Abstract
A ground-based network of spectral observations is useful for ecosystem monitoring and validation of satellite data. However, these observations contain inherent uncertainties due to the change of sunlight conditions. This study investigated the impact of changing solar zenith angles and diffuse/direct light conditions on the consistency of vegetation indices (normalized difference vegetation index (NDVI) and green-red vegetation index (GRVI)) derived from ground-based spectral measurements in three different types of cropland (paddy field, upland field, cultivated grassland) in Japan. In general, the vegetation indices decreased with decreasing solar zenith angle. This response was affected significantly by the growth stage and diffuse/direct light conditions. The decreasing response of the NDVI to the decreasing solar zenith angle was high during the middle growth stage (0.4 < NDVI < 0.8). On the other hand, a similar response of the GRVI was evident except in the early growth stage (GRVI < 0). The response of vegetation indices to the solar zenith angle was evident under clear sky conditions but almost negligible under cloudy sky conditions. At large solar zenith angles, neither the NDVI nor the GRVI were affected by diffuse/direct light conditions in any growth stage. These experimental results were supported well by the results of simulations based on a physically-based canopy reflectance model (PROSAIL). Systematic selection of the data from continuous diurnal spectral measurements in consideration of the solar light conditions would be effective for accurate and consistent assessment of the canopy structure and functioning.
Highlights
And large-scale observations of agroecosystems by remote sensing are crucial for food and environment security [1,2,3,4]
A part of the fluctuations of normalized difference vegetation index (NDVI) and the green-red vegetation index (GRVI) observed in Figure 3 would be attributable to the interaction of solar azimuth angle with crop row orientation, the other parts might have been caused by other environmental factors such as rain and birds [1]
We investigated the impact of the changing solar zenith angle and diffuse/direct light conditions on the consistency of vegetation indices (NDVI and GRVI) derived from ground-based spectral measurements in three kinds of croplands
Summary
And large-scale observations of agroecosystems by remote sensing are crucial for food and environment security [1,2,3,4]. High spatial and temporal resolutions are required at the same time. In many Asian countries, high spatial resolution is critical because agricultural fields are small and land use is mosaic. High-spatial-resolution optical satellites are used for mapping of the protein content and the full ripe stage of rice in a large number of individual fields [5,6]. Despite the constellation of satellites, the probability of image acquisition at specific target periods is still unsatisfactory for timely mapping. Low-spatial-resolution optical satellite sensors, such as the Terra and Aqua Moderate Resolution Imaging Spectroradiometer (MODIS), SPOT-VEGETATION (SPOT-VGT), and NOAA Advanced Very High Resolution
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