| The transition zone affected by summer monsoon is a typical transition zone of ecosystem and fragile zone of ecological environment.The research on the process of surface water and heat transfer and the influence of environmental and ecological factors on energy distribution in this special region is an urgent scientific problem to be solved.This paper selects six vegetation types in the transition zone affected by summer monsoon-alpine meadow,desert grassland,corn farmland,semi-arid grassland,potato field and wheat field,to deeply understand the energy balance and distribution process of different types of underlying surface in summer,and analyze the diurnal,seasonal and interannual variation of Bowen ratio in this region.The regulatory effects of influencing factors on the process of water and heat transfer in different ecosystems are explored from two aspects of climatic environment and physiology and ecology.On this basis,the long-time series of farmland ecosystem observation data are used to reveal the response of the above factors to the law of land surface water and heat exchange under different dry and wet conditions of the same ecosystem.This study has important reference for revealing the relationship between the transitional area of the transition zone and the parametric relationship between the transition zone ecosystems and the improved land process.The main conclusions are as follows:(1)The surface energy of each ecosystem is not balanced,and the turbulent flux is underestimated compared with the available energy.In the different types of underlying surfaces in the summer transition zone,the surface energy closure of semi-arid grassland is close to the ideal condition,reaching 91%,followed by wheat farmland ecosystem(74%).There is little difference in energy closure between desert grassland(69%)and alpine meadow(71%).The surface closure degree of potato farmland(61%)is second only to corn farmland(62%).Long-term observation results show that the land energy closure degree of semi-arid farmland ecosystem(89%)is similar to that of semi-humid farmland ecosystem(81%).(2)In the typical underlying surface of the transition zone,the maximum values of net radiation,latent heat flux,sensible heat flux and soil heat flux appeared in corn farmland(130.33 W?m ~2),alpine meadow(81.45 W?m~2),semi-arid grassland(42.56W?m ~2)and desert grassland(4.32 W?m~2),respectively.For the farmland underlying surface in the Loess Plateau for a long time,the daily peak value of net radiation and sensible heat flux in Dingxi and Qingyang is very close.However,there is a great difference in the latent heat flux between the two places,and the peak value of the latent heat flux in the semi-moist area is almost twice that of semi-arid zones.The daily peak value of soil heat flux in the semi-humid area is about 2 to 3 of that in the semi-arid area.(3)In the transition zone,the latent heat fluxes of corn farmland,alpine meadow,potato farmland and wheat farmland play a dominant role in energy distribution.Desert grassland and semi-arid grassland heat the atmosphere through sensible heat flux.Compared with the energy distribution of different types of underlying surface in summer,the growing season of semi-arid farmland ecosystem in the Loess Plateau is affected by the fluctuation of monsoon precipitation,and the sensible heat flux and latent heat flux are alternately dominant in the energy distribution.The available energy in the semi-humid area is mainly consumed by the latent heat flux.The latent heat flux in the growing season in the semi-humid region of the Loess Plateau is 2.4 times of the sensible heat flux,but they are almost the same in the semi-arid region in the same period.(4)Sensible heat flux of desert grassland and semi-arid grassland in summer is the main consumption item of available energy,while latent heat flux is the main consumption item of other underlying surface.Compared with the observed results in the growing season,the sensible heat flux in Dingxi in the semi-arid region is the main consumption of available energy,and its Bowen ratio fluctuates around 1 even in the monsoon period with concentrated precipitation.For Qingyang in semi-humid area,the latent heat flux plays a dominant role in the energy distribution in summer(the average Bowen ratio is 0.71),and the sensible heat plays a dominant role in the energy distribution in the other three seasons(Bowen ratio is 1.15-5.85).(5)From the meteorological factors of land surface water and heat exchange in the image,the main influencing factors of different underlying surface Bowen ratios in the transition zone are saturated water vapor pressure difference and ground-air temperature difference.The main environmental factors affecting the Bowen ratio of farmland ecosystem under different dry and wet conditions on the Loess Plateau are Ta,VPD,shallow SWC and precipitation.The negative correlation between Bowen ratio and Ta in the Loess Plateau is consistent with the theoretical analysis,but this law is better in the semi-humid region than in the semi-arid region.And under drought conditions,the correlation between VPD and Bowen ratio in semi-humid area is more significant.The Bowen ratio of farmland ecosystem in this region decreases with the increase of SWC,especially in semi-arid areas,because the process of land surface water and heat exchange in semi-arid areas is more sensitive to the change of SWC.(6)From the ecological factors affecting the Bowen ratio in the growing season,there is a good correlation between NDVI and turbulent fluxes on the typical underlying surface of the transition zone,in which the latent heat flux increases with NDVI,while the sensible heat flux has a negative correlation with NDVI.The regression equation coefficients of Bowen ratio and NDVI of each typical underlying surface also decrease exponentially with the mean value of NDVI.The relationship between the two is actually the influence of ecological factors on evapotranspiration,that is,NDVI regulates the transpiration process of the ecosystem by affecting Gs,and then plays an important role in regulating the Bowen ratio of the ecosystem.In the seasonal and interannual variation of water and heat exchange,the relationship between Priestley-Taylor coefficient(?)and Bowen ratio obeys the power function law,and there is a significant correlation in the farmland ecosystem of the Loess Plateau.~2 in semi-humid and semi-arid areas is 0.62 and 0.72 respectively.In addition,the Bowen ratio of farmland ecosystem in the Loess Plateau decreases with the increase of NDVI,and the relationship between them is more significant in the semi-humid region(~2=0.40),but the response of Bowen in the semi-arid region is faster than that in the semi-arid region.There is a negative exponential relationship between Gs and Bowen ratio,and the trend of Bowen ratio decreasing with the increase of Gs in semi-arid area is more obvious than that in semi-humid area. |
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