Characteristics of surface albedo in subtropical paddy rice fields

As a key input parameter of numerical climate models and surface energy balance equations, surface albedo affects climatic systems on the earth. Data observed on rice paddy surface albedo not only better depict energy distribution between ground and atmosphere in paddy fields, but also provide more accurate parameters values for land surface models. It also provides authentication for remote sensing inversions of surface albedo and therefore better explains the effects of land use/cover change mechanism on global climate change. Using CNR4 measured surface albedo in paddy fields at Lishui Experiment Station in 2016, we analyzed the characteristics of surface albedo of paddy field. In the study, we combined the analysis of surface albedo with temperature, humidity, wind speed, wind direction and other meteorological data. Some other possible factors affecting surface albedo in paddy fields were also analyzed. The correlation coefficient between surface albedo and temperature reflected the effect of land surface albedo on temperature. The correlation between surface albedo and meteorological factors (e.g., humidity and solar radiation) was used to study the relationship among surface albedo, temperature and humidity. The results showed that surface albedo had a U-type of distribution for sunny days, which was lowest at noon, highest in the afternoon and morning, but also affected by precipitation. Surface albedo on sunny days was asymmetrical, which was mainly caused by dew, wind speed and wind direction. As actual air temperature was close to dew point temperature at night, more dew settled on leaves in the morning. When solar altitude angle was small, the scattering effect of dew resulted in higher surface albedo in the morning than in the afternoon. When solar altitude angle was high, dew on leaf surfaces gradually evaporated. Southwest winds influenced the tilting of crop leaves, resulting in higher surface albedo in the afternoon than in the morning. Surface albedo was higher on sunny days than on cloudy and rainy days. The correlation coefficient between surface albedo and outgoing shortwave radiation was highest (0.670, P < 0.01) on sunny days, while the correlation coefficient between surface albedo and relative humidity was highest (-0.480, P < 0.05) on cloudy days. The trend in surface albedo during the growing season initially increased and then decreased. Surface albedo was highest during grain-filling to mature stage of rice and was lowest during transplanting to tillering stage. The correlation coefficient among surface albedo, humidity and shortwave radiation for rice growing season was high. Also the correlation coefficients between surface albedo and temperature, and then between humidity and shortwave radiation were not the same for different growth stages. During grain-filling period, the correlation among surface albedo, solar shortwave radiation and humidity was more significant, all at P < 0.01 significance level. Surface albedo changed greatly at tillering and jointing stages, and were significantly affected by climatic factors.