Diurnal change in trees as observed by optical and microwave sensors: the EOS synergism study

The EOS Synergism Study examined the temporal variability of the optical reflectance and microwave backscatter due to diurnal change in canopy properties of interest to eco- system modelers. The experiment was designed specifically to address diurnal changes in canopy water status (including water potential and content) that relate to transpiration. Multispectral optical and multifrequency, multipolarization microwave measurements were acquired using boom-truck- based systems over a 2-week period in August 1987. Sensor and canopy properties were collected around the clock. The canopy under study was a walnut orchard in the San Joaquin Valley of California. The results demonstrate large diurnal variations in the di- electric properties of the tree (the stem, or bole, specifically) that in turn produce significant diurnal changes in the micro- wave backscatter. The change in dielectric constant is related to the diural change in water potential, although the specific physical or physiological relationship has not yet been estab- lished. A diurnal change in optical reflectance could be attrib- uted primarily to sun angle-view angle change. Although some changes in canopy spectral properties were also observed, these could not be distinguished from edge effects of the canopy. 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. Microwave sensors, although able to image day or night, are sensitive to the diurnal pattern of change in vegeta- tion canopies and, therefore, should be in sun synchronous or- bits with a node crossing time selected to optimize sensitivity to the diurnal patterns (4-6 p.m. for this canopy and season). Furthermore, comparison of the long-term spaceborne syn- thetic aperture radar (SAR) data sets collected with SAR in different equator crossing time orbits must consider potential diurnal variation in the surface properties and the effects on the backscatter. Optical reflectance is affected more by sun-an- glehiew angle variations than a diurnal change in canopy water properties. Therefore, based on the results of this study, opti- cal sensors should be placed in orbits which minimize the influ- ence of clouds on terrestrial data acquisition.