Using the Web Soil Survey (WSS)

The Web Soil Survey (WSS) is a great source for soil data and information. The data is generated by the National Cooperative Soil Survey and the site is operated by the USDA Natural Resources Conservation Service (NRCS). The WSS site provides access to a number of related links: topographic, climate, and watershed boundary maps; soil series descriptions; ortho imagery; land use; and historical soil surveys. I was blown away by the capabilities of the Web Soil Survey and would like to share my experience. I've included several images captured during my interaction, all of which are credited to the WSS.

Once launched, you must first select an area of interest (AOI). Choose this using the interactive map's polygon tools, a keyword, state and county, address, latitude and longitude, or by importing a GIS shapefile. Use the Quick Navigation tools to search for a specific Soil Survey Area by state and county, the Public Land Survey System, Bureau of Land Management offices, Department of Defense installations, Forest Service offices, National Park Service locations, or by the 8-digit hydrologic unit.

Since this was my first time using the WSS, I decided to keep it simple and search for Soil Survey Areas in San Diego, CA. I've been focusing on this area lately for its history of wildland fires and drought. Three of the largest wildland fires in California's history occurred in San Diego County, two of which since 2003 (1). Southern California has also been abnormally dry compared to historical conditions and is currently classed as D0 on the U.S. Drought Monitor (2). This classification is associated with slowing crop growth and lingering water deficits (2). There were two results from my search and I chose the San Diego County Soil Survey Area (CA 638):

The Soil Map tab shows you all soil units, acreage, and their percentage of the AOI. I discovered that there are 2,215,625.2 acres in the San Diego County survey area (3). The three dominant unit types were acid igneous rock land, AcG (6.9% of AOI); Cieneba very rocky course sandy loam, CmrG (5.8% of AOI); and Cieneba-Fallbrook rocky sandy loam, CnG2 (4.7% of AOI) (3). Clicking on each unit in the Map Unit Legend allowed me to view its setting, composition, and properties:

The Soil Data Explorer tab is the real powerhouse of the WSS. There are five sections within this tab: Intro to Soils, Suitabilities and Limitations for Use, Soil Properties and Qualities, Ecological Site Assessment, and Soil Reports. Note: You can add this data to the free shopping cart at any time. It can be downloaded in tabular format or as spatial data for use in ArcGIS. Here's an idea of the topics offered within each section:

Intro to Soils

Suitabilities and Limitations for Use

Soil Properties and Qualities

Ecological Site Assessment

Soil Reports

I investigated a number of these tools, but I'd only like to share a few with you here. The first was under Land Management on the Suitabilities and Limitations tab: Fire Damage Susceptibility.

I wanted to know how vulnerable to fire the dominant units in this area are. AcG has an unweathered bedrock profile, so fire damage isn't applicable (3). The WSS has not rated CmrG, but CnG2 is rated with a high susceptibility to fire damage (3). In fact, 74.5% of the San Diego County survey area is rated as moderate or highly susceptible to fire (3).

I wanted to dig deeper and understand what exactly about the dominant soil types made them so prone to fire. On the Soil Properties and Qualities tab, I thought I'd investigate Soil Health: Available Water Capacity (AWC) of the surface layer. AWC is an indicator of the soil's ability to retain water (3). It is measured in centimeters of water per centimeter of soil and is directly related to the amount of water available for plant use. The lower the AWC, the drier the conditions. AcG has no rating because solid rock cannot hold water in this way, but for CmrG and CnG2 the AWC rating is quite low at 0.10 (3).

On the Ecological Site Assessment tab, I also wanted to investigate the type of ecosystems that might be found in the loamy CmrG and CnG2 soil. Unfortunately, there was no plant community data available on this tab. It did at least indicate that this region was a rangeland ecological site, so I went back to the Intro to Soils tab for more information. It explained that rangelands can consist of grasslands, deserts, tundra, alpine plant communities, savannas, wet meadows, and freshwater and coastal marshes (3).

It was easy to determine which are common in my AOI based on the AWC of the dominant soil types, but I confirmed it by referencing the rangeland composition (Soil Reports tab > Vegetative Productivity > Rangeland Productivity and Plant Composition). Chamise is prolific in xeric sites, which is why the WSS states that it's the most common species of chaparral found in the San Diego County Soil Survey Area (4). It is considered a "fire-type vegetation" whose succession depends on fire for proliferation (4). 

Otay Mountain Wilderness, 2005 (https://interwork.sdsu.edu)

In an effort to correlate this information to my prior research, I referenced the map I generated for California's Level IV ecoregions and wildland fires (see the original post here). I found that there were six wildland fires in my AOI in 2018: one in the Santa Ana Mountain region and four in the Diegan Western Granitic Foothills region, one of which extended into the Morena/Boundary Mountain Chaparral region (5). As it turns out, the chamise and mixed chaparral are dominant in each of these level IV ecoregions (6). Using the Web Soil Survey, I was able to conclude that edaphic factors have a significant influence on San Diego County's arid conditions and prevalence of wildland fires.

Resources:

1. California Department of Forestry. (2019). Stats and Events. Retrieved July 4, 2019, from https://www.fire.ca.gov/stats-and-events/
2. National Drought Mitigation Center. (2019, June 21). California. Retrieved July 4, 2019, from https://www.drought.gov/drought/states/california
3. NRCS. (2019, April 9). Web Soil Survey Home. Retrieved July 4, 2019, from https://websoilsurvey.sc.egov.usda.gov/App/HomePage.htm
4. US Fish & Wildlife Service: FEIS. (2019). Index of Species Information: Adenostoma fasciculatum. Retrieved from https://www.fs.fed.us/database/feis/plants/shrub/adefas/all.html
5. Shuck, J. (2019, June 27). California's Wildland Fires: 2000 vs. 2018. Retrieved from https://ca-climatechange-biodiversity.blogspot.com/2019/06/using-arcgis-to-create-maps-is-pretty.html
6. Griffith, G. E., Omernik, J. M., Smith, D. W., Cook, T. D., Tallyn, E., Moseley, K., & Johnson, C. B. (2016, February 23). Ecoregions of California. Retrieved from https://pubs.er.usgs.gov/publication/ofr20161021