The Impact of Sunlight Conditions on the Consistency of Vegetation Indices in Croplands—Effective Usage of Vegetation Indices from Continuous Ground-Based Spectral Measurements

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 OPEN ACCESSRemote Sens. 2015, 7 14080 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.

Illumination Geometry and Flying Height Influence Surface Reflectance and NDVI Derived from Multispectral UAS Imagery

“…NDVI was lower in the flight nearest solar noon, which agrees with the trend observed by Brede et al in UAS surveys of a tropical rainforest ecosystem [30]. Ishihara et al found a similar effect, sometimes with a sharp drop at solar noon, in continuous ground-based measurements of three types of cropland in Japan [45]. On the other hand, Assmann et al found an increase in NDVI with solar elevation [13], suggesting that the NDVI response may be site-specific.…”

Estimation of area- and mass-based leaf nitrogen contents of wheat and rice crops from water-removed spectra using continuous wavelet analysis

“…The leaf-level analysis concentrated on chemical signals and laid the foundation of water removal application for scaling up to the canopy level. To better understand the mechanism at canopy level, the strategy should be examined in detail by considering other external factors such as canopy structure [ 57 , 58 ], soil background [ 59 , 60 ], solar zenith angle [ 61 , 62 ], view zenith angle [ 7 , 43 ], and diffuse/direct light condition [ 63 ]. Finally, the signal to noise ratio should also be accounted for in case the nitrogen-related second overtones are suppressed by the spectral noise from instruments and atmospheric interference [ 10 ].…”

Assessing Correlation of High-Resolution NDVI with Fertilizer Application Level and Yield of Rice and Wheat Crops Using Small UAVs

“…Because the reflectance values of plants vary in response to environmental changes in sunshine irradiance, solar angle [19], soil background, and water glint [20,21], reducing sampling noise due to these environmental changes for UAV-based measurement is crucial for accurate and stable data acquisition. This is particularly so for long-term measurement during an entire crop-growing season.…”