Estimation of surface reflection effects on upwelling radiance field measurements in turbid waters

The contribution of surface reflection effects to above-water upwelling radiance measurements has been estimated from field optical data recorded in turbid coastal and estuarine waters. The water-leaving radiance signal was determined from underwater measurements and compared to the total upwelling radiance signal measured above the water surface with the sensor pointing towards the nadir, then adopting a recommended oblique viewing position. The observed differences are representative of surface reflection effects at the air/water interface (predominately sun and sky glint). These effects are analysed for different illumination conditions (cloud cover and solar zenith angle). Results are presented at four wavelengths representative of the visible and near-infrared spectral domains. The contribution of surface reflection effects to above-water upwelling radiance measurements is highly variable and always significant (typically 50% in the visible and higher at short and near-infrared wavelengths). It is more stable (consistent value), but higher under diffuse light conditions. The correction of surface reflection effects is problematic because the percentage of sky radiance reflected at the air/water interface (ρ) is unknown most of the time. An approximate correction (assuming a 0.02 ρ value) leads to large errors in the retrieved water-leaving signal. These errors are greatly minimized when considering a radiance ratio between two wavelengths.