E.T. Kanemasu

Intercepted photosynthetically active radiation estimated by spectral reflectance

Abstract Interception of photosynthetically active radiation (PAR) was evaluated relative to greenness and normalized difference [MSS (7 − 5)/(7 + 5)] for five planting dates of wheat for 1978–1979 and 1979–1980 at Phoenix, Arizona. Intercepted PAR was calculated from leaf area index and stage of growth. Linear relationships were found with greenness and normalized difference with separate relationships describing growth and senescence of the crop. Normalized difference was significantly better than greenness for all planting dates. For the leaf area growth portion of the season the relation between PAR interception and normalized difference was the same over years and planting dates. For the leaf senescence phase the relationships showed more variability due to the lack of data on light interception in sparse and senescing canopies. Normalized difference could be used to estimate PAR interception throughout a growing season.

Relating the microwave backscattering coefficient to leaf area index

Abstract This paper examines the relationship between the microwave backscattering coefficient of a vegetation canopy, σ can 0 , and the canopys leaf area index (LAI). The relationship is established through the development of one model for corn and sorghum and another for wheat. Both models are extensions of the cloud model of Attema and Ulaby (1978). Analysis of experimental data measured at 8.6, 13.0, 17.0, and 35.6 GHz indicates that most of the temporal variations of σ can 0 can be accounted for through variations in green LAI alone, if the latter is greater than 0.5.

Estimation of Total Above-Ground Phytomass Production Using Remotely Sensed Data*

Remote sensing potentially offers a quick and nondestructive method for monitoring plant canopy condition and development. In this study, multispectral reflectance and thermal emittance data were used in conjunction with micrometeorological data in a simple model to estimate above-ground total dry phytomass production of several spring wheat canopies. The fraction of absorbed photosynthetic radiation (PAR) by plants was estimated from measurements of visible and near-infrared canopy reflectance. Canopy radiation temperature was used as a crop stress indicator in the model. Estimated above-ground phytomass values based on this model were strongly correlated with the measured phytomass values for a wide range of climate and plant-canopy conditions.

Estimates of leaf area index from spectral reflectance of wheat under different cultural practices and solar angle

Two methods (a regression formula and an indirect technique) were used to assess the influence of several management practices and solar illumination angles on the estimates of leaf area index (LAI) using red and near infrared reflectance measurements from wheat (Triticum aestivum L. and T. durum) experiments conducted. In all three experiments, the management treatments affected the seasonal trend in spectral response of wheat canopies. The effect of each treatment was shown in the estimated LAIs that were based on the measurements of canopy reflectance. Solar illumination angle also affected the spectral properties of the canopies, and hence the estimated LAIs. For both methods, good agreements were obtained between the measured and estimated LAIs over a range of 0.0–6.0 LAI, with major differences only for LAI greater than 6.0.

Seasonal canopy reflectance patterns of wheat, sorghum, and soybean

Abstract Reflectance characteristics of agronomic crops are of major importance in the energy exchanges of a surface. In addition, unique reflectance patterns may be an aid in crop identification by means of remote sensing. Our study suggests that the ratio of the reflectances of the 545-nm to the 655-nm wavebands provides information about the viewed surface, regardless of the crop. The reflectance ratio is less than unity early and late in the growing season. For all crops studied, the ratio closely followed crop growth and development and appeared to be more desirable than the near-infrared reflectance as an index of growth.

Interception and use efficiency of light in winter wheat under different nitrogen regimes

In an identical experiment conducted at Mandan (ND), Manhattan (KS) and Lubbock (TX), the influence of the environment and nitrogen (N) fertility upon light interception efficiency (ei) and light use efficiency (ec) of winter wheat (Triticum aestivum L.) were examined using remotely sensed canopy reflectance data to estimate ei. Treatments consisted of two cultivars, four levels of applied N and three levels of irrigation. Increased N application resulted in increased ei, with only secondary effects on ec. Whole season values of ec did not differ significantly between sites or between crops grown under different N regimes. However, ec did change through the season, increasing from an average of 1.5 during the double ridge-to-terminal spikelet stage to an average of 3.8 during the terminal spikelet-to-anthesis stage and finally decreasing to an average of 3.1 during the anthesis-to-soft dough stage. These changes in ec corresponded to changes in the mean temperatures for each growth period.

Leaf-area estimates from spectral measurements over various planting dates of wheat

Abstract Several vegetative indices were analysed for their sensitivity and stability to green-leaf-area index (LAI) changes over various planting dates and irrigation frequencies of wheat grown at Phoenix, Arizona, from 1978 to 1980. Seasonal patterns of greenness showed that values saturated at LAI values above 4·0 did not return to the pre-emergence bare-soil value at senescence, and were not uniquely related to LAI over the various planting dates. Regressions of individual MSS band reflectances against LAI also showed that there was not a unique relation between any of the bands and LAI. However, the near-infrared/red reflectance ratio was stable over all planting dates and could be used successfully over a number of years and locations

Assessing grassland biophysical characteristics from spectral measurements

Abstract Remote sensing offers a potential alternative to tedious hand sampling as a means of monitoring vegetation condition and estimating productivity over large areas of grasslands. This study was conducted to assess the use of spectral reflectance measurements in estimating grass canopy leaf area index (LAI) and total above ground green phytomass. Spectral reflectance measurements were made on a tallgrass prairie during 1983 and 1984 with two multiband radiometers. Green leaf-area index and dry matter accumulations (green above-ground phytomass) were measured on the area monitored by the radiometer. Three indices—near-infrared to red ratio, greenness, and normalized difference—were computed from spectral reflectance data. The direct relationships between these spectral reflectance indices and grass biophysical parameters (LAI and phytomass) were site-dependent and year-specific. Indirect methods of estimating LAI and phytomass from estimates of absorbed, phytosynthetically active radiation, based on measurements of grass canopy spectral reflectance, were found to be more consistent across treatments for the two years of this study.

Thermal scanner measurement of canopy temperatures to estimate evapotranspiration

Abstract Temperatures of soybean ( Glycine max L.), sorghum ( Sorghum bicolor L.), and millet ( Panicum miliaceum L.), measured with a thermal scanner flown at altitudes of 610 and 1220 m, were used in a resistance form of the energy-balance equation to estimate evapotranspiration ( ET ). Atmospheric attenuation produced errors of 1 to 6 °C in the scanner measurements. Errors from neglecting emissivity corrections were less than 1.0 °C. A correction procedure relating temperature error to atmospheric precipitable water was applied to the scanner measurements. Estimates of ET using the corrected temperatures differed from lysimetric measurements by -0.40 to 0.1 7 ly/min. It was found that accurately extrapolating scanner measurements to ground level was essential for estimating ET .

Effect of carbon dioxide on sorghum yield, root growth, and water use

Abstract The concentration of atmospheric carbon dioxide (CO 2 ) is rising. The effect of higher than ambient levels of CO 2 on plants grown in the sub-humid central Great Plains of the U.S.A. has not been investigated. Therefore, an experiment was conducted at Manhattan, Kansas, to study the effect of elevated levels of CO 2 on grain sorghum [ Sorghum bicolor (L.) Moench]. During the summer of 1984, the sorghum was grown in rhizotrons in which root and shoot growth could be monitored throughout the growth cycle. The tops of the plants were enclosed in plastic chambers, which contained one of four concentrations of CO 2 : 330 (ambient), 485, 660, and 795 μl 1 −1 . Enriched CO 2 delayed the boot, half bloom, and soft dough stages. Sorghum grown at elevated concentrations of CO 2 yielded more roots and shoots than plants grown with 330 μl 1 −1 . At all soil-profile depths, root numbers and weights were higher at elevated CO 2 than at ambient CO 2 . However, water use per unit dry matter of leaf, stem, root, and grain was decreased 13, 30, 31, and 29%, respectively, in plants grown at 795 μl 1 −1 CO 2 compared to plants at 330 μl 1 −1 CO 2 . Although elevated CO 2 levels increased the stomatal resistance and leaf temperature, an increase in leaf area indices resulted in a lower canopy resistance.