David E. Escobar

Vegetation Indices in Crop Assessments

Abstract Vegetation indices (VI), such as greenness (GVI), perpendicular (PVI), transformed soil adjusted (TSAVI), and normalized difference (NDVI), measure the photosynthetic size of plant canopies and portend yields. A set of equations, called spectral components analysis (SCA), that interrelates VI or cumulative seasonal VI (∑VI), leaf area index (I), fractional photosynthetically active radiation (FPAR, dimensionless), cumulative daily PAR energy absorbed (∑APAR, MJ/m2), above-ground dry photomass (DM, g/m2), and economic yield (Y, g /m2) are presented and used to analyze data from two studies conducted in 1989. In one study we made boll counts and percent plant cover measurements in a salt-affected cotton field on 60 m grid intervals and calculated GVI, PVI, TSAVI, and NDVI at the grid intersections for SPOT-1 HRV and videography scenes. The four VI from SPOT accounted for more of the variation in the lint yield estimated from the boll counts (75–76%) than those from videography (61–63%), but VI from the different systems each accounted for 67% of the variation in plant cover. All relations were linear. In the other study, reflectance factors and FPAR were measured periodically during the season in corn planted at three densities (7.7, 5.4, and 3.1 plants/m2). FPAR could be estimated from NDVI and PVI, respectively, by FPAR = −0.344 + 0.229exp (1.95 NDVI) (r2 = 0.973) and FPAR = 0.015 + 0.036(PVI) (r2 = 0.956). Methods of obtaining FPAR and their effect on the efficiency of conversion of APAR to DM are illustrated and discussed. The data demonstrate how SCA unifies and strengthens the scientific basis of VI interpretations.

Photographic and videographic observations for determining and mapping the response of cotton to soil salinity

Better ways are needed to assess the extent and severity of soil salinity in fields in terms of economic impact on crop production and effectiveness of reclamation efforts. Procedures to help meet these needs were developed from soil salinity, plant height and boll counts, and digitized color infrared aerial photography and videography acquired during midboll set development stage for four salt-affected cotton (Gossypium hirsutum, L.) fields in the San Joaquin Valley of California. Unsupervised classijication procedures were used to produce seven-category spectral maps by field. Regression equations were developed from salinity measurements in the surface 30 cm (EC1) at 100-200 sample sites per field and the photography and videography digital counts at those same sites. The equations were used to estimate the salinity of each of the approximately 100,000 pixels per field, and the salinity categories corresponding to the spectral ones were mapped. The spectral classification maps and the estimated salinity maps corresponded well. Boll counts, made at about 20 sites perjield, were converted to lint yield and regressed on NDVl from both the photography and videography; the correlation coefficient (r) was 0.72 for video and 0.73 for the photographic data. Lint yields decreased by 43 f 10 kg ha-’ per dS mm1 increase in ECl, or

Airborne laser measurements of rangeland canopy cover and distribution.

52 f 12 ha-’ at current market prices. Our results illustrate very practical ways to combine image analysis capability, spectral observations, and ground truth to map and quantify the severity of soil salinity and its effects on crops.

Mapping leafy spurge (Euphorbia esula) infestations using aerial photography and geographic information systems

Studies were made at 2 rangeland areas in south Texas to measure canopy cover and distribution with an airborne laser profiler. In a comparison of laser and ground measurements of canopy cover on the same eighteen 30.5-m segments at the Yturria area, laser measurements of canopy cover ranged from 1 to 89% and were correlated significantly (r2 = 0.89) with ground measurements (1 to 88%) on the same eighteen 30.5-m segments. Comparisons of laser measurements of canopy cover for 500- and 940-m segments with an average of three 30.5-m ground measurements of canopy cover made within these segments were also significantly correlated (r2 = 0.95). Topography, vegetation height, and spatial distribution of canopy cover for 6- to 7-km flightlines were also measured with the laser profiler. Airborne laser measurements of land surface features can provide quick and accurate measurements of canopy cover and distribution for large areas of rangeland. Accurate and timely data on the amount and distribution of plant cover are valuable for understanding vegetation characteristics, improving estimates of infiltration, erosion, and evapotranspiration for rangeland areas, and making decisions for managing rangeland vegetation.

Airborne video systems for agricultural assessment

Abstract Leafy spurge is a troublesome weed on the northern Great Plains of the United States that chemicals and grazing management have not controlled. Remote sensing and geographic information system (GIS) technology have been used to detect and monitor numerous grassland related problems. The objectives of this study were to use both technologies to map and quantify the extent of leafy spurge within Theodore Roosevelt National Park and to provide information for managing the infestation. Analysis of the data indicated that 702 ha of the 18,680 ha park were infested by leafy spurge; however, leafy spurge populations occurring under dense woody canopies, in deep stream channels, and on steep slopes were not always detected. Infestations were especially dense in the western and southeast portions of the park. Most infestations were restricted to riparian zones and smaller drainage channels. Leafy spurge infestations decreased exponentially as distance from stream channels increased (r2=0.98). The signific...

Estimating grassland phytomass production with near-infrared and mid-infrared spectral variables

Abstract Remote sensing data has not been used to its fullest potential for management of natural resources largely because these data are not readily available. Video remote sensing has been proposed as an alternative to provide near-real-time information about natural resources. This paper reviews the status and development of airborne video imaging systems and their application for resource management, with special emphasis on agriculture. Video imagery has been used to detect or assess a variety of agricultural variables such as plant species, chlorosis, grass phytomass levels, cotton and alfalfa root rot infestations, wind erosion, soil moisture and irrigated crops, soil drainage and salinity, and insect pests. The digitization and computer processing of video imagery has also been demonstrated. Presently video does not have the detailed resolution of film, but it can provide farm managers with immediately available remote sensing data that can allow them to make quick decisions concerning their operations.

Assessment of grassland phytomass with airborne video imagery

Abstract Near-infrared and mid-infrared spectroradiometric reflectance variables were studied for estimating grass phytomass production within grass plots fertilized with five rates of nitrogen. Nine difference vegetation indices, five single wavelengths, and one waveband were correlated with phytomass. Among the 15 variables studied, TM5 (1550–1750 nm), reflectance at the 1100 nm (R1100), 1650 nm (R1650), and 2200 nm (R2200) wavelengths, and five indices [R1100–R1200, R1100–R1650, (R1100–R1300)-TM5, R1100–R2200, and R1200–R1650] had significant coefficients of determination (r 2 ) ranging from 0.83 to 0.92. The spectral variables involving the mid-infrared wavelengths generally yielded the highest r 2 coefficients. These results suggest the potential for estimating grass phytomass production over large and inaccesible rangeland areas using Thematic Mapper (TM5 and TM7) satellite data.

Utilizing airborne video imagery in kenaf management and production

Airborne video imagery was evaluated for assessing phytomass production within grass plots fertilized with five rates of nitrogen. Video imagery was taken with two black-and-white video cameras—one visible light and the other visible/near-infrared light sensitive. Red (0.644–0.656-μm) and infrared (0.815–0.827-μm) narrowband filters were used with the visible and visible/near-infrared light sensitive cameras, respectively. Hand-held red and infrared radiometric reflectance and phytomass measurements were made on the day that imagery was obtained. Red and infrared digital video data were obtained from the plots using an image processor. The plots were studied on two dates: 15 April and 22 May 1985. On 15 April, three phytomass levels could be distinguished within the infrared video image. Moreover, a infrared/red ratio video composite produced on an image processor enhanced differences among nitrogen treatments to the extent that generally four levels of phytomass could be identified. Coefficients of determination (r2), obtained by regressing red, infrared, and infrared/red reflectance, and red, infrared, and infrared/red digital video data on phytomass measurements were significant statistically. Imagery acquired on 22 May, however, showed few differences among treatments. This may have been caused by plant phenological and canopy architectural differences among the grass species. Nevertheless, video imagery should be a useful tool to assess phytomass production on rangelands when grasses are actively growing.

A video system to demonstrate interactions of near-infrared radiation with plant leaves

Abstract Kenaf ( Hibiscus cannabinus L.) is a rapid-growing nonwood fiber crop that can attain heights in excess of 5 m during a 150–200 day season. The tall kenaf coupled with the high plant densities used in commercial production can make visual detection of crop stress difficult. Airborne video imagery was evaluated for assessing kenaf production at three different locations. Results indicated that airborne video imagery had good potential for detecting crop stress resulting from Phymatotrichum root rot and the root-knot nematode/soil-borne fungi complex. Video imagery from all three locations indicated that remote sensing technology can provide an effective means for detecting and monitoring crop injury and disease movement in kenaf production. Airborne video imagery should be useful for detecting problem areas in large-scale plantings of kenaf. Identification of these potential problem areas should be beneficial to producers in regards to crop management.

A twelve-band airborne digital video imaging system (ADVIS)

Abstract A video system has been developed that displays results of interactions of near-infrared radiation in the 0.78 to 1.1 μm waveband with plant leaves. The system has a video camera, video monitor, cassette player, and cassette recorder. Three examples from the literature that have been shown to affect light reflectance were demonstrated successfully with the video system: leaf water infiltration, leaf maturation, and leaf stacking. The video system has potential use to facilitate teaching, research, and applied remote sensing.