mean velocity flows in a particular reach. By selecting the reach and opening the attribute table.

Figure 5 – Use of NHDPlus relationships to find out the mean velocity at a particular reach

Using the related table and browsing to the “flowlineattributesflow” table the data for the selected stream can be invoked and found.

Other Enhancements

The NHDPlus consists of 6 enhancements:

(1) The Value Added Attribute (VAA) table is populated with a number of attributes to analyze, more quickly, the flow network; NHDPlus offers VAA tools which are specifically designed for easy navigation on the flow network using specific attribute fields called VAA fields. Tools are available at http://www.horizon-systems.com/nhdplus/tools.php Tool tutorials are available at ftp://ftp.horizon-systems.com/NHDPlus/exercises/NHDPlus-Exercise4-NavigatingWithVAAsandFlowTable.pdf

(2) Catchment polygons define the drainage area of each individual NHD flowline with known flow direction based on elevation data which has been modified to create more accurate results;

(3) Improvements to the medium resolution NHD network geometry to produce more accurate flow navigation and improvements to the naming of water features; this means there are less disjointed or broken network and help more seamless navigation. This is also enables more accurate delineation of a watershed.

(4) Estimates of mean annual streamflow and velocity for each flowline with known flow direction.

(5) Flow Accumulation grids derived from elevation models; and

(6) Land use, precipitation and temperature data associated with each flowline catchment. This is vital information which earlier would have needed to be obtained separately and even then would require interpolation to find values for individual reaches. NHDPlus offers this data for each flowline catchement. All one needs is to use the relationship classes to reach the desired node.