Simulation Of Maize Evapotranspiration Considering CO₂ Effects

Evapotranspiration?ET?,which is composed of two separate processes of water loss to atmosphere from the soil surface by evaporation and from the plant canopy via transpiration,and it plays a key role in energy and water balance of agricultural systems,and is also a critical process in the terrestrial hydrological cycle.Accurate estimation or measurement of ET is of great significance for improving water use efficiency and optimizing the structure of regional water use.Importantly,ET is not only influenced by physiological factors,but also affected by environmental variables,especially,the change of atmospheric CO₂ concentration will influence a variety of processes,and particularlythe exchange of water and gas fluxes between plant and atmosphere.Recent observational studies have shown that elevated atmospheric CO₂ concentration have significant implications for water availability through its physiological effects on plant function associated with the increased water use efficiency.Several experimental studies showed that elevated atmospheric CO₂ concentration trends to reduce stomatal conductance and consequently lower transpiration rate per unit leaf area at leaf scales.At regional scales,the physiological processes associated with elevated atmospheric CO₂ can increase canopy leaf area via direct CO₂ fertilization and indirect increased water availability resulting from reduced stomatal conductance.Under the background of global change with the increase of air temperature and CO₂ concentration,elevated atmospheric CO₂ concentration and water shortage has seriously restricted the regional sustainable development of agriculture.In order to fully reveal the response regular of Maize to climatic factors and provide some suggestions for the future adjustment of planting structure,future change of land use and the change of CO₂ concentration in the arid oasis area of Northwest of China.In our study,we use the Maize in the middle oasis area of Heihe River Basin of Northwest China,based on the current evapotranspiration modelsand Maize data of three eddy covariance observations from May to September in 2012,coupled the canopy resistance models of Maize considering the influence of atmosphere CO₂ concentrationand non-considering the influence of atmosphere CO₂ concentration into the evapotranspiration models,such as one layered model and dual-layered model,and then the simulated data were compared with the observed data from eddy covariance observation site.Considering the influence of atmosphere CO₂ concentration and non-considering the influence of atmosphere CO₂ concentration into the evapotranspiration models were the highlights of this article,as follows:?1?The simulated values from different canopy resistance models were compared with measured data from eddy covariance site.Two verification points indicated that among these canopy resistance models,the simulated data from Irmak model that considering the influence of atmosphere CO₂ concentration has a better performance and it has strong regional applicability and spatial portability.The model also simulated the response processes of canopy resistance to environmental variables and reflected the effect of the variations in atmosphere CO₂ concentration on ET.?2?Sensitivity analysis of the improved Irmak model showed that canopy resistance and ET were the most sensitive to the net radiation and relative humidity,followed by air temperature,leaf area index and atmospheric CO₂ concentration.?3?Different growing stage has different regular of water consumption for Maize,it is unreasonable using a set of parameters when we use the dual layered S-W model to simulate Maize transpiration and soil evaporation.Some study results showed that if we use a set of parameters in a whole growing stage,it will produce large error.So it is necessary to divided the whole growing stage into some different small growing stage while we simulating the ET.?4?We coupled two canopy resistance models of Maize considering the influence of atmospheric CO₂ concentration and non-considering the influence of atmospheric CO₂ concentration into the S-W?Shuttleworth-Walance?model,and divided the whole growing stage of Maize into three stage by it's different water consumption regular.The ET of maize on half and daily scales were simulated by using the modified S-W model?MSW?.we validated the performance of the two MSW model in three different growing stage with measured data from three eddy covariance sites.The results showed that the simulated ET of Maize from MSW model that considering the influence of CO₂ concentration in every growing stage of three different site was in good agreement with the observed data from eddy covariance.?5?Sensitivity analysis of the MSW model that considering the influence of CO₂ concentration showed that the ET of Maize was more sensitive to canopy resistance??? and aerodynamicresistance of canopy to reference height??? in different growing stage.Therefore,it is verynecessary to pay more attention to determine the resistance parameters in the different growing stage when calculating the ET of Maize by using the MSW model considering the influence of CO₂ concentration.