Characterisation of the apparent reflectance of a concrete calibration surface over different time scales

This paper describes results from a detailed field spectral measurement campaign, designed to assess the temporal stability of calibration surface reflectance factors (ρλ). The results have wider implications for those using field measurements of reflectance in stand-alone studies, or in a calibration/validation context. A dual-field-of-view VNIR spectroradiometer system measured temporal changes in nadir ρλ at a concrete calibration surface. Reflectance factor variations were assessed in relation to the solar zenith angle (SZA), using the modified Walthall et al. (1985) model. Superimposed on low-magnitude variations in ρλ with SZA, were larger differences caused by environmental effects. Seasonal changes were caused by activation of a vegetative bloom which caused ρλ to vary by up to 16% (670nm). A smaller amount of variation was associated with changes in total solar radiation (kWm). This was an ‘apparent’ effect, caused by the directional anisotropy of the surface response, in relation to changing distributions of irradiance. A quantitative surface response model was derived empirically, and successfully used to predict concrete ρλ. The results highlight fundamental differences between inherent and apparent reflectance, and demonstrate that the reflectance factor response of a weathered concrete calibration surface was predictable over a range of timescales.