Blaine L. Blad

Simple equation to approximate the bidirectional reflectance from vegetative canopies and bare soil surfaces

A simple equation has been developed for describing the bidirectional reflectance of some vegetative canopies and bare soil surfaces. The equation describes directional reflectance as a function of zenith and azimuth view angles and solar azimuth angle. The equation works for simulated and field measured red and IR reflectance under clear sky conditions. Hemispherical reflectance can be calculated as a function of the simple equation coefficients by integrating the equation over the hemisphere of view angles. A single equation for estimating soil bidirectional reflectance was obtained using the relationships between solar zenith angles and the simple equation coefficients for medium and rough soil distributions. The equation has many useful applications such as providing a lower level boundary condition in complex plant canopy models and providing an additional tool for studying bidirectional effects on pointable sensors.

Multisite analyses of spectral-biophysical data for corn.

Abstract Spectral-biophysical functional relations that hold across experiments and environments are needed for economically important crops. Thus, reflectance factors and leaf area index (LAI) measurements for sorghum [ Sorghum bicolor (L.) Moench] experiments conducted at Bushland, Texas (102.2°W, 35.2°N), Lubbock, Texas (101.8°W, 33.7°N), and Weslaco, Texas (98.0°W, 26.2°N) over a 7-year period were related using two-parameter power and exponential equations for four common vegetation indices (VI): normalized difference (NDVI), perpendicular (PVI), near-infrared to red ratio (RVI), and transformed soil-adjusted (TSAVI). The objective was to produce recommendable empirical equations for estimating LA1 of sorghum from spectral vegetation indices using observations pooled across locations. During both the pre- and post-maximum leaf area index portions of the growing season, power and exponential forms were equally good for estimating LAI from TSAVI and NDVI and gave coefficients of determination, R 2 , that ranged from 0.76 to 0.83. However, for PVI and RVI the relations were not as nonlinear and the power form accounted for more of the variation ( R 2 = 0.74–0.82) than the exponential form ( R 2 = 0.69–0.78). Infinite reflectance for sorghum was estimated from the pooled data to be 4.0% in the RED band and 49.7% in the NIR band. Coefficients of exponential relations for sorghum were similar to those for corn reported earlier. Although soils, agronomic treatments, sun angles, and instruments differed among locations, it was possible to develop general relations for estimating leaf area of sorghum from four of the most commonly used spectral vegetation indices.

Turbulent Exchange Coefficients for Sensible Heat and Water Vapor under Advective Conditions

Abstract Results are presented of micrometeorological measurements made over alfalfa and soybeans under conditions of sensible heat advection at Mead, Neb. The sensible heat advection phenomenon reported here is of a regional rather than a local nature. The exchange coefficient for sensible heat (KH) is found to be generally greater than the exchange coefficient for water vapor (KW). This result contradicts the usual assumption of equality of KH and KW under nonadvection (lapse or unstable) conditions when the net transfer of both sensible heat and water vapor are away from the earths surface. Under advective conditions, however, heat and water vapor are transferred in opposite directions. Our results are supported by Warhafts (1976) recently published theoretical analysis in which he concludes that the greatest departure of KH/KW from unity will occur when temperature and humidity gradients are of opposite sign.

A critical review of light models for estimating the shortwave radiation regime of plant canopies

Abstract A general review of geometrical and statistical light models is presented in this paper. In the geometrical approach plant shapes are simulated by various geometrical forms described by characteristic dimensions. Geometrical models may be divided into two classes — those which consider individual shapes and those which consider an arrangement of shapes. In the statistical approach the location of plant elements is parameterized by various distributions. The type of leaf dispersion in space is the most important consideration in statistical models. Four different types of leaf dispersion are considered in this review: regular leaf dispersion, clumped leaf dispersion, random leaf dispersion, and variable leaf dispersion. Hypotheses which underlie the various random and more generalized types of statistical light models are presented. Several models in the literature are discussed in terms of these assumptions. Although, in many cases, required plant data and actual light measurements in the field are grossly inadequate for experimental verification of light models, it appears that the light regime in plant canopies can be adequately described by those models already available. However, for the most part, these models are very complex and a synthesis of these fundamental models into workable expressions that can be used by agronomists, crop ecologists and others concerned with breeding plants for more efficient interception of light is needed.

Prairie grassland bidirectional reflectances measured by different instruments at the FIFE site

Land surface reflectance measurements were acquired during the First ISLSCP Field Experiment (FIFE) field campaigns using a variety of ground-based and airborne spectral radiometers. To examine the validity of the assumption that the values acquired by the several different instruments and teams were interchangeable, the surface radiation measurement teams converged on a common site for 1 day during the fifth intensive field campaign (IFC 5) in 1989. The instruments compared for near-surface measurements included two ground-based Spectron Engineering SE59Os, one helicopter-mounted SE590, one ground-based and one helicopter-mounted Barnes modular multiband radiometer (MMR), and the portable apparatus for rapid acquisitions of bidirectional observations of land and atmosphere (PARABOLA) field radiometer. Comparisons were made for nadir measurements over a range of solar zenith angles and a range of off-nadir viewing angles. The bidirectional reflectances from the different instruments were generally found to be quite comparable. For example, for a 52° solar zenith angle, the nadir red and near-infrared spectral reflectance factors ranged from 3.5 to 4.5% and 28.2 to 31.9%, respectively. At the smaller solar zenith angles, however, the differences were somewhat greater (red, 4.5–6.1%; near-infrared (NIR), 25.0–28.9%), and the coefficients of variation for the samples taken by all of the instruments increased. Off-nadir viewing caused major departures from nadir bidirectional reflectances (30% reflectance at nadir compared with 55% at 60° off nadir in the NIR, for example), but all of the instruments captured the effects reasonably well. Spectral vegetation indices were found to have a considerable dependence on both solar zenith angle and sensor viewing angle. In spite of the general agreement between most instruments and teams, the lack of a more consistent band-to-band agreement resulted in appreciable differences in the spectral vegetation index values, which could potentially affect the accuracy and precision of remote sensing assessments of biophysical parameters.

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.

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.

Lysimetric Calibration of the Bowen Ratio-Energy Balance Method for Evapotranspiration Estimation in the Central Great Plains

Abstract Most reports show good agreement between evapotranspiration (ET) rates estimated by the Bowen Ratio-Energy Balance (BREB) method and rates measured with lysimeters, although underestimation by the BREB model has occasionally been reported. This study was conducted to evaluate the performance of the BREB technique in the climatic conditions characteristic of the central Great Plains, a region where a significant proportion of the energy consumed by evapotranspiration is supplied from advected sensible heat. Agreement between the BREB method and lysimetric measurements of ET is good during non-advective periods but during advective periods the BREB model underestimates ET by about 20%. Data collected in this study suggest that the difference is due primarily to an inequality of the exchange coefficients for beat (Kh) and water vapor (Kw). In the development of the BREB method these coefficients are assumed to be identical but our results indicate that the ratio Kh/Kw is greater than 1 for the stabl...

Estimation of shortwave hemispherical reflectance (albedo) from bidirectionally reflected radiance data

Abstract Albedo is the ratio of reflected solar radiation from a surface to that incident upon it. Due to the spatial and temporal resolution of satellite remote sensing instruments, many formulations have been developed to convert remotely sensed data into estmates of albedo. Most of these equations depend upon the assumption of isotropic reflection and, therefore, use only nadir measurements; only in recent years have investigators attempted to model the anisotropic nature of terrestrial surfaces. A Barnes Modular Multiband Radiometer (MMR) was used to collect remotely sensed data from prairie vegetation at seven view zenith angles in the solar principal plane. An equation to estimate albedo from bidirectional reflectance data is proposed and evaluated in this paper. The estimates of albedo were greater than values obtained with simultaneous pyranometer measurements: a more conventional approach. The overestimation was systematic. Potential sources of error are discussed and include: 1) extrapolation of the bidirectional reflectance data out to a view zenith angle of 90°; 2) use of inappropriate weighting coefficients in the numerator of the albedo equation; 3) surface shadowing caused by the A-frame intrumentation used to measure the incoming and outgoing radiation fluxes; 4) errors in estimates of the denominator of the proposed albedo equation (i.e., incoming shortwave radiation); and 5) a “hot spot” contribution in bidirectional data measured by the MMR.

Plant and air temperatures in differentially-irrigated corn☆

Abstract Studies by Jackson et al. (1977) and Idso et al. (1977) indicate that wheat is not stressed for water unless leaf temperature exceeds air temperature. One objective of the study reported here was to determine relationships between leaf temperature and air temperature within corn canopies as a function of water stress. A second objective was to evaluate the effects of varying levels of plant water stress on crop temperature. A third objective was to establish the relationship between plant water stress and crop temperature in corn ( Zea mays L.) with the aim of providing practical water resource management tools. Meteorological, physiological and phenological measurements were made in nine plots of corn, grown on Valentine fine sand (Typic ustipsamment) at the Sandhills Agricultural Laboratory located near Tryon, Nebraska. Each plot received one of seven different irrigation treatments. Canopy temperatures were measured with an infrared thermometer at midday throughout the growing season. Air and leaf temperature measurements were made on an hourly basis with thermocouples. Physiological and phenological observations were made weekly. The average midday difference in canopy temperature between stressed and non-stressed areas was as large as 7.0°C. In fully-irrigated plots, the standard deviation of midday canopy temperature was about 0.3°C but in non-irrigated areas it reached, at times, 4.2°C. It is concluded that a standard deviation of temperature in a plot exceeding 0.3°C signals that some plants are experiencing water stress. This behaviour can indicate the need for irrigation. Daily profiles of leaf and air temperature in stressed and non-stressed canopies were found to be similar. Profiles tended to be lapse before crop cover was complete and inverted later in the season. At any level within the stressed canopy, plants were warmer than at the same level within the non-stressed canopy. The midday temperature of sunlit leaves of non-stressed and moderately stressed plants was generally 1–2°C below air temperature. The temperature of sunlit leaves in severely stressed plants was as much as 4.6°C above air temperature. It was observed that corn plants may be subject to water stress and still be cooler than air temperature.