| With global warming and global water shortage,many researchers and research institutions more focus on the water and carbon cycles in terrestrial ecosystem.Farmland ecosystem is the closest with human activities across all the ecosystems.It not only promotes the development of social economy,but also affects the atmospheric water and carbon cycles.It is of great significance to quantify the characteristics of water and carbon cycles in farmland ecosystem,which can promote the water resources management and carbon sequestration and emission reduction.In this study,we selected the grape field in the northwest arid area of China,as the research object.We measured meteorological data and water-carbon fluxes between2017 and 2018 from eddy covariance system and soil carbon flux automatic observation system.Based on above observed data,our paper analyzed the characteristics of energy and carbon fluxes,the relationships between water-carbon fluxes and environmental factors,and the law of water-carbon coupling on different time scales.In addition,we evaluated the performance of the simulated water and carbon fluxes by CEVSA model,and predicted the changes of water and carbon in the grape field using CEVSA model in different scenarios.The main conclusions are as follows:(1)From May to October 2017,the energy distribution had obvious daily changes in grape field.The net radiation flux(Rn),sensible heat flux(H),latent heat flux(LE)and soil heat flux(G)increased and then decreased,and the Rn,H and LE reached the maximum at about 14:00.The maximum value of G appeared after 14:00.Energy fluxes changes are different in the growth season from 2017 to 2018.The chenges of Rn and LE are the biggest,and the trends of change are single peak.However,the change of H is opposite to that of LE.When LE is the largest in thegrowing season,H is negative,which is related to the transformation of net radiation flux among the components.(2)The daily trend of net ecosystem exchange(NEE)and gross primary productivity(GPP)in grape fields is basically the same.At night,there was no photosynthesis and low carbon sequestration.The grape field showed weak carbon source.After sunrise,the temperature and light increased,subsequently the photosynthetic capacity increased,which results in strong sink in grape field.The daily variation of carbon fluxes in different months shows obvious seasonal characteristics.The daily variation of carbon fluxes is different in May and October.In June,the carbon flux at noon is the largest,followed by July and August,and the smallest in October.During the growing season,the change of carbon fluxes has obvious seasonal changes.It shows weak carbon sink at the germination stage and carbon sink in rapid growth stage in the grape filed,and the carbon sequestration continues to increase until the end of growth.(3)The response of evapotranspiration(ET)to environmental factors on a daily scale is different from that of net ecosystem exchange(NEE).Photosynthetically active radiation(PAR)is the main environmental factor affecting the daily change of ET,followed by temperature.At the daily scale,the correlation between soil temperature and NEE is the strongest,followed by photosynthetic effective radiation,and the correlation between saturated water vapor pressure difference is the smallest.At the half hour scale,the response of ET and NEE to environmental factors is more consistent,and both of them were positively correlated with PAR(P<0.01).They are circularly distributed with temperature and saturated water vapor pressure difference,the correlation is relatively small.There are obvious seasonal differences between ET,NEE and environmental factors.There is a linear positive correlation between the water and carbon fluxes at the canopy scale,indicating that there is a significant coupling between the water and carbon fluxes at the canopy scale.(4)The simulation results of CEVSA model are compared with the observation data,and the results show that the simulation of water and carbon fluxes by CEVSA model is effective.Based on different climate change scenarios,the model predicts the change characteristics of water-carbon fluxes in farmland.The results show that ET is no-sensitive to the change of net radiation,but sensitive to the change of relative humidity,temperature and precipitation.The sensitivity of GPP and NEE tometeorological variables was basically the same,but the change of net radiation has no impact on them.GPP and NEE were positively correlated with the change of relative humidity and precipitation.The effect of temperature on GPP and NEE is different from other variables.The simulated carbon flux at the current temperature is larger than that at ±2??,which indicates that the current temperature may be closer to the optimal temperature of grape photosynthesis. |
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