Mark A. Mesarch

Relations between Directional Spectral Vegetation Indices and Leaf Area and Absorbed Radiation in Alfalfa

Abstract Sensors on satellite platforms with extreme view angles have been increasingly used to analyze regional and global vegetation cover and productivity because of frequent observations. This study, using experimental and theoretical methods, analyzed variations in vegetation indices with sun-view geometry as a means of understanding the sensitivity of relations beween vegetation indices and the biophysical properties, the leaf area index (LAI), and the instantaneous fraction of absorbed photosynthetically active radiation (fAPAR). Canopy bidirectional reflectance factors (BRFs) of an alfalfa crop were measured and simulated at a variety of solar and view zenith angles. Also, fAPAR, LAI, and leaf optical properties were measured. Measured and simulated canopy reflectances agreed generally within 1% (absolute). Normalized difference and simple ratio vegetation indices (NDVI and SRVI, respectively), derived from BRFs, varied with view and solar zenith angles. The minimum for near-infrared (NIR) BRFs and relatively high red BRFs generally occurred near nadir, resulting in some of the lowest vegetation index values. Highest VI values were generally obtained at forward view angles. Variation of NDVI with sun-view-geometry was greatest at LAs 2. Measured reflectances indicate that relations between NDVI and LAI and between SRVI and fAPAR were curvilinear across all solar and view zenith angle combinations in the solar principal plane, whereas relations between SRVI and LAI and between NDVI and fAPAR varied from linear to curvilinear. Analyses revealed that vegetation indices at large view zenith angles were poorly correlated with fAPAR, whereas those at small zenith angles were strongly correlated. In general, vegetation indices were more sensitive to fAPAR than to LAI, which is attributed to the fact that fAPAR is a radiation quantity, whereas LAI is nonlinearly related to radiation. Regression of fAPAR with VI values derived from combinations of red and NIR BRFs from similar and nonsimilar directions indicates that the highest correlation is in near-nadir and backscatter directions. However, further investigation into variations of relations between remotely sensed observations and canopy attributes and into the usefulness of off-nadir in extracting information is recommended.

Biophysical properties affecting vegetative canopy reflectance and absorbed photosynthetically active radiation at the FIFE site

Leaves of the dominant grass species of the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) site reflect and transmit radiation in a similar manner to other healthy green leaves. Visible reflectance factors (RFs) and transmittance factors (TFs) were lower for older leaves than younger leaves except during senescence, when RF and TF values were higher. Near-infrared (NIR) RF values increased and TF values decreased with leaf age, with the reverse occurring as the leaf underwent senescence. Leaf optical properties were not found to be dependent on leaf water potential in the range from −0.5 to −3.0 MPa. Canopy bidirectional reflectance factor (BRF) values generally increased with increasing view zenith angle (θυ). Maximum values were in the backscatter direction, whereas BRF values in the visible region were lowest at oblique off-nadir θυ in the forward scatter direction and at or near nadir in the NIR region. Solar principal plane BRF values varied most at large solar zenith angles (θs). Visible and mid-infrared canopy BRF values decreased and NIR BRF values increased with leaf area index (LAI). Soil BRF distributions in the solar principal plane varied slightly with θs and θυ and varied considerably for wet and dry surfaces. Spectral vegetation indices (SVIs) varied with θs and θυ; values were lowest in the backscatter direction and highest in the forward scatter direction. The fraction of absorbed photosynthetically active radiation (APAR) increased with increasing θs. APAR had a strong linear relationship to nadir-derived SVI values but not to oblique off-nadir-derived SVI values. The relatively small dependence of off-nadir SVI values on θs should allow daily APAR values to be estimated from measurements made at any time of the day.

A Revised Measurement Methodology for Conifer Needles Spectral Optical Properties: Evaluating the Influence of Gaps between Elements☆

Abstract Gaps are unavoidable when compositing small or narrow plant parts (e.g., conifer needles, twigs, narrow leaves, and leaflets) on sample holders in preparation for measuring spectral optical properties. The Daughtry et al. (1989) (A new technique to measure the spectral properties of conifer needles. Remote Sens. Environ. 27:81–91.) method of measuring conifer needle optical properties utilizes a relatively large gap fraction (approximately 0.3–0.6) and needles painted black on one surface of the sample from which the gap fraction of the sample is indirectly determined. Following this protocol typically results in distortions in optical properties, including underestimates in transmittance (sometimes negative values), and only one surface of the sample can be measured. The objectives of this article are to: 1) evaluate the influence of gaps between sample elements (conifer needles, twigs, narrow leaves and leaflets) on optical properties calculated with the published equations from Daughtry et al. (1989) and 2) revise the original Daughtry et al. method for optical property measurements by using an image-analysis to directly measure the gap fraction and use both surfaces of the sample. We achieve these objectives by reviewing the theory and investigating the effects of gaps by measurements of an inert photographic film material, fir needles, and mesquite leaflets. Tests to estimate the transmittance of film samples (film) and foliage (fir needles, mesquite leaflets) indicate that a relatively small gap fraction (less than 0.20) reduces the occurrence of computed negative transmittance values, reduces the variation in computed values, and yields values expected for the “true” or “nongap” transmittance. Employing the image analysis along with reduced gap fractions decreased the variance of measurements and permitted measurements of both surfaces per sample, thus reducing the time required by making half as many samples as originally required by Daughtry et al.

An improved goniometer system for calibrating field reference-reflectance panels

Abstract Field reference-reflectance panels need an initial calibration and periodic recalibration to ensure valid field reflectance data. The field calibration method proposed by Jackson et al. (1987) is an affordable means to accomplish this provided that a field goniometer system is available. Design and construction details for such a system are described.

Estimating net radiation with remotely sensed data: Results from KUREX‐91 and FIFE studies∗

Net radiation (Rn) is the major source of energy for evaporating water, heating the soil and air, and photosynthesis. The objective of this study is to estimate this important parameter with various models that have been developed to estimate the radiation balance components with remotely sensed data, and readily available meteorological data. Data used in this paper were collected over grassland vegetation during the FIFE‐87, ‐88, ‐89 studies and the KUREX‐91 study. For all studies estimated values of Rn were within about 10% of measured Rn. For the KUREX‐91 study, measured and estimated Rn agreed to within about 1%. Improvement in a model(s) to estimate the reflected shortwave flux would provide an even better estimate of Rn since in all studies the reflected radiation stream was overestimated compared to the measured values. There was no clear trend for under or over‐estimation of incoming short‐wave radiation from study to study. Components of the long‐wave balance were estimated with low mean relativ...

Comparing biophysical properties of the Streletskaya Steppe Reserve and the Konza Prairie

Reflected radiation by a crop canopy can be influenced by changes in the biological processes and physical attributes of the canopy. The study was conducted to characterize the effective leaf area index (LAI) and leaf angle distribution of the Streletskaya Steppe Reserve of the Russian Republic and the Konza Prairie in Kansas and to determine dependence of leaf optical properties on leaf water potentials of some dominant species in discrete wavebands in the visible and near‐infrared (NIR) (spanning 400–1000 nm range). Biophysical properties were measured in July 1991 during the Kursk Experiment (KUREX‐91) and in the summer of 1989 during the First ISLSCP Field Experiment (FIFE‐89). Leaf area index, leaf angle distribution, mean tilt angle, canopy height, leaf optical properties, and leaf water potential were measured. Generally, the KUREX‐91 steppe sites were characterized by high leaf area index and an uniform leaf angle distribution, while the FIFE‐89 prairie sites were characterized by low leaf area in...

Absorbed photosynthetically active radiation and sun‐view geometry effects on remote sensing relationships∗

Quantifying the amount of photosynthetically active radiation (PAR) absorbed by vegetation is an essential consideration for determining useful vegetative photosynthetic capacity and surface conductance values for regional and global carbon cycle studies. This study was conducted to compare absorbed photosynthetically active radiation at the FIFE‐89 Konza prairie sites to that of the KUREX‐91 steppe grassland sites and to investigate variations in relationships between absorbed PAR and spectral vegetation indices derived from bidirectional reflectance factors. Incoming, reflected and transmitted PAR were measured from which fractions of reflected, transmitted and absorbed PAR were computed at selected FIFE prairie and KUREX steppe sites. Fractions of direct and diffuse PAR transmitted through canopies were estimated. Fractions of absorbed PAR were much lower at the FIFE sites (ranging from 0.35 to 0.65) than those at KUREX (ranging from 0.75 to 0.95) which can be explained by differences between leaf area...

Estimation of the longwave radiation balance components: KUREX‐91 and FIFE studies∗

Estimates of incoming longwave radiation flux densities (R1?) obtained using several empirical and theoretical algorithms were compared to flux densities measured during the FIFE‐87 and FIFE‐88 experiments. Statistical analysis showed that all models gave values that were greater than the measured values with biases ranging from 25 to 55Wm−2. The incoming longwave radiation models were corrected to reduce or eliminate these biases and applied to independent data sets obtained during FIFE‐89 and KUREX‐91. With these adjustments, the models showed significant improvement in their ability to accurately estimate Rl?. The algorithms of Brunt and Brutsaert performed the best of all models and gave consistently good results for the FIFE‐89 and KUREX‐91 studies. Compared to the Brunt equation, the Brutsaert equation had a higher d value and smaller random and systematic errors. Both models had mean relative errors of less than 2% for the KUREX‐91 study and about 5% for the FIFE‐89 study. Outgoing (emitted) longwa...

Biophysical Properties of the Streletskaya Steppe Reserve

Biophysical properties were measured at the Streletskaya Steppe Reserve of the Russian Republic in July 1991 for the Kursk Experiment (KUREX-91) and at the Konza Prairie in Kansas during the summer of 1989 for the First ISLSCP Field Experiment (FIFE89). Leaf area index, leaf angle distribution, mean tilt angle, canopy height, leaf spectral properties, and leaf water potential data are presented. Comparisons between the leaf spectral properties of the two experiments are discussed.

BOREAS TE-12 SSA Shoot Geometry Data

The Boreal Ecosystem-Atmospheric Study (BOREAS) TE-12 (Terrestrial Ecology) team collected shoot geometry data in 1993 and 1994 from aspen, jack pine, and black spruce trees. Collections were made at the Southern Study Area Nipawin Fen Site (SSA FEN), Young Jack Pine (YJP), Old Jack Pine (OJP), Old Aspen (OA), Young Aspen (YA), Mixed Site (MIX), and Old Black Spruce (OBS) sites. A caliper was used to measure shoot and needle lengths and widths. A volume displacement procedure was used to measure the weight of the shoot or twig submerged in water. The data files are available on a CD-ROM (see document number 20010000884), or from the Oak Ridge National Laboratory (ORNL) Distributed Active Archive Center (DAAC).