Soil Conservation

To preserve and restore stream environment zones and limit impervious land cover in the Region.

Percent of Land Coverage Within Land Capability Class 4 (allow up to 20% impervious coverage)

Relevance - This indicator measures the percent of land coverage on different land capability classes as described by Bailey (Bailey, 1974) and updated with the most recent soil survey by the Natural Resources Conservation Service (NRCS) in 2007 (Loftis, 2007; USDA-NRCS, 2007). Impervious cover is a primary indicator of land disturbance. Excessive impervious surface within a watershed contributes to sediment and nutrient inputs to Lake Tahoe and its tributaries impairing water quality, altering surface hydrology and groundwater recharge regimes. The results are often negative impacts on soil health, fisheries, wildlife habitat and vegetation growth. Impervious surfaces include hard coverage such as roads, buildings, driveways, and parking lots, and soft coverage with soil compaction as a result of use, but where no structure is in place.
Human and Environmental Drivers - Impervious cover is created through use or development on natural lands. This could be for commercial, residential, recreational, and other activities, and encompasses the spectrum of human uses that involve physical modification of the environment. The economy plays a large role in the housing market and the business environment, which are both among the most important drivers of new land coverage in the basin.

2015 Threshold Evaluation


- Considerably Better Than Target
- See summary in Table 1. All land capability classes are in attainment except for class 1b and class 2. Since August 2010, 19.09 acres of hard impervious cover have been permitted within the basin and 10.4 acres of cover in class 1b have been removed. Commodity transfers by private parties as part of the Transfer of Development Rights Program accounted for 8.08 acres removed from class 1b and 2.45 acres were removed by the CTC and NDSL. No estimate is provided for changes in unpermitted impervious cover. Unpermitted cover refers to cover that may have been added or created in the Region, for which no permit was acquired.


- Little or No Change
- See summary in Table 2. The percent change relative to target across all land capability classes over the four-year analysis period was 0.01 percent. The largest percent change relative to target was the 1.84 percent decrease in the 1b class, which would be classified as moderate improvement (see methodology section). Percent change relative to target for all other land capability classes was less than 0.07 percent indicating little or no change. The added coverage amounts to an increase of 0.01 percent in coverage basin-wide, which indicates little or no change overall.


- Moderate
- View Details Below

Methods not provided


Percent cover in each Bailey land capability class relative to the allowable coverage in class.

Percent cover in each Bailey land capability class relative to the allowable coverage in class.

Connecting Actions to Outcomes


Intermediate Results


Parcel level verification of sites’ land capability class is the primary mechanism by which TRPA and partners ensure that development in the basin adheres to the Bailey system and that excess coverage is not added within a land capability class. Removal of coverage from sensitive lands is primarily facilitated by Environmental Improvement Program (EIP) implementation partners and the California and Nevada land banks through the Excess Coverage Mitigation Program. In addition to the actions of EIP partners, the Transfer of Development Rights Program is a central part of TRPA’s growth management system and an important strategy used to attain multiple environmental thresholds, by providing an incentive to transfer coverage to less sensitive lands. Within the program, if 10 tourist accommodation units (TAUs) were removed from a SEZ and transferred to a town center, an additional 20 TAUs would be awarded for this transfer, for a total of 30 TAUs (i.e. 1:3 transfer ratio). TRPA is actively seeking new mechanisms to encourage removal of excess coverage on sensitive lands. For example, the TRPA Governing Board recently took action to allow coverage transfers across Hydrologically Related Area (HRA) boundaries. With this update, coverage can be transferred across HRA boundaries if it is removed from sensitive lands in one HRA and sent to lands that are non-sensitive and not located along the shoreline of Lake Tahoe. In addition, the new Excess Coverage Mitigation Program usage requirements state that the land banks should give preference to the retirement of coverage in sensitive lands. Both actions were taken at the Dec. 16, 2015 meeting of the TRPA Governing Board.
Physical removal of impervious cover is readily verifiable and contributes to standard attainment. Standard TRPA permitting conditions for projects that remove cover, include the requirement the natural hydrologic function and services are restored after cover is removed. Site restoration is field verified as well prior to a project being determined complete.

The class 1b target is not expected to be attained in the foreseeable future, given the magnitude of change needed and public funding levels. The rate of coverage removal from class 1b lands over the last four years averaged 2.5 acres annually. At this rate the 1b target would not be attained for 264 years. Continuing this rate over the next four years would result in the removal of an additional 10 acres from class 1b. Impervious cover in class 2 lands is currently 43 acres above the target level. Removing 43 acres of cover is potentially achievable within 10 to15 years if sufficient funding is available and focused on attainment of the class 2 target. While it is conceivable that the class 2 target could be achieved if actions were targeted solely to promote attainment of the individual threshold standard, project prioritization should also consider the potential for projects to contribute to threshold gains in any number of categories. For example, consideration of the multiple threshold benefits may result in prioritizing restoration projects in SEZ lands (which are generally class 1b) because their restoration may also result in greater benefits to water quality, wildlife and vegetation thresholds than restoration efforts on class 2 lands. Such a prioritization framework is more consistent with the intent of land capability classes, which are described as a means to achieve a range of objectives rather than an end point in and of themselves. “Criteria were developed for classifying lands – according to their inherent physical capability to provide for use without endangering achievement of the goals established in the Bi-State Compact (P.L. 91-148) for protection of environmental qualities of the basin (Bailey, 1974).”
2019 Threshold Evaluation Report.

Applicable Standard

- Bailey Land Coverage Coefficients - Class 4 (20%)
- N/A


The base impervious coverage layer for the Region was sourced from a LiDAR survey completed in August 2010. LiDAR is a remote sensing technology that uses laser and light refraction to image objects and terrain. The 2010 LiDAR analysis mapped the extent of hard and soft impervious cover in the Region. The cost of acquiring LiDAR data for the Region makes quadrennial LiDAR surveys infeasible. To assess change in impervious cover without the benefit of new LiDAR imagery, information collected from project permitting by TRPA and partners was used to determine added/new coverage. Land capability as defined in the 2007 soil survey was used as the primary unit to measure coverage in a land capability class, both because it was used in the 2011 Threshold Evaluation Report (TRPA, 2012b) and because it is more detailed than the 1974 Bailey report (Loftis, 2007). Information about coverage removed was provided by the CTC, NDSL and the Parcel Tracker.


Map image not provided

Confidence Details

- Moderate. The 2011 Threshold Evaluation Report assessed the accuracy of the impervious cover map as moderate (TRPA, 2012b). The rigorous tracking and permitting process of TRPA and partners in the Region is extensive and would yield a high confidence if assessed independently.
- High. Even though a statistical analysis was not used to test if trends were significant, there is high confidence in the cumulative accounting of acres of cover added in each land capability class.
- Moderate. Overall confidence takes the lower of the two confidence determinations.


Evaluation of the effectiveness of the individual policies or programs implemented to facilitate removal of coverage is challenging because of the diversity of contributing factors, but is essential to informing program design and should remain a priority.

Detailed LiDAR analysis like the one conducted in 2010 is the most reliable method for determining the extent of impervious cover in the Region and should be acquired at regular intervals. The next LiDAR analysis could also be used to assess the accuracy of coverage estimates obtained through permit accounting. To improve the accuracy of the Region-level maps of land capability, information collected during parcel land capability verifications could be integrated into the gross scale maps of land capability.

The 2007 NRCS soil survey, produced at field verified scale of 2.5 acres is significantly more detailed than the previous Bailey map at a 10-acre scale, is the highest quality and most accurate estimate of basin-level land capability classes. It is the best available information for use by TRPA as the base map for standard evaluation and its adoption should be considered. Its adoption was also recommended in the 2011 Threshold Evaluation Report (TRPA, 2012b).

Attainment of the impervious cover standard for the 1b land capability class would require the removal and/or relocation of 659 acres of impervious cover, roughly 8.3percent of all impervious cover in Region. Removal or relocation of this magnitude may be infeasible in a reasonable time-frame. It would also likely require removal and buyout (with transfers or retirement) of large portions of existing private development (residential, tourist, commercial) in the Region’s local communities.

Existing land use policies and regulations should continue to evolve and may need to be amended to better facilitate the transfer and restoration of urban development-oriented coverage from less suitable land capability classes that historically supported wetland and meadow vegetation to areas with a greater land capability.

Additional Figures

Status determination by land capability class including percent of target
(~163 KB)
Credit: TRPA
Status determination by land capability class including percent of target
Summary of trend determination by land capability class
(~166 KB)
Credit: TRPA
Summary of trend determination by land capability class

Historic Evaluations

View Evaluation Legend