Validation of a spectral correction procedure for sun and sky reflections in above-water reflectance measurements.

A three-component reflectance model (3C) is applied to above-water radiometric measurements to derive remote-sensing reflectance R-rs(lambda)3C provides a spectrally resolved offset Delta(lambda) to correct for residual sun and sky radiance (Rayleigh-and aerosol-scattered) reflections on the water surface that were not represented by sky radiance measurements. 3C is validated with a data set of matching above-and below-water radiometric measurements collected in the Baltic Sea, and compared against a scalar offset correction Delta. Correction with Delta(lambda) instead of Delta consistently reduced the (mean normalized root-mean-square) deviation between R-rs(lambda) and reference reflectances to comparable levels for clear (Delta: 14 : 3 +/- 2 : 5 %, Delta(lambda) : 8 : 2 +/- 1 : 7 %), partly clouded (Delta : 15 : 4 +/- 2 : 1 %,Delta(lambda) : 6 : 5 +/- 1 : 4 %), and completely overcast (Delta: 10 : 8 +/- 1 : 7 %, Delta(lambda) : 6 : 3 +/- 1 : 8 %) sky conditions. The improvement was most pronounced under inhomogeneous sky conditions when measurements of sky radiance tend to be less representative of surface-reflected radiance. Accounting for both sun glint and sky reflections also relaxes constraints on measurement geometry, which was demonstrated based on a semi-continuous daytime data set recorded in a eutrophic freshwater lake in the Netherlands. R-rs(lambda) that were derived throughout the day varied spectrally by less than 2 % relative standard deviation. Implications on measurement protocols are discussed. An open source software library for processing reflectance measurements was developed and is made publicly available.