Measurement And Modeling Of Water And Carbon Fluxes Over A Winter-wheat/summer-maize Cropland In The Dry Semi-humid Regime

Guanzhong Plain is one of the most important grain production area in Northwest China.Due to water shortage,the contradiction between the large amount of irrigation water requirement and the limited water resources was become more serious.Under the background of climate change,how to improve water use efficiency and optimize the allocation of water resources is one of the most arresting study areas.The energy,water and carbon fluxes are the critical processes of ecohydrological processes in agroecosystem.In this study,based on the in-situ measurement of energy,water,and carbon fluxes,we analyzed the ecohydrological processes over a classical agroecosystem and established an ecohydrological model which can used to model the processes between atmosphere and agroecosystem.The new ecohydrological model was evaluated based on the eddy flux data.The results of this study are as follows:1. Seasonal and inter-annual variation of energy,water,and carbon fluxes over the winter-wheat/summer-maize croplandEddy covariance?EC?system observations of water and energy exchanges were made over a typical irrigated winter wheat/summer maize rotation field in the Guanzhong Plain from June 2013 to June 2018.Total ET for each crop year was 627 mm-775 mm.For both crops,ET increased rapidly in conjunction with rapid canopy development and reached peak values when the canopy was fully developed.The ET decreased as the canopy senesced.Maximum daily ET values ranged from 5.5 to 6.0 mm day^-1 during the wheat growing season and from 5.2 to 6.7 mm day^-1 during the maize growing season.Canopy conductance?G_c?can reach up to 19.5 mm s^-1 and 29.5 mm s^-1 during the maize and wheat growing seasons,respectively.The threshold value of G_caffecting Priestley-Taylor coefficient???was determined to be about 10 mm s^-1 for both maize and wheat,above which the ratio of ET/ET_eq was unaffected by canopy conductance.For both maize and wheat,daily Bowen-ratio???values were between 0.1 and 0.2 when LAI reached maximum values.The?values ranged from 1.5 to 2.9 during the rotation period.The average?over the five years of the study was 0.59 for maize and 0.53 for wheat.Environmental factors explained 82.6%and 85.9%of the variability in ET during the maize and wheat growing seasons,respectively.Net radiation?R_n?and vapor pressure deficit?VPD?significantly,directly,and positively affected G_c,but ET was only significantly and directly affected by R_n.The gross primary productivity?GPP?of summer maize was higher than that of winter wheat,but the net ecosystem carbon exchange?NEE?of summer maize was smaller than that of winter wheat due to the ecosystem respiration?Re?of summer maize was much higher than that of winter wheat.2. The calculation of crop coefficient curve for the summer maize in Guanzhong PlainBased on a four-year observation period using an EC system,we derived the local crop coefficient curve and studied evapotranspiration partitioning in a summer maize cropland.Evapotranspiration partitioning have significant seasonal variability.In the initial stage,soil evaporation accounted for 78%–85%of the ET.With the development of the canopy,the proportion of soil evaporation in ET decreased gradually.In the mid growing stage,the soil evaporation was very small,and the transpiration accounted for 54%–81%of the ET.The crop coefficients derived by local datasets were 0.57,1.01,and 0.50 in the initial,mid,and late stages,respectively.The locally developed K_c curve can provide a good prediction of the crop water requirement.The variability in crop coefficient at different regions was high,which was attributed to the differences in climate and measurement methods.Different measurement methods can result in great variability of the crop coefficient.The K_c calculated using ET datasets from lysimeter measurements ranged from 1.20–1.53,which was significantly higher than the Kc calculated using EC datasets.Although lysimeters are widely used as a standard method to measure ET,lysimeters commonly overestimate K_c and crop evapotranspiration due to their limited representative area.3. Establised an ecohydrological model based on SPAC systemA fundamental understanding of coupled energy,water and carbon flux is vital for obtaining the information of ecohydrological processes and functioning under climate change.The coupled model,SCOPE?STEMMUS,integrating radiative transfer,photochemistry,energy balance,root system dynamic,and soil moisture and soil temperature dynamic,has been proven to be a practical model to simulate detailed land surface processes such as evapotranspiration and NEE.The performance of the coupled SCOPE?STEMMUS model in ET partitioning was improved due to the comprehensive radiative transfer scheme in SCOPE.The coupled model has been successfully applied in a maize field.Through the inter-comparison of SCOPE,SCOPE?SM,STEMMUS,and SCOPE?STEMMUS,we concluded that the coupled STEMMUS?SCOPE can be used to investigate vegetation states under water stress conditions,and to simultaneously understand the dynamics of soil heat and mass transfer,as well as the root growth.However,there are some needs for further studies to enhance the capacity of STEMMUS?SCOPE in understanding ecosystem functioning.Frist of all,the estimation of soil boundary condition especially during the irrigation period,which has significant influence on the simulation of soil temperature,needs further improvement.Second,the soil respiration model used in SCOPE,which neglected currently the effect of soil moisture,should be upgraded in the coupled model.Nevertheless,the SCOPE?STEMMUS may be used as an observation operator to assimilate remote sensing data such as solar-induced chlorophyll fluorescence,to improve the estimation of water and carbon fluxes.SCOPE?STEMMUS could also be used to investigate regional or global land surface processes,especially in arid and semi-arid regions,due to its sensitivity to water stress conditions.