Lynn F. Fenstermaker

Hyperspectral mixture modeling for quantifying sparse vegetation cover in arid environments.

Abstract A linear mixture model based on calibrated, atmospherically corrected Probe-1 hyperspectral imagery was compared with three vegetation indices to test its relative ability to measure small differences in percent green vegetative cover for areas of sparse vegetation in arid environments. The goal of this research was to compare multispectral and hyperspectral remote sensing approaches for detecting human disturbance of arid environments. The normalized difference vegetation index (NDVI) was tested using both narrow and broad bandwidths. Broadband NDVI provided results r 2 =0.63 similar to NDVI derived from individual hyperspectral channels r 2 =0.60 . While the soil-adjusted vegetation index (SAVI) was designed as an improvement to NDVI for sparse vegetation, in this study SAVI performed significantly worse than NDVI r 2 =0.51 . The modified soil-adjusted vegetation index (MSAVI) provided an insignificant improvement over NDVI r 2 =0.64 . Linear mixture modeling provided significantly better results, r2 of 0.74. Cross-validation was used to test the significance of differences between the various methods and to determine the standard error associated with each method. Results suggest that any improvements provided by adjusted vegetation indices over NDVI may be strongly dependent on those adjustments being derived from local conditions. The use of a linear mixture model with multiple soil endmembers appears to provide the best method for quantifying sparse vegetative cover. Though present in small amounts, a single plant species, Krameria erecta, was strongly correlated with residuals of the mixture model. Inclusion of a spectral endmember for this species increased the r2 of the fit with percent green cover to 0.86. However, it is not clear if the explained variation was actually due to K. erecta or a correlated phenomena. Problems were also identified with the use of multiple vegetation endmembers.

Monitoring Vegetation Phenological Cycles in Two Different Semi-Arid Environmental Settings Using a Ground-Based NDVI System: A Potential Approach to Improve Satellite Data Interpretation

In semi-arid environmental settings with sparse canopy covers, obtaining remotely sensed information on soil and vegetative growth characteristics at finer spatial and temporal scales than most satellite platforms is crucial for validating and interpreting satellite data sets. In this study, we used a ground-based NDVI system to provide continuous time series analysis of individual shrub species and soil surface characteristics in two different semi-arid environmental settings located in the Great Basin (NV, USA). The NDVI system was a dual channel SKR-1800 radiometer that simultaneously measured incident solar radiation and upward reflectance in two broadband red and near-infrared channels comparable to Landsat-5 TM band 3 and band 4, respectively. The two study sites identified as Spring Valley 1 site (SV1) and Snake Valley 1 site (SNK1) were chosen for having different species composition, soil texture and percent canopy cover. NDVI time-series of greasewood (Sarcobatus vermiculatus) from the SV1 site allowed for clear distinction between the main phenological stages of the entire growing season during the period from January to November, 2007. NDVI time series values were significantly different between sagebrush (Artemisia tridentata) and rabbitbrush (Chrysothamnus viscidiflorus) at SV1 as well as between the two bare soil types at the two sites. Greasewood NDVI from the SNK1 site produced significant correlations with chlorophyll index (r = 0.97), leaf area index (r = 0.98) and leaf xylem water potential (r = 0.93). Whereas greasewood NDVI from the SV1 site produced lower correlations (r = 0.89, r = 0.73), or non significant correlations (r = 0.32) with the same parameters, respectively. Total percent cover was estimated at 17.5% for SV1 and at 63% for SNK1. Results from this study indicated the potential capabilities of using this ground-based NDVI system to extract spatial and temporal details of soil and vegetation optical properties not possible with satellite derived NDVI.

Multiscale assessment of green leaf cover in a semi-arid rangeland with a small unmanned aerial vehicle

Spatial variability in green leaf cover of a semi-arid rangeland was studied by comparing field measurements on 50 m crossed transects to aerial and satellite imagery. The normalized difference vegetation index was calculated for 2 cm resolution images collected with a multispectral digital camera mounted on a radio-controlled helicopter, as well as a 30 m resolution Landsat Thematic Mapper image. Variograms of green cover from these two sources show that the range of influence for spatial autocorrelation extended to a distance of approximately 200 m. Field transects that are much smaller than the extent of this spatial autocorrelation are more likely to fall within local deviations from the mean landscape condition. A sampling scheme that exceeds the spatial scale of these localized deviations is shown to reduce sample variance and require fewer sampling locations to reach a given level of measurement uncertainty. The time and cost of more spatially extensive sampling at each location may be less than deploying to a larger number of locations with smaller transects, and unmanned aerial vehicles may be a valuable tool in extending current field sampling strategies for quantifying the health of shrub-dominated rangelands.

Monitoring Soil Erosion of a Burn Site in the Central Basin and Range Ecoregion: Final Report on Measurements at the Gleason Fire Site, Nevada

The increase in wildfires in arid and semi-arid parts of Nevada and elsewhere in the southwestern United States has implications for post-closure management and long-term stewardship for Soil Corrective Action Units (CAUs) on the Nevada National Security Site (NNSS) for which the Nevada Field Office of the United States Department of Energy, National Nuclear Security Administration has responsibility. For many CAUs and Corrective Action Sites, where closure-in-place alternatives are now being implemented or considered, there is a chance that these sites could burn over at some time while they still pose a risk to the environment or human health, given the long half lives of some of the radionuclide contaminants. This study was initiated to examine the effects and duration of wildfire on wind and water erodibility on sites analogous to those that exist on the NNSS. The data analyzed herein were gathered at the prescribed Gleason Fire site near Ely, Nevada, a site comparable to the northern portion of the NNSS. Quantification of wind erosion was conducted with a Portable In-Situ Wind ERosion Lab (PI-SWERL) on unburned soils, and on interspace and plant understory soils within the burned area. The PI-SWERL was used to estimate emissions of suspendible particles (particulate matter with aerodynamic diameters less than or equal to 10 micrometers) at different wind speeds. Filter samples, collected from the exhaust of the PI-SWERL during measurements, were analyzed for chemical composition. Based on nearly three years of data, the Gleason Fire site does not appear to have returned to pre burn wind erosion levels. Chemical composition data of suspendible particles are variable and show a trend toward pre-burn levels, but provide little insight into how the composition has been changing over time since the fire. Soil, runoff, and sediment data were collected from the Gleason Fire site to monitor the water erosion potential over the nearly three-year period. Soil hydrophobicity (water repellency) was noted on burned understory soils up to 12 months after the fire, as was the presence of ash on the soil surface. Soil deteriorated from a strong, definable pre-fire structure to a weakly cohesive mass (unstructured soil) immediately after the fire. Surface soil structure was evident 34 months after the fire at both burned and unburned sites, but was rare and weaker at burned sites. The amount of runoff and sediment was highly variable, but runoff occurred more frequently at burned interspace sites compared to burned understory and unburned interspace sites up to 34 months after the burn. No discernible pattern was evident on the amount of sediment transported, but the size of sediment from burned understory sites was almost double that of burned and unburned interspace soils after the fire, and decreased over the monitoring period. Curve numbers, a measure of the runoff potential, did not indicate any obvious runoff response to the fire. However, slight seasonal changes in curve numbers and runoff potential and, therefore, post-fire runoff response may be a function of fire impacts as well as the time of year that precipitation occurs. Site (interspace or understory) differences in soil properties and runoff persisted even after the fire. Vegetation data showed the presence of invasive grasses after the fire. Results from analysis of wind and water coupled with the spatial analysis of vegetation suggest that wind erosion may continue to occur due to the additional exposed soil surface (burned understory sites) until vegetation becomes re-established, and runoff may occur more frequently in interspace sites. The potential for fire-related wind erosion and water erosion may persist beyond three years in this system.