Engineering topics

Weber, James A.

Publications and source records attributed to Weber, James A..

2 records

NASA NTRS1991

Diurnal change in trees as observed by optical and microwave sensors - The EOS Synergism Study

The EOS (Earth Observing System) Synergism Study examined the temporal variability of the optical and microwave backscatter due to diurnal change in canopy properties of interest to ecosystem modelers. The experiment was designed to address diurnal changes in canopy water status that relate to transpiration. Multispectral optical and multifrequency, multipolarization microwave measurements were acquired using boom-truck-based systems over a two-week period. Sensor and canopy properties were collected around the clock. The canopy studied was a walnut orchard in the San Joaquin Valley of California. The results demonstrate a large diurnal variation in the dielectric properties of the tree that in turn produces significant diurnal changes in the microwave backscatter. The results suggest that permanently orbiting spaceborne sensors such as those on EOS should be placed in orbits that are optimized for the individual sensor and need not be tied together by a tight simultaneity requirement on the order of minutes to hours for the purpose of monitoring ecosystem properties.

Way, Jobea↗

NASA NTRS1991

Diurnal water relations of walnut trees - Implications for remote sensing

Leaflet water content (WC), relative water content (RWC), and water potential, Phi(lf) were measured as indicators of diurnal change in tree water status in an experimental walnut orchard receiving two irrigation treatments: 100 and 33 percent of potential evapotranspiration (PET). Diurnal change was greatest in Phi(lf) throughout the experimental period, with minima occurring each day in early to mid-afternoon and maxima between midnight and sunrise. Leaflet WC and RWC were lower in the afternoon than at night, but had greater variability so that the diurnal pattern was not as clear. Comparison between the pattern of Phi(lf) and dielectric constants (DCs) measured from probes inserted 2 cm into a tree hole showed that both declined nearly in parallel in the morning. Phi(lf) recovered more rapidly than DC in the afternoon. This temporal discrepancy could be caused by cavitation of xylem elements in the vicinity of the DC probe. Microwave backscatter for L- and X-bands also measured diurnal variation that had local minima in the afternoon, but the pattern among wavelength and polarization signatures was complex.

Weber, James A.↗