亚热带地区水稻田地表反照率变化特征

利用地面实测资料研究稻田地表反照率,一方面可以更好地刻画以稻田为主要土地利用方式的流域地气之间的能量分配过程;另一方面,可以为陆面模式提供更为准确的参数值,以及为遥感反演的地表反照率提供验证,从而为更好地解释土地利用/覆被变化对全球气候变化的影响机制提供参考。本文利用江苏省农业科学院溧水试验基地四分量仪测得的2016年稻田地表反照率数据,分析了稻田地表反照率特征,并结合同期观测的太阳短波辐射、温度、湿度、风速、风向等气象数据,进行相关性分析,识别影响稻田地表反照率的主要气象因子,为进一步量化地表反照率与温度及湿度等的参数化关系提供参考。结果表明:晴天稻田地表反照率整体上呈"U"型分布,中午较低,下午和上午较高。晴天稻田地表反照率在一天内的变化呈不对称特性,其不对称性主要是由露水和风速、风向引起。太阳高度角较小时,露水的散射作用使得上午时分的地表反照率值较下午高;而太阳高度角较大时,西南风促使作物叶面倾斜,从而使得下午的地表反照率值较上午高。稻田晴天地表反照率值较阴雨天高。地表反照率在晴天与出射短波辐射相关系数最高(0.670,P0.01),在阴天与相对湿度之间的相关程度最高(-0.480,P0.05)。在整个观测期间,稻田生长季内地表反照率呈现先升高后降低的趋势,地表反照率最高值出现在灌浆期到成熟期之间,插秧到分蘖期之间最低,其中灌浆期地表反照率与太阳短波辐射及湿度间的相关程度较高,并且均通过了P0.01显著性检验。分蘖期和拔节期是水稻生长季内地表反照率变化较快的两个生育期,并受气象因素的显著影响。

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.