| Land use is one of the main ways for human to change the surface environment,which can cause a series of climatic,ecological and hydrological effects.Among them,cropland conversion is a typical land use/cover change(LUCC).In the past few hundred years,in order to meet the needs of the increasing population for food and fiber,land use change is dominated by agricultural reclamation,with the arable land area greatly increased and the woodland area greatly reduced.In recent years,with the development of social economy,especially the rise of the second and third industries,the marginal benefit of agricultural land is gradually reduced,the agricultural employed population is reduced,the phenomenon of cropland conversion is increasingly common,and the cultivated land is gradually converted into woodland.Due to the differences between cultivated land and woodland in surface albedo,leaf area index,vegetation canopy height and other aspects,agroforestry land conversion has a significant impact on surface radiation energy balance,water budget,surface roughness and so on,which has important climatological significance.(1)This paper first reveals the spatial-temporal characteristics and spatial distribution of the formation factors of cultivated land abandoned in China on a national scale.There are 163 counties(cities)in the abandoned area,covering more than 20 provinces in China.The distribution is concentrated in mountainous areas in the south of China,and the distribution pattern is similar to the "T" shape with 90 degrees of sinistral rotation.The vertical axis covers Gansu,Sichuan,Chongqing,Guizhou and Yunnan from north to south.The horizontal axis is located in the middle and lower reaches of the Yangtze river,covering Hubei,Hunan and Anhui from west to east.The form of abandoned space--the formation of "T" shape changes with time.Before 2010,it mainly appeared in the east-west direction of the middle and lower reaches of the Yangtze river,and after that,it mainly appeared in the south-north direction.(2)Based on Google Earth high-definition images,200 pairs of pure pixels of cultivated land and forest land are identified on the scale of 1km for the typical abandoned area-Yangtze River Basin and its adjacent areas.Then,using MODIS landdata products,the differences of land surface temperature(LST),leaf area index(LAI)and Albedo between cultivated land and forest land are compared and analyzed.The results showed that LAI reached its maximum in summer(June,July and August)in the study area.Cultivated land was generally lower than forest land,and the peak value and valley value appeared almost at the same time.FVC is similar to LAI,but the peak period lags behind LAI.Albedo in visible light band shows that valley value appears in summer,and cultivated land is higher than forest land.The highest albedo in near infrared band of cultivated land and forest land occurs at different times,with staggered peaks.The highest albedo of cultivated land occurs in spring,while that of forest land occurs in autumn.(3)In order to further reveal the climate effect of farmland conversion to forest land after abandonment,this paper localized the parameters and carried out simulation tests using localized WRF model.The results show that the ecological restoration after abandonment has no obvious cooling effect on temperature and has no obvious influence on regional precipitation and precipitation days.However,it has a significant impact on the rainfall intensity in some areas,such as the increase of heavy rain and precipitation in central Anhui,which may increase the probability of disaster events. |
