The above map shows the results of the Wildfire Hazard analysis. This analysis used LANDFIRE data and the FlamMap model to estimate wildfire hazard. Adjustments were made in areas of mortality. The results show that the highest elevation areas have the lowest wildfire hazard. The adjustments and other assumptions used in the model are available on the project files page (see link on sidebar).


The above map shows the wildfire hazard ranking based on the FlamMap analysis.

High-severity fires can cause changes in watershed components that can dramatically change runoff and erosion processes in watersheds. Water and sediment yields may increase as more of the forest floor is consumed (Wells et al. 1979, Robichaud and Waldrop 1994, Soto et al. 1994, Neary et al. 2005, and Moody et al. 2008) and soil properties are altered by soil heating (Hungerford et al. 1991).

The data used was the U.S. Department of Agriculture - Natural Resources Conservation Service (NRCS) STATSGO soils data. STATSGO soils data is available for the entire watershed and is relatively coarse soils data, created at a scale of 1:250,000. SSURGO soils data does not cover all the watershed but is available at a scale that generally ranges from 1:12,000 to 1:63,360.

The soil analysis used a combination of two standard erodibility indicators: the inherent susceptibility of soil to erosion (K factor) and land slope derived from USGS 30m digital elevation models. The K factor data (kwfact or Kw) from the STATSGO or SSURGO spatial databases was combined with a slope grid using Natural Resources Conservation Service (USDA NRCS 1997) slope-soil relationships to create a classification grid divided into slight, moderate, severe and very severe erosion hazard ratings.

The map above is the categorized soil erodibility map for the Arkansas Headwaters Watershed. The categorization was adjusted due to two watersheds (Lake Fork-Clear Creek and Headwaters Clear Creek) that were rated much higher than all of the other watersheds. Those two watersheds were given the highest scores and the rest of the watersheds recategorized.

Watershed steepness or ruggedness is an indicator of the relative sensitivity to debris flows following wildfires (Cannon and Reneau 2000). The more rugged the watershed, the higher its sensitivity to generating debris flows following wildfire. The ruggedness of a watershed is calculated here using a formula from Melton (1957) that uses the watershed area and height, or change in elevation.

The map above is the categorized ruggedness for the Arkansas Headwaters Watershed. It generally shows that while much of the watershed is quite steep, some watersheds (Trout Creek and Tennessee Creeks) are flatter than the others.

Roads can convert subsurface runoff to surface runoff and then route the surface runoff to stream channels, increasing peakflows (Megan and Kidd 1972, Ice 1985, and Swanson et al. 1987). Therefore, watersheds with higher road densities have a higher sensitivity to increases in peak flows following wildfires. Road density in miles of road per square mile of watershed area were used as an indicator of flooding risk. Roads data need to be consistent within the fifth-level watershed to allow for appropriate comparisons during prioritization. Therefore, the U.S. Census Bureau’s Tiger database for a consistent roads layer. Other data layers are being looked at to see if they would provide more accurate results.

The map above is the categorized road density for the Arkansas Headwaters Watershed. It generally shows that much of the watershed has low road densities. It also displays some expected differences in road density throughout the watershed.

Comments and Responses

Comment 1:

A comment was submitted about road density, particularly related to Box Creek. The comment was that the road density seemed too high unless it was related to roads in a housing development that would not affect hydrologic changes following wildfires.

We looked into this comment and found that there was a high road density in several watersheds due to towns or housing developments. Box Creek and City of Leadville - Arkansas River were skewing the categorization and were adjusted down manually. The revised analysis is shown in the map above. The road density for those two watersheds is shown below.

Comment 2:

At the second stakeholder meeting several comments were provided about a concern that the roads situation in this watershed is more complex than that displayed by the roads data. Particularly, mining roads were mentioned as well as other roads in the forest and not on the maps.

We evaluated several different roads layers and have looked at various methods of adjusting the roads analysis to fit the concerns of the stakeholders. The method that was used involved a comparison of the Tiger roads data with conditions on the ground. This was done by looking at some vegetation type mapping and using Google Earth to look for roads that were not in the roads data and evaluate roads that were in subdivisions and towns. Adjustments were then made to the roads density calculations based upon those evaluations. The adjusted road density ranking is shown in the map above.

The Flooding and Debris Flow Risk is the combination of ruggedness and road density. The procedure from the Front Range Watershed Work Group determined that slope should have a higher value than road density in this ranking. The effect of road density on post-wildfire effects was determined to be more variable than slope. For example, an area with a shallow slope and high road density would have little influence on post-wildfire erosion. The determination that slope would have a higher value than road density was based on professional judgment, experience and the results of the Upper South Platte Watershed Assessment Test Case. This assumption was used in the initial calculation of Flooding/Debris Flow Risk for the Arkansas Headwaters watershed.

The map above is the categorized Flooding/Debris Flow Risk for the Arkansas Headwaters Watershed. It generally shows that areas of the watershed with high road densities and high ruggedness rank high in this combined factor. The best way to look at this map is to look at a single watershed on the ruggedness and road density maps, noting the rankings on each. Then look at this map and see how they resulted in the final ranking for this component.

The Flooding/Debris Flow ranking map has been modified based on the most recent changes in the road density analysis.