| As one of the three major terrestrial vegetation ecosystems,the water-heat and carbon cycle of farmland ecosystem is greatly affected by human farming activities.Especially in the arid irrigation area of northwest China,the long-term water-lifting irrigation activities have formed a relatively unique waterheat and carbon exchange process and change characteristics of farmland ecosystem.The evolution characteristics of water-heat and carbon flux in farmland ecosystem and the environmental response mechanism are still unclear.Therefore,it is necessary to study the characteristics,budget characteristics and control mechanism of the water-heat and carbon cycle in the typical farmland ecosystem in the arid irrigation area of northwest China.This paper took the wheat and corn farmland ecosystems in Jingtaichuan electric power irrigation area of Gansu Province as the research object,and used the eddy correlation technology to carry out experimental observation of their water-heat and carbon flux.Based on the observation data processing,the variation characteristics and budget characteristics of the waterheat and carbon flux in this type of ecosystem at different time scales were analyzed,combined with the changes of micrometeorological factors.The transport mechanism and main control factors of the waterheat and carbon flux were discussed,and the coupling simulation model of the water heat carbon flux was constructed.The research is of great significance to realize the optimal allocation of water resources,carbon sequestration and emission reduction,and water and salt regulation in the arid irrigation area of Northwest China.The main research contents and results are as follows:(1)The median absolute deviation method was used to control the quality of the half-hour flux data,and the marginal distribution sampling method was used to interpolate the missing data.Based on the principle of energy balance,the energy closure verification of the observed eddy flux data was carried out.The results showed that the overall energy closure was 0.72 during the observation period,0.74 in the growing season,0.76 in the growing season of wheat,and 0.79 in the growing season of maize.The energy closure of the flux observation data was within a reasonable range,and the flux contribution area was within the target study area.The quality of the observation data was reliable and representative.(2)Based on the observed data of eddy correlation system,the daily and seasonal variation characteristics,energy budget and seasonal distribution characteristics of each energy flux were analyzed.The control mechanism of surface parameters and environmental factors on evapotranspiration was revealed,and the direct and indirect effects of environmental factors on evapotranspiration were determined by path analysis method.Based on the analysis of the spatiotemporal distribution of soil waterheat and salt,the response relationship between soil salt and water and heat was discussed.The results showed that the average daily variation of each energy flux showed an inverted "U" shape,and there was temperature inversion at night in summer.The net radiation(Rn)is mainly dissipated in the form of latent heat(LE)in the growing season,and sensible heat(H)in the non-growing season.The evapotranspiration(ET)in growing season was mainly driven by meteorological factors such as net radiation and temperature,while the ET in non-growing season was mainly driven by canopy conductivity(Gc).The influence threshold of Gc on ET,PT coefficient and decoupling coefficient was 11 mm s-1.Air temperature(Ta)and soil temperature(Ts)contributed the most to the seasonal change of ET in wheat and maize,respectively.The profile distribution of soil salt in the growing season showed a "surface aggregation" type,and the soil salt content had strong linear relationship with soil water storage and soil temperature at different stages in wheat and maize planting years,respectively.The soil conductivity at different depths of 0～60 cm met the exponential relationship with groundwater depth.(3)Based on the potential water use efficiency theory of ecosystem,the evapotranspiration components were separated.The water consumption laws of wheat and maize in different growth stages were revealed,and the characteristics of water exchange in deep soil were analyzed by water balance in closed field.According to the measured evapotranspiration,the coefficient curves of wheat and maize crops suitable for this area were determined,and the seasonal variation of evapotranspiration components and crop coefficient and the main controlling factors were discussed.The results showed that the seasonal changes of plant transpiration(T)and ET were similar.There was a significant positive correlation between T and Rn and leaf area index(LAI),and a significant positive correlation between E and Rn and Ts.The proportion of plant transpiration and soil evaporation was similar in wheat season,and soil evaporation was the main evapotranspiration in maize season.The field water utilization coefficients of wheat and maize in planting years were 0.601 and 0.367,and the deep water exchange mainly occurred in winter irrigation and spring irrigation.The threshold values of Gc for E were 8 mm s-1(wheat)and 10 mm s-1(maize),respectively.Crop coefficients of wheat and maize at different growth stages were 0.20 and 0.25 in the early growth stage,0.72 and 0.96 in the middle growth stage,and 0.61 and 0.35 in the late growth stage.E during the growing season was mainly driven by Gc,and T was mainly driven by LAI.The threshold of influence of Gc on Kc,Ke and Kcb was 7 mm s-1(wheat)and 6 mm s-1(maize),respectively.(4)The net ecosystem carbon exchange(NEE)measured by night respiration was divided into total primary productivity(GPP)and ecosystem respiration(Re),and the characteristics of carbon flux changes at different time scales and their responses to environmental factors were analyzed.The carbon flux budget of wheat and maize was discussed,and the carbon balance was analyzed by source and sink.Canopy scale water use efficiency(WUE)was calculated using the GPP derived from the decomposition of ET and NEE observed by vorticity,and the variation characteristics of WUE in different forms were analyzed.The results showed that the monthly average diurnal variation of NEE in wheat and maize field ecological system showed a U-shaped,while the GPP and Re showed an inverted U-shaped variation.Temperature(Ta or Ts)was the most important factor contributing to the change of carbon flux,and there was an exponential positive correlation between Re and temperature.The Re of maize wsa more sensitive to temperature rise than wheat.During the growing season,the total budget of GPP,Re and NEE in wheat ecosystem was 956.4 g C m-2,657.1 g C m-2 and-299.4 g C m-2,and the total budget of GPP,Re and NEE in maize ecosystem was 1540.6 g C m-2,1444.5 g C m-2 and-96.1 g C m-2,that is,both wheat and maize were carbon sinks.Considering the carbon emissions caused by grain harvesting,wheat was carbon sinks and maize was carbon sources,with the same carbon budget characteristics on an annual scale.The average water use efficiency(WUE)of wheat and maize in the growing season was 2.94 g C kg-1 H2O and 3.82 g C kg-1 H2O,respectively.The WUE of maize as C4 crop was higher than wheat,the WUE of farmland in the study area is low.(5)The SCOPE-STEMMUS coupling model was constructed based on the photochemical model(SCOPE)and the soil water heat conduction model(STEMMUS)of ecosystem land surface process simulation.The model was calibrated based on the in-situ observation data of the observation station,and the water-heat carbon fluxes in the growing season of wheat and corn fields were simulated.The results showed that the SCOPE-STEMMUS coupling model had high accuracy in the simulation of water heat and carbon in wheat and corn fields in arid salinized irrigation area.The deviation of simulation results were different at different growth stages of crops,and the simulation accuracy of energy and water carbon flux in wheat fields was slightly higher than that in maize fields.On the scale of growing season,the simulation error of water flux in wheat and maize ecosystem was less than ±10%,and that of GPP was about ±5%.The coupled model can be used as an effective method to simulate water,heat and carbon fluxes of land surface processes in this type of ecosystem. |
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