UNCERTAINTIES
Uncertainties are associated with all observations, including radiation measurements. For example, vegetation heterogeneity can lead to non-uniform radiation interception, and global radiation measurements made under direct sunlight were found to be lower (about 10%) than those above the canopy (Tuzet et al., 1997). On the other hand, radiometer readings are greatly influenced by weather. For example, water droplets on sensors can absorb long-wave radiation, resulting in large deviations in long-wave radiation readings during rain or dewfall events (Michel et al., 2008). This explains the relatively weak relationship between RI and GPP at the daily scale. However, these sensor-induced deviations are much less significant at the annual scale, making the annual estimations of GPP more reliable.