Michael Cosh, William White, Andreas Colliander, Thomas Jackson, John Prueger, Brian Hornbuckle, E. Raymond Hunt, Heather McNairn, Jarrett Powers, Victoria Walker, Paul Bullock
Vegetation water content (VWC) is an important land surface parameter that is used in retrieving surface soil moisture from microwave satellite platforms. Operational approaches utilize relationships between VWC and satellite vegetation indices for broad categories of vegetation, i.e., “agricultural crops,” based on climatological databases. Determining crop type–specific equations for water content could lead to improvements in the soil moisture retrievals. Data to address this issue are lacking, and as a part of the calibration and validation program for NASA’s Soil Moisture Active Passive (SMAP) Mission, field experiments are conducted in northern central Iowa and southern Manitoba to investigate the performance of the SMAP soil moisture products for these intensive agricultural regions. Both sites are monitored for soil moisture, and the calibration and validation assessments had indicated performance issues in both domains. One possible source could be the characterization of the vegetation. In this investigation, Landsat 8 data are used to compute a normalized difference water index for the entire summer of 2016 that is then integrated with extensive VWC sampling to determine how to best characterize daily estimates of VWC for improved algorithm implementation. In Iowa, regression equations for corn and soybean are developed that provided VWC with root mean square error (RMSE) values of 1.37 and 1.10 kg / m2, respectively. In Manitoba, corn and soybean equations are developed with RMSE values of 0.55 and 0.25 kg / m2. Additional crop-specific equations are developed for winter wheat (RMSE of 0.07 kg / m2), canola (RMSE of 0.90 kg / m2), oats (RMSE of 0.74 kg / m2), and black beans (RMSE of 0.31 kg / m2). Overall, the conditions are judged to be typical with the exception of soybeans, which had an exceptionally high biomass as a result of significant rainfall as compared to previous studies in this region. Future implementation of these equations into algorithm development for satellite and airborne radiative transfer modeling will improve the overall performance in agricultural domains.
Remote sensing with small unmanned aircraft systems (sUAS) has potential applications in agriculture because low flight altitudes allow image acquisition at very high spatial resolution. We set up experiments at the Oregon State University Hermiston Agricultural Research and Extension Center with different platforms and sensors to assess advantages and disadvantages of sUAS for precision farming. In 2013, we conducted an experiment with 4 levels of N fertilizer, and followed the changes in the normalized difference vegetation index (NDVI) over time. In late June, there were no differences in chlorophyll content or leaf area index (LAI) among the 3 higher application rates. Consistent with the field data, only plots with the lowest rate of applied N were distinguished by low NDVI. In early August, N deficiency was determined by NDVI, but it was too late to mitigate losses in potato yield and quality. Populations of the Colorado potato beetle (CPB) may rapidly increase, devouring the shoots, thus early detection and treatment could prevent yield losses. In 2014, we conducted an experiment with 4 levels of CPB infestation. Over one day, damage from CPB in some plots increased from 0 to 19%. A visual ranking of damage was not correlated with the total number of CPB or treatment. Plot-scale vegetation indices were not correlated with damage, although the damaged area determined by object-based feature extraction was highly correlated. Methods based on object-based image analysis of sUAS data have potential for early detection and reduced cost.
Fuel moisture content (FMC), an important variable for predicting the occurrence and spread of wildfire, is the ratio of foliar water content and foliar dry matter content. One approach for the remote sensing of FMC has been to estimate the change in canopy water content over time by using a liquid-water spectral index. Recently, the normalized dry matter index (NDMI) was developed for the remote sensing of dry matter content using high-spectral-resolution data. The ratio of a spectral water index and a dry matter index corresponds to the ratio of foliar water and dry matter contents; therefore, we hypothesized that FMC may be remotely sensed with a spectral water index divided by NDMI. For leaf-scale simulations using the PROSPECT (leaf optical properties spectra) model, all water index/NDMI ratios were significantly related to FMC with a second-order polynomial regression. For canopy-scale simulations using the SAIL (scattering by arbitrarily inclined leaves) model, two water index/NDMI ratios, with numerators of the normalized difference infrared index (NDII) and the normalized difference water index (NDWI), predicted FMC with R2 values of 0.900 and 0.864, respectively. Leaves from three species were dried or stacked to vary FMC; measured NDII/NDMI was best related to FMC. Whereas the planned NASA mission Hyperspectral Infrared Imager (HyspIRI) will have high spectral resolution and very high signal-to-noise properties, the planned 19-day repeat frequency will not be sufficient for monitoring FMC with NDII/NDMI. Because increased fire frequency is expected with climatic change, operational assessment of FMC at large scales may require polar-orbiting environmental sensors with narrow bands to calculate NDMI.
Crop rotation is one of the important decisions made independently by numerous farm managers, and is a critical variable in models of crop growth and soil carbon. In Iowa and much of the Midwestern United States (US), the typical management decision is to rotate corn and soybean crops for a single field; therefore, the land-cover changes each year even though the total area of agricultural land-use remains the same. The price for corn increased from 2001 to 2010, which increased corn production in Iowa. We tested the hypothesis that the production increase was the result of changes in crop rotation in Iowa using the annual remote sensing classification (the cropland data layer) produced by the United States Department of Agriculture, National Agricultural Statistics Service. It was found that the area planted in corn increased from 4.7 million hectares in 2001 to 5.7 million hectares in 2007, which was correlated with the market price for corn. At the county level, there were differences in how the increase in corn production was accomplished. Northern and central counties had little land to expand cultivation and generally increased corn production by converting to a corn-corn rotation from the standard corn-soybean rotation. Southern counties in Iowa increased corn production by expanding into land that was not under recent cultivation. These changes affect the amount of soil carbon sequestration.
Fuel moisture content (FMC) is an important variable for predicting the occurrence and spread of wildfire. Foliar FMC
was calculated as the ratio of leaf foliar water content (Cw) and dry matter content (Cm). Recently, the normalized dry
matter index (NDMI) was developed for the remote sensing of Cm using high-spectral resolution data. This study
explored the potential for remote sensing of FMC using the ratio of various vegetation water indices with NDMI. For
leaf-scale simulations, all index ratios were significantly related to FMC. For canopy-scale simulations, ratio indices of
the normalized difference infrared index (NDII) and normalized difference water index (NDWI) with NDMI predicted
FMC with R2 values of 0.900 and 0.864, respectively. NDII/NDMI determined from leaf reflectance data had the
highest correlation with FMC. Further investigation needs to be conducted to evaluate the effectiveness of this approach
at canopy scales with airborne remote sensing data.
Crop residue (or plant litter) on the soil surface can decrease soil erosion and runoff and improve soil quality. Quantification of crop residue cover is required to evaluate the effectiveness of conservation tillage practices as well as the extent of biofuel harvesting. Remote sensing techniques can provide reliable assessment od crop residue cover over large fields. With Landsat Thematic Mapper bands, crop residues can be brighter or darker than soils depending on soil type, crop type, moisture content, and residue age. With hyperspectral reflectance data, relatively narrow absorption features, centered near 2100 and 2300 nm, can be detected that are associated with cellulose and lignin concentrations. These features are evident in reflectance spectra of crop residues, but not in reflectance spectra of soils. Our objectives were to: (1) estimate crop residue cover using remotely sensed data over an agricultural site in central Iowa, and (2) evaluate alternative, less labor-intensive sampling schemes for acquiring crop residue cover surface reference data. We acquired EO-1 Hyperion imaging spectrometer data over agricultural fields in central Iowa shortly after planting in May 2004 and 2005. Crop residue cover was also measured in corn and soybean fields using line-point transects. The cellulose absorption index (CAI), which measured the relative intensity of the absorption feature near 2100 nm, was calculated using three relatively narrow bands centered at 2030, 2100, and 2210 nm. Results showed that crop residue cover was linearly related to CAI. Changes in the slopes of the regression line from year to year were related to scene moisture conditions. Tillage intensity classes corresponding to conventional tillage (≤ 30% cover) and conservation tillage (> 30% cover) were correctly identified in 75-82% of the fields. In addition, by combining information from previous season’s crop classification with crop residue cover after planting, an inventory of soil tillage intensity by previous crop was generated for the whole Hyperion scene for each year. Inventories and maps of tillage intensity are required for field- and watershed scale models to evaluate management practices that maximize production and minimize environmental impact.
Leaf and canopy water contents provide information for leaf area index, vegetation biomass, and wildfire fuel moisture
content. Hyperspectral retrievals of leaf and canopy water content are determined from the relationship of spectral
reflectance and the specific absorption coefficient of water over the wavelength range of a water absorption feature.
Vegetation water indices such as the Normalized Difference Water Index [NDWI = (R850 - R1240)/(R850 + R1240)] and
Normalized Difference Infrared Index [NDII = (R850 - R1650)/(R850 + R1650)] may be calculated from multispectral
sensors such as Landsat Thematic Mapper, SPOT HRG, or MODIS. Predicted water contents from hyperspectral data
were much greater than measured water contents for both leaves and canopies. Furthermore, simulated spectral
reflectances from the PROSPECT and SAIL models also had greater retrieved leaf and canopy water contents compared
to the inputs. Used simply as an index correlated to leaf and canopy water contents, hyperspectral retrievals had better
predictive capability than NDII or NDWI. Atmospheric correction algorithms estimate canopy water content in order to
estimate the amount of water vapor. These results indicate that estimated canopy water contents should have a
systematic bias, even though this bias does not affect retrieved surface reflectances from hyperspectral data. Field
campaigns in a variety of vegetation functional types are needed to calibrate both hyperspectral retrievals and vegetation
water indices.
Leaf chlorophyll content is an important variable for agricultural remote sensing because of its close relationship to leaf
nitrogen content. We propose a triangular greenness index (TGI), which calculates the area of a triangle with three
points: (λr, Rr), (λg, Rg), and (λb, Rb). TGI was correlated with chlorophyll content using a variety of leaf and plot
reflectance data. However, indices using the chlorophyll red-edge (710-730 nm) generally had higher correlations. With
broad bands, TGI had higher correlations than other indices at leaf and canopy scales. Simulations using a canopy
reflectance model indicate an interaction among TGI, leaf area index (LAI) and soil type at low crop LAI, whereas at
high crop LAI, TGI was only affected by leaf chlorophyll content. Excess nitrogen fertilizer causes numerous
environmental problems, nitrogen management using remote sensing will help balance fertilizer applications with crop
nitrogen requirements.
The water in green vegetation is detectable using reflectances in the near infrared and shortwave infrared. Canopy water
content is estimated from the product of leaf water content and leaf area index (LAI). The Normalized Difference
Infrared Index [NDII = (R850 - R1650)/(R850 + R1650)] was found to be strongly related to canopy water content using
various moderate resolution sensors (Landsat TM, ASTER, AWiFS) during the SMEX02, SMEX04, SMEX05, and
OTTER experiments. With the high temporal resolution of MODIS, changes in canopy water content may perhaps be
used to estimate plant water stress and wild-fire potential. However, the low spatial resolution of MODIS does not allow
the relationship between NDII and canopy water content to be determined experimentally. The objective of this study is
to validate the expected relationship of canopy water content with NDII by the standard LAI data product from MODIS;
the quotient is the expected leaf water content which will vary by land-cover type. Maximum NDII for 2000-2007 was
calculated from the MODIS standard surface reflectance data products and compared to maximum MODIS LAI for the
same years. Mean leaf water content from MODIS was not significantly different from leaf data for most land cover
types. However the large standard deviations indicated that canopy water content from NDII is not currently accurate for
monitoring the incipient stages of plant water stress.
Vegetation water content is an important biophysical parameter for estimation of soil moisture from microwave
radiometers. One of the objectives of the Soil Moisture Experiments in 2004 (SMEX04) and 2005 (SMEX05) were to
develop and test algorithms for a vegetation water content data product using shortwave infrared reflectances. SMEX04
studied native vegetation in Arizona, USA, and Sonora, Mexico, while SMEX05 studied corn and soybean in Iowa,
USA. The normalized difference infrared index (NDII) is defined as (R850 - R1650)/(R800 + R1650), where R850 is the
reflectance in the near infrared and R1650 is the reflectance in the shortwave infrared. Simulations using the Scattering by
Arbitrarily Inclined Leaves (SAIL) model indicated that NDII is sensitive to surface moisture content. From Landsat 5
Thematic Mapper and other imagery, NDII is linear with respect to foliar water content with R2 = 0.81. The regression
standard error of the y estimate is 0.094 mm, which is equivalent to about a leaf area index of 0.5 m2 m-2. Based on
modeling the dynamic water flow through plants, the requirement for detection of water stress is about 0.01 mm, so
detection of water stress may not be possible. However, this standard error is accurate for input into the tau-omega
model for soil moisture. Therefore, NDII may be a robust backup algorithm for MODIS as a standard data product.
One of the goals of applied remote sensing is to map locations of invasive weeds. However, differences in plant cover and leaf area index (LAI) alter canopy reflectance, making detection of a single species difficult. Variation in canopy reflectance may be simulated using the Scattering by Arbitrarily Inclined Leaves (SAIL) model. Simulated reflectances are used to calculate spectral angles to determine the separability of an invasive weed from co-occurring vegetation. Leafy spurge is a noxious invasive weed with yellow-green flower-bracts. Spectral angles from SAIL model simulations show that flowering leafy spurge may be detected when LAI is greater than 1.0 and flower-bract cover is greater than 10%. A threshold of 3.5 deg (0.061 radians) was determined to provide the best separation between leafy spurge and co-occurring vegetation. To test this prediction, the Spectral Angle Mapper was used to classify leafy spurge using AVIRIS, Landsat ETM+ and SPOT data. Classification accuracy was inversely related to simulated spectral angles from the SAIL model analyses. Using canopy reflectance models and spectral angles may help identify those invasive species that are potentially detectable by remote sensing, and may indicate the conditions where detection will be problematic based on variation of LAI, cover and other variables.
Remote sensing is used to show the actual distribution of distinctive invasive weeds such as leafy spurge (Euphorbia esula L.), whereas landscape modeling can show the potential distribution over an area. Geographic information system data and hyperspectral imagery [NASA JPL's Airborne Visible Infrared Imaging Spectrometer (AVIRIS)] were collected for Devils Tower National Monument in northeastern Wyoming, USA. Leafy spurge was detected in the AVIRIS imagery using the Spectral Angle Mapper with a 74% overall accuracy. The areas of leafy spurge presence and absence were compared to the predictions of the Weed Invasion Susceptibility Prediction (WISP) model. Over the area of the AVIRIS imagery, about 8% of the landscape was covered by leafy spurge, whereas 23% of the landscape has the potential to be invaded. Using kappa analysis, the agreement between remote sensing and landscape modeling was 30%, which was significantly less than expected by chance, indicating model errors. Detailed analysis of individual data layers showed that only a few of the predictor variables were required. Elimination of non-significant predictor variables reduced the area predicted to be susceptible to 13%, and increased the accuracy of the predictions to 81%. Remote sensing was a powerful addition to landscape modeling because the entire landscape was used for the analysis increasing its statistical power, whereas field data collection would be limited in scope and would be more costly.
Precision farming relies on the cost effectiveness of collecting and interpreting data, which describes the variations of agricultural conditions such as crop stresses, nutrient deficiencies, water stresses, or pest infestation. Hyperspectral remote sensing from satellites and airborne sensors can be a way to obtain data needed to develop site-specific farming management strategies. The primary objective of the hyperspectral applications in precision farming is to provide farmers with a technology, which can detect specific crop conditions that can be used to program variable-rate applications. Applications of water, pesticides, and fertilizer can be tailored to the needs of the agricultural crops, based on the conditions reflected on the imagery. This paper presents an experimental study performed in Beltsville, Maryland for assessing the plant density and nutrient uptake of corn using a simple photographic method from a model airplane versus obtaining hyperspectral imagery from an airborne sensor. The hyperspectral sensor utilized in this study was the AISA sensor. These remote sensors can measure the temperature of plants; or to be more specific, they can measure how much energy plants emit at the visible and near-infrared wavelengths of the spectrum, such as water and vegetation.
Management of semi-arid rangelands in the western United States for sustainability requires objective methods for monitoring large areas; the goal of the Wyoming Hyperspectral Imagery Pilot Project was to determine if hyperspectral remote sensing can provide the capability for rangeland assessment. Airborne Visible-Infrared Imaging Spectrometer (AVIRIS) data obtained during the Geology Group Shoot over the Nature Conservancy's Red Canyon Ranch near Lander, WY, were compared with Landsat Thematic Mapper data for classification of vegetation communities. Using the same training areas, supervised classification from the two sensors were significantly different. The amount of vegetation cover from unconstrained linear spectral unmixing was highly correlated to normalized difference vegetation index. The flight lines were east-west, vegetation on the north side of the image had significantly higher reflectances compared to similar vegetation on the south side of the image, possibly due to differences in the bidirectional reflectance distribution function. These results indicate hyperspectral imagery can provide better data on community composition, but equivalent information on the amount of vegetation. Thus, infrequent collection of AVIRIS data combined with other sensors provides an optimal solution for monitoring rangelands.
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