| The loess plateau is the largest high-quality apple producing area in the world.Its planting area and total output are the first in the world.Apple has become the pillar industry of local ecological and economic development.However,due to the lack of local rainfall and uneven distribution,with the transpiration of fruit trees consumes water strongly leads to the shortage of water resources in the orchard,which seriously affects the development of the apple industry.Through the method of model simulation,we can quantitatively describe the water cycle process of SPAC system in mountain apple orchard,explore the effect of soil dry air on soil moisture transport,and reveal the mechanism of orchard soil water and vapor flux,which can provide certain theoretical basis and technical support for optimizing orchard water resources management measures.In this study,data was obtained through field experiments in orchards,and a soil-plant-atmosphere continuum(SPAC)model under the influence of different evapotranspiration calculation methods was constructed based on the STEMMUS(Simultaneous Transfer of Energy,Mass and Momentum in Unsaturated Soil)model.Based on this model,a systematic study of the above problems was carried out,and the following progress was preliminarily made:(1)The STEMMUS model better simulates the dynamic processes of soil water,heat and evapotranspiration in mountain orchards.The output of the SPAC model under the influence of three evapotranspiration calculation methods based on the STEMMUS model,FAO56-PM(FAO56),Penman-Monteith(PM),and Shuttleworth-Wallace(SW),was analyzed and compared.The model output results can meet the model reliability standards during the calibration period(2018)and the verification period(2019).The soil moisture simulation results of the PM method performed better than the FAO56 method and the SW method.The model varied of the PM method in the range of 10.4%?27.3% in normalized root mean square error(NRMSE)and in the range of 0.26?0.87 in consistency index(d).The simulated values of soil temperature obviously overestimated the measured values in the calibration periods,while the simulated values and the measured values were highly consistent during the verification period.The simulated values and measured values of apple tree transpiration of the three evapotranspiration methods can maintain good consistency.Among them,the simulation results of the FAO56 method are superior to the PM method and the SW method.The model varied of the FAO56 method in the range of 20.0%?21.6% in NRMSE and in the range of 0.91?0.93 in d.Affected by the experimental observation data and model settings,the soil evaporation simulation results performed poorly overall.(2)Clarified the effect of soil dry air on the rainfall infiltration process of loess.In a single rainfall,the increment of soil moisture content in the soil profile of the coupled model with the dry air mechanism taken into account was obviously lower than that in the soil profile of the single-phase model without taking the dry air mechanism into account after rainfall.The results indicate that the coupled model has a lower infiltration rate and the soil air hinders rainwater infiltration.On the day of and after rainfall,the coupled model varied in the range of 0.19%?0.57% in NRMSE and in the range of 0.94?0.95 in d,the single-phase model varied of in the range of 0.48%?1.30% in NRMSE and in the range of 0.87?0.88 in d.The simulation accuracy of coupled model is significantly higher than the single-phase model.Under different rainfall intensity conditions,the model has different simulation accuracy in different rainfall periods due to the influence of input parameter error and variable measurement error.But the coupled model simulation results are better than the single-phase model,the error difference is between 0.4% and 2.9%.It indicates that the coupled model is more reasonable than the single-phase model to simulate the rainwater infiltration process in the loess area.(3)The mechanism of spatial and temporal variation of soil water and vapor fluxes in orchards was preliminarily revealed.Affected by the soil matrix potential gradient and soil temperature gradient,the soil vapor flux has a diurnal variation,which increases during the day and decrease at night.Before the rainfall,the soil water flux and soil vapor fluxe are basically close to zero flux.The rainfall leads to a downward gradient of soil matrix potential,which causes the isothermal soil liquid water flux and soil vapor flux to increase rapidly into the soil.Soil vapor is more sensitive to changes in matrix potential than soil liquid water.After rainfall,the flux gradually decreases with the depth of the soil layer.The flux of isothermal soil liquid water affected by soil matrix potential is 5 orders of magnitude higher than that of soil vapor,while the flux of thermal soil vapor affected by soil temperature is 6 orders of magnitude higher than that of soil liquid water,indicating that soil liquid water transport mainly driven by the soil matrix potential gradient,the soil vapor transport is mainly driven by the soil temperature gradient.In summary,this study describes the SPAC water cycle process of apple orchards in the Loess Plateau based on the STEMMUS model,clarifies the effect of soil dry air on the infiltration process of the loess rainwater,and initially reveals the mechanism of soil water and vapor flux transport in the orchard to provide a theoretical basis for water resources management in apple orchards on the Loess Plateau. |
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