| This experiment was conducted in the typical farmland of the North China Plain-winter wheat/summer maize field from 2013 to 2015. The exchange of CO2, water and heat between crop field and atmosphere was measured by using eddy covariance system, meanwhile the soil evaporation and respiration were measured with micro-lysimeter and soil respiration system, respectively. By continuous observation of three years, the dynamics of CO2, water and heat fluxes of winter wheat/summer maize field and their response to the influencing factors were systematically studied. And the applicability of two models based on Penman-Monteith model on prediction evapotranspiration from winter wheat and summer maize field was analyzed. The key parameters in models were calibrated with meteorological data, and the calibrated models were validated by comparing the predicted ET and measured ET, hoping to get a practical model to predict ET accurately and help us to understand water requirement of different stage of corp.The main conclusions of this study are as follows:1. Energy distribution of winter wheat/summer maize fieldFluxes of net radiation, sensible heat, latent heat and soil heat had obviously diurnal variation, and presented an inverted U-shape. Each energy flux reached the maximum at about12:00. The distribution of net radiation flux in every energy component was closely related with the characteristics of the underlying surface. During summer maize seaason, the smallest part about 5.3% of energy was stored in the soil and heated the soil, the largest part about57.3% of energy was used in evaportranspiration, and the average Bowen ratio was about 0.43.During winter wheat season, about 4.4% and 57.5% of energy was transferred into soil heat flux and latent heat flux, respectively. And the average Bowen ratio was 0.34 during wheat season.2. The variation characteristics of CO2 flux of winter wheat/summer maize field(1) CO2 flux had obvious diurnal variation, the variation tendency of CO2 fluxes was consistent in different growth stage and like a U-shape, the maximum of CO2 absorption occurred at about 13:00, and the peak value in each growth stages were obviously different.(2) CO2 flux had a obviously seasonal variation. Wheat field in the initial and wintering stages worked as carbon source, and changed into carbon sink from Mar.10 in the development stage with the maximum of-0.45 mg?m-2?s-1. Maize field changed into carbonsink from July 20 in the development stage, and reached the maximum value about-0.3mg?m-2?s-1 in 2014 and-0.6 mg?m-2?s-1 for 2015 in the middle and late August. The peak diffusion of CO2 was occurred in June and October which was in the alternative period of wheat and maize.(3) CO2 flux of wheat field was correlated with photosynthetically active radiation(PAR)and net radiation, CO2 flux of maize field had good relation with wind speed and net radiation,and fitted quadratic polynomial model. Soil CO2 efflux had obvious correlation with 10 cm soil temperature and 5cm soil water content and fitted bivariate compound model.3. The characteristic of evapotranspiration(ET) from winter wheat/summer maize field(1) ET from crop field had obvious diurnal and seasonal variation. ET was almost zero from 18:00 to 6:00; the maximum of ET with about 0.6mm?h-1 for wheat and 0.7 mm?h-1 for maize occurring at about 12:00-13:00. The daily ET of summer maize firstly increased to the maximum in the midseason or late season stage and then decreased. While the daily ET of winter wheat slowly decreased during the initial and wintering stages, reached the minimum value in the late of January, and then increased to the maximum in April or May, and then decreased slowly. Total ET of maize season in 2013, 2014 and 2015 was 275.9,272.9 and266.6mm,respectively; Precipitation for the same periods was 300.7,355.8 amd 115.8mm.Total ET of wheat season in 2013-2014 and 2014-2015 was 408.2mm and 353.1mm,respectively; Precipitation for the same periods was only 281.9mm and 103.5mm, respectively.Soil evaporation was about 38.6% and 30% of ET for maize and wheat crop field,respectively.(2) ET from maize field was correlated well with LAI which fitted exponential model, and followed by net radiation and PAR which fitted quadratic polynomial model. While ET from wheat field had significant correlation with net radiation and PAR, and both fitted quadratic polynomial model. Soil evaporation from maize field mainly correlated with PAR, 5cm soil water content and net radiation which all fitted quadratic polynomial model; Soil evaporation from wheat field had good relation with PAR and net radiation which fitted quadratic polynomial model, and negatively linear regression with air humidity.(3) Water use efficiency(WUE) had obvious diurnal and seasonal variation, the difference of WUE in different stages and growth seasons was very distinct. The seasonal variation of WUE presented obvious single apex, which firstly increased and then decreased.From the development stage, crop WUE increased to the peak in the late of April of wheat season or August of maize season with the maximum value of 18 g·kg-1 and 19 g·kg-1 in wheat and maize seasons, respectively. And then decreased to the minimum value about 10 days ahead of the harvest.4. Simulation research on ET from winter wheat/summer maize field(1) Both FAO-PM model and R-K model which were based on Penman-Monteith model were calibrated with data collected by using eddy covariance system at Jiaozhou bay station.And daily ET were predicted with the corrected FAO-PM and R-K models, and compared with observed ET. The ET for each growth stage of crop was also predicted with these two models. The results showed that the performance of FAO-PM model was better than that of R-K model. The prediction of ET for different growth stages with R-K model was obviously underestimated.(2) A new data processing method wea proposed in this paper to calibrate R-K model,,and then corrected R-K model was validated on the basis of data collected in summer maize and winter wheat seasons. It was obviously showed that this method greatly increased the performance of R-K model in predicting daily ET and the accuracy of the ET in different growth stages of summer maize; As to winter wheat, the model performance was not as satisfactory as that of summer maize, the difference was obvious between two years. |
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