Characteristics And Impact Factors Of Surface Albedo In Subtropical Paddy Rice Basin

The surface albedo controls the net radiation energy reaching the earths surface,reflecting the energy distribution between the surface and atmosphere.As an important input parameter in many land surface climate models and surface energy balance equations,the surface albedo is the key variable that affects the earth's climate system.The surface albedo has great significance to the exchange of water and heat between the surface and the atmosphere,and directly or indirectly affects the global and regional climate.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 parameter 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.Remote sensing is an effective method to obtain the surface albedo of large area,which can reflect the characteristics of the surface albedo on the regional scale.The MCD43A3 surface albedo of MODIS has high spatial and temporal resolution.The use of ground measured data is helpful to analyze the inversion accuracy of MCD43 A3 data.This paper first analyzes the surface albedo in paddy field in the Qinhuai River basin,using the ground test data to verify the MCD43A3 data,and then combining MCD43A3 data and vegetation index MOD13A1,analyzed the 2001-2016 years change characteristics and correlation of the Qinhuai River basin surface albedo and vegetation index NDVI.The main conclusions of this paper are follows.(1)On the field scale,surface albedo had a U-type of distribution for sunny days,which was lowest at noon,highest in the aftermoon 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.On the field scale,the condition of the underlying surface in the same area is relatively uniform,and the surface albedo is greatly influenced by the weather factors,such as solar radiation and humidity,which vary with time and weather conditions.The correlation coefficient between surface albedo and outgoing shortwave radiation was highest on sunny days,while the correlation coefficient between surface albedo and relative humidity was highest on cloudy days.The surface albedo in the growing season of rice field is higher than that in the non-growing season.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 above p<0.01 significance level.Surface albedo changed greatly at tillering and jointing stages,and was significantly affected by climatic factors.(2)At the regional scale,the land use types of different regions are quite different,and the surface albedo is greatly influenced by the types of land cover.MCD43A3 white and black sky albedo basically have the same trend with the measured surface albedo,the white sky albedo is more close to the actual surface albedo,the deviation of white and black sky albedo is less than 0.05 and the precision can meet the basic requirements of application and analysis of surface albedo.(3)At the regional scale,for 2001-2016 years,the surface albedo of the Qinhuai River basin changed little,but the difference in different areas was large.The surface albedo increased significantly in the fast growing area of urbanization.In the 2001-2016 years the NDVI showed a decreasing trend,and the decrease was more obvious in the fast developing area.The NDVI showed an increasing trend in the edge of the basin and forest.On the time scale,,except for the water area,the expansion of urban land changed the farmland with more vegetation to the urban land with high albedo,the surface albedo and NDVI showed a negative correlation in the past 2001-2016 years.Except for the water area,according to the gradient of city,farmland and forest,the surface albedo and NDVI showed the opposite trend in the years of 2005 and 2015,in the original city land on the basis of further improve the level of city,the shield effect by the high density and the height of the city constructions can make the surface albedo decrease.The correlation between surface albedo and NDVI in 2005 and 2015 showed that when the NDVI was less than 0.45,the surface albedo did not change significantly with the increase of NDVI.When the NDVI value was greater than 0.45,the surface albedo decreased with the increase of NDVI.