Potentials and limits of vegetation indices for LAI and APAR assessment

In the field of remote sensing applications, scientists have developed vegetation indices (VI) for qualitatively and quantitatively evaluating vegetative covers using spectral measurements. The spectral response of vegetated areas presents a complex mixture of vegetation, soil brightness, environmental effects, shadow, soil color and moisture. Moreover, the VI is affected by spatial‐temporal variations of the atmosphere. Over forty vegetation indices have been developed during the last two decades in order to enhance vegetation response and minimize the effects of the factors described above. This paper summarizes, refers and discusses most of the vegetation indices found in the literature. It presents different existing classifications of indices and proposes to group them in a new classification.

A review of vegetation indices

with the Agricultural Research Service (ARS) and various government agencies and private institutions have provided a great deal of fundamental information relating spectral reflectance and thermal emittance properties of soils and crops to their agronomic and biophysical characteristics. This knowledge has facilitated the development and use of various remote sensing methods for non-destructive monitor- ing of plant growth and development and for the detection of many environmental stresses which limit plant productivity. Coupled with rapid advances in computing and position- locating technologies, remote sensing from ground-, air-, and space-based platforms is now capable of providing detailed spatial and temporal information on plant response to their local environment that is needed for site specific agricultural management approaches. This manuscript, which empha- sizes contributions by ARS researchers, reviews the biophysi- cal basis of remote sensing; examines approaches that have been developed, refined, and tested for management of water, nutrients, and pests in agricultural crops; and as- sesses the role of remote sensing in yield prediction. It con- cludes with a discussion of challenges facing remote sens- ing in the future.

/pdf/remote-sensing-for-crop-management-323qhffpqs.pdf

Remote Sensing for Crop Management

An evaluation is made of the remotely sensed surface temperature (Ts)/normalized difference vegetation index (NDVI) relationship in studies of the influence of biome type on the slope of Ts/NDVI, and of the automation of the process of defining the relationship so that the surface moisture status can be compared with Ts/NDVI at continental scales. The analysis is conducted using the NOAA AVHRR over a 300 x 300 km area in western Montana, as well as biweekly composite AVHRR data. A strong negative relationship is established between NDVI and Ts over all biome types.

/pdf/developing-satellite-derived-estimates-of-surface-moisture-1jzims5in8.pdf

Developing Satellite-derived Estimates of Surface Moisture Status

ABSTRACf Thi s paper reviews various applications of remote sensing and image anal ysis in plant path ology. It describes technical methods and their possibilities, but also emphasizes the biological prerequisites and restrictions of prac tical appli­ cations. The subject area comprises many nondestructive and noninvasive methods to detect and assess plant diseases and stress objectiv ely and cost-ef­ fectively, even though to date we cannot identify the specific cause of the damage. As a supplement to conventional methods , these new methods have great potential to facilitate and increase accuracy and precision in plant patho­ logical researc h.

Remote sensing and image analysis in plant pathology

An attempt has been made to relate hand-held radiometer measurements, and airborne multispectral scanner readings, with both different wheat stand densities and grain yield. Aircraft overflights were conducted during the tillering, stem extension and heading period stages of growth, while hand-held radiometer readings were taken throughout the growing season. The near-IR/red ratio was used in the analysis, which indicated that both the aircraft and the ground measurements made possible a differentiation and evaluation of wheat stand densities at an early enough growth stage to serve as the basis of management decisions. The aircraft data also corroborated the hand-held radiometer measurements with respect to yield prediction. Winterkill was readily evaluated.

Spring wheat-leaf phytomass and yield estimates from airborne scanner and hand-held radiometer measurements†

Airborne measurements were made over controlled oil-spill test sites to evaluate various techniques, utilizing reflected sunlight, for detecting oil on water. The results of these measurements show that (1) maximum contrast between oil and water is in the uv and red portions of the spectrum; (2) minimum contrast is in the blue-green; (3) differential polarization appears to be a very promising technique; (4) no characteristic absorption bands, which would permit one oil to be distinguished from another, were discovered in the spectral regions measured; (5) sky conditions greatly influence the contrast between oil and water; and (6) highest contrast was achieved under overcast sky conditions.

Airborne optical detection of oil on water.

In an attempt to evaluate the relationship between hand-held infrared thermometers and aircraft thermal scanners in near-level terrain and to quantify the variability of surface temperatures within individual fields, ground-based and aircraft thermal sensor measurements were made along a 50-km transect on 3 May 1979 and a 20-km transect on 7 August 1980 These comparisons were made on fields near Davis, California Agreement was within 1 C for fields covered with vegetation and 36 C for bare, dry fields The variability within fields was larger for bare, dry fields than for vegetatively covered fields In 1980, with improvements in the collection of ground truth data, the agreement was within 1 C for a variety of fields

Variability of surface temperature in agricultural fields of central California

Values of directional and hemispherical emittance of twelve coatings were needed in support of a spacecraft experiment. Laboratory measurements were made by two calorimetric and four reflectance techniques and with two portable devices designed for field or laboratory operation. The measurement results are compared, primarily on the basis of hemispherical emittance values deduced from each; and the limitations and uncertainties of each technique are summarized.

A comparison of infrared-emittance measurements and measurement techniques.

Skylight polarization, which varies with the position of the sun in the sky, influences the contrast of oil on water. Good contrast is most consistently obtained by viewing in azimuth directions toward or away from the sun. Contrast is enhanced by imaging selected polarization components and by taking the difference between orthogonal polarization components.

https://science.sciencemag.org/content/sci/180/4091/1170.full.pdf

J. P. Millard

Papers

Spring wheat-leaf phytomass and yield estimates from airborne scanner and hand-held radiometer measurements†

Airborne optical detection of oil on water.

Variability of surface temperature in agricultural fields of central California

A comparison of infrared-emittance measurements and measurement techniques.

Polarization - A key to an airborne optical system for the detection of oil on water.