Long term measurement network for FIFE

The objectives were: to obtain selected instruments which were not standard equipment on the Portable Automated Mesometeorological (PAM) and Data Control Platform (DCP) stations; to assist in incorporation of these instruments onto the PAM and DCP stations; to help provide routine maintenance of the instruments; to conduct periodic instrument calibrations; and to repair or replace malfunctioning instruments when possible. All of the objectives were or will be met soon. All instruments and the necessary instrument stands were purchased or made and were available for inclusion on the PAM and DCP stations before the beginning of the IFC-1. Due to problems beyond control, the DCP stations experienced considerable difficulty in becoming operational. To fill some of the gaps caused by the DCP problems, Campbell CR21-X data loggers were installed and the data collected on cassette tapes. Periodic checks of all instruments were made, to maintain data quality, to make necessary adjustments in certain instruments, to replace malfunctioning instruments, and to provide instrument calibration. All instruments will be calibrated before the beginning of the 1988 growing season as soon as the weather permits access to all stations and provides conditions that are not too harsh to work in for extended periods of time

Measurement of surface physical properties and radiation balance for KUREX-91 study

Biophysical properties and radiation balance components were measured at the Streletskaya Steppe Reserve of the Russian Republic in July 1991. Steppe vegetation parameters characterized include leaf area index (LAI), leaf angle distribution, mean tilt angle, canopy height, leaf spectral properties, leaf water potential, fraction of absorbed photosynthetically active radiation (APAR), and incoming and outgoing shortwave and longwave radiation. Research results, biophysical parameters, radiation balance estimates, and sun-view geometry effects on estimating APAR are discussed. Incoming and outgoing radiation streams are estimated using bidirectional spectral reflectances and bidirectional thermal emittances. Good agreement between measured and modeled estimates of the radiation balance were obtained

Calibration of field reference panel and radiometers used in FIFE 1989

Remote sensing of the earth's surface features involves the measurement of reflected solar radiation and the interpretation of the data in biophysical terms. Reflected radiation is a function of the surface properties and incident solar irradiance. The amount of radiation reflected from a surface is compared to the amount of solar radiation received at the surface as a means of comparing information from different times of day as well as for different days of the year. Thus, it is imperative to calibrate the instruments used to measure the incoming and reflected radiation

Southeastern U.S. Vegetation Response to ENSO Events (1989–1999)

El Niño/Southern Oscillation (ENSO) is considered one of the most powerful forces driving anomalous global weather patterns. Large-scale seasonal precipitation and temperature changes influenced by ENSO have been examined in many areas of the world. The southeastern United States is one of the regions affected by ENSO events. In this study, remote sensing detection of vegetation response to ENSO phases is demonstrated with one-kilometer biweekly Normalized Difference Vegetation Index (NDVI) data (1989–1999) derived from the Advanced Very High Resolution Radiometer (AVHRR). The impacts of three ENSO phases, cold, warm and neutral, on vegetation were analyzed with a focus on two vegetation cover types, two seasons and two geographic regions within the southeastern U.S. Significant ENSO effects on vegetation were found in cropland and forest vegetation cover types based on image and statistical analysis of the NDVI data. The results indicate that vegetation condition was optimal during the ENSO neutral phase for both agricultural and natural vegetatio

Improving the calibration of silicon photodiode pyranometers

Reliable measurements of global irradiance are essential for research and practical applications. Silicon photodiode pyranometers (SiPs) offer low-cost sensors to measure direct and diffuse irradiance despite their non-uniform spectral response over the 300–1000 nm spectral range. In this study, non-adjusted linear and adjusted calibrations were applied at different times of the year to determine sources of estimated errors in global irradiance due to the two calibration approaches, calibration time, and sensor age. 16 SiPs, along with two standards, measured incident global irradiance over a 5-year period under a range of sky conditions. Sensors performed best in the months in which they were calibrated when using the linear calibration approach. With the solar zenith angle adjusted calibration approach, certain calibration months provide a defendable validation for the following 12 months [ranging an average of 13.5–17.4 W m−2 standard error (SE)], while other calibration months do not provide consistent results and sometimes result in very poor validation (31.1–242.7 W m−2 SE). Older sensors (greater than 6 years) in general become more sensitive to solar zenith angle and their response drifts over time, while newer SiPs performed better than older sensors. Calibrations which accounted for solar zenith angle effects improved global irradiance estimates for older SiPs. For the Lincoln NE location, the appropriate calibration is in spring or late summer, regardless of calibration approach. These results indicate that solar zenith angle correction is not needed for largely diffuse components under cloudy conditions, so that in the future, a “smart” calibration may be possible, where diffuse radiation fractions are known

Measuring and modeling near-surface reflected and emitted radiation fluxes at the FIFE site

Information is presented pertaining to the measurement and estimation of reflected and emitted components of the radiation balance. Information is included about reflectance and transmittance of solar radiation from and through the leaves of some grass and forb prairie species, bidirectional reflectance from a prairie canopy is discussed and measured and estimated fluxes are described of incoming and outgoing longwave and shortwave radiation. Results of the study showed only very small differences in reflectances and transmittances for the adaxial and abaxial surfaces of grass species in the visible and infrared wavebands, but some differences in the infrared wavebands were noted for the forbs. Reflectance from the prairie canopy changed as a function of solar and view zenith angles in the solar principal plane with definite asymmetry about nadir. The surface temperature of prairie canopies was found to vary by as much as 5 C depending on view zenith and azimuth position and on the solar azimuth. Aerodynamic temperature calculated from measured sensible heat fluxes ranged from 0 to 3 C higher than nadir-viewed temperatures. Models were developed to estimate incoming and reflected shortwave radiation from data collected with a Barnes Modular Multiband Radiometer. Several algorithms for estimating incoming longwave radiation were evaluated and compared to actual measures of that parameter. Net radiation was calculated using the estimated components of the shortwave radiation streams, determined from the algorithms developed, and from the longwave radiation streams provided by the Brunt, modified Deacon, and the Stefan-Boltzmann models. Estimates of net radiation were compared to measured values and found to be within the measurement error of the net radiometers used in the study

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

s ensors 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 re$ectances agreed generally within 1% (absolute). Normalized dijj5erence 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 value-s were generally obtained at forward view angles. Variation of NDVI with sun-view-geometry was greatest at LAls ~2, whereas the range in SRVI was greatest for LAls\u3e2. Measured reflectances indicate that relations between NDVl and LA1 and between SRVl and fAPAR were curvilinear across all solar and view zenith angle combinations in the solar principal plane, whereas relations between SRVI and LA1 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 NlR 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 of-nadir in extracting information is recommended. OElsevier Science Inc., 199