Road Density (subbasin)
Parameter Description: Road density (km/km2, mi/mi2) is calculated at the scale of individual subbasins, commonly HUC 6th field in NetMap. Users can import their own subbasins or use the ones (HUC 6) bundled with the watershed databases. Users can import their own road layer, although most NetMap watershed datasets will come bundled with a road layer.
Data Type: polygon
Field Name: RoadDens; Common name: Road Density (Polygon)
Units: Road Density: L/L2 (km/km2 or mi/mi2); Road Crossing Density: number//L2 (km/km2)
NetMap Module/Tool: Roads (Transportation/Energy)
Model Description:
Road density (mi/mi2, km/km2) is often used as a surrogate for road related impacts in a watershed (disruption of drainage, increased erosion, fish migration barriers etc.). Typically, road density is calculated at the scale of entire watersheds or subbasins. Road density can be calculated for all features (e.g., all roads) or only for certain types of roads (i.e., logging roads or gravel roads and not paved roads).
In NetMap, road density also can be calculated at the scale of individual stream segments, or at any scale defined by the stream network, using “Drainage Wings” (see next section).
Figure 1. (1) NetMap’s Road Density tool allows a user to calculate road density at scale of subbasin polygons (1). (2) Road density can also be evaluated at the scale of individual channel segments (see next section). (3) User selects all road features in the file or just a subset (for example gravel roads). (4) Units of measurement are selected (km km-2 or mi/mi2).
Figure 2. Road density in NetMap calculated at the scale of Hydrologic Unit Code 6th field (12 digit) subbasins. Note how these values are very different compared to road density calculated at the scale of individual channel segments (see next section).
Figure 3. Calculating road density at the stream segment scale provides a much higher resolution mapping of variations in road density. For example, road density in the Clearwater Basin that is calculated at the scale of HUC 6th field subbasins ranged between 0.5 km/km2 and 4 km/km2 (top). In contrast road density measured at the stream segment scale ranged between zero and 100 km/km2.
Other parameters that are created when running the road density tool include:
-road crossing density
-road crossing count
-road length per basin
Figure 4. One of the additional outputs when running NetMap’s road density tool is the road-stream crossing per subbasin.
