Parametrization Of Resistances For Modeling Crop Evapotranspiration In The Open Field And Greenhouse Environment

Accurate estimation of crop evapotranspiration(ET_c)is key to the development of reasonable irrigation scheduling and improve the water use efficiency in agricultural production.However,due to the differences between the micro-meteorological parameters and the water and heat transport dynamics under open field and greenhouse environment,the existing ET_c models are still uncertain in their application.Therefore,it is necessary to select appropriate ET_c model,determine the key parameters for the corresponding model under different climatic and vegetated conditions,and evaluate and verify the applicability of the model to improve the accuracy of ET_c simulation.In this paper,we conducted field experiments for the simulations of ET_c of tea plants in the open field and cucumber plants in the greenhouse environment.The actual ET_c,micro-meteorological data and crop growth physiology were measured,and analyzed the characteristics of soil water content and heat fluxes in different environments.In order to provide a basis for accurate simulation of ET_c under open field and greenhouse environment,we simulated ET_c by modifying key parameters based on different models,and evaluated and verified the applicability of different models through measured values.The main contents and conclusions in this thesis are as follows:(1)According to the experimental observation data during the crop growing periods under open field and greenhouse environment,the variation characteristics of meteorological conditions and the water and heat fluxes were analyzed.High air temperature and humidity and low wind speed was found in greenhouse.The solar radiation,relative humidity and wind speed in the open field are higher than those in the greenhouse,while the air temperature is lower than that in the greenhouse.The latent heat flux of greenhouse cucumber is small at the initial stages,accounting for only 10.1%of net radiation,while it is 87.5%for sensible heat flux.During the crop development stage,the latent heat flux increased rapidly,accounting for 44.5%of the net radiation,and the latent heat flux in the mid-season stage accounted for 79.2%of the net radiation.The latent heat flux and sensible heat flux at the late season stage accounted for 38.3%and 61.1%of the net radiation,respectively.The energy expenditure of the tea filed in different seasons was mainly latent heat flux.The latent heat flux in summer,autumn,winter and spring seasons accounted for63.2%,59.6%,55.3%and 64.2%of the net radiation,respectively.The soil heat flux accounted for a small proportion of net radiation under both open field and greenhouse environment.(2)The response relationship between daily ET_c and meteorological factors in different growing stages of greenhouse cucumber was analyzed,and the crop coefficient values in different growing stages were determined.The greenhouse cucumber ET_c was positively correlated to solar radiation,air temperature and water vapor pressure,but negatively correlated to relative humidity,and had a low correlation with wind speed.During the whole crop growth stages,the correlations degree between ET_c and meteorological factors was:solar radiation>air temperature>water vapor pressure difference>relative humidity>wind speed.The crop coefficients of greenhouse cucumber determined in this study were 0.12,0.81,and 0.46 in the initial,mid-season,and mid-season stages,respectively.The crop coefficients of greenhouse cucumber were lower than the values recommended by FAO-56 under open field conditions.(3)Under the greenhouse environment,we constructed the aerodynamic and canopy resistance parameters for the Penman-Monteith model,and evaluated the applicability of the Penman-Monteith model for simulating ET_c in different growth stages of greenhouse cucumber.The results showed that the Penman-Monteith model can accurately simulate the ET_c in mid-season stage.For the other growing stages,the Penman-Monteith model overestimated the cucumber ET_c,but there was a good correlation between the simulated and measured ET_c.The canopy resistance was large and constant at night,and the value decreased rapidly during sunrise,and kept stable at 9:00-15:00.The convection type in the greenhouse was mixed convection for most of the time,when a constant aerodynamic resistance was applied(200-250 s/m),the Penman-Monteith model can accurately simulate the ET_c at the crop mid-season stage.(4)In the open field,the Penman-Monteith and Priestley-Taylor models were used to simulate ET_c of a tea field,and different sub-models were evaluated and verified based on the measured ET_c values by the Bowen ratio energy balance system.In the case of the Penman-Monteith model,the coefficients a and b were 1.06 and 0.29 in the Katerji-Perrier canopy resistance sub-model,respectively,which can accurately simulate the ET_c of the tea field,but the Penman-Monteith model overestimated the ET_c when the Todorovic canopy resistance sub-model was used.The Priestley-Taylor model needs fewer input parameters than the Penman-Monteith model,and the application is simpler.The coefficient?in the Priestley-Taylor model was 1.20.The?showed a strong annual cycle,and that this annual cycle can be explained by variations in the soil water content and water vapor pressure deficits,in which high water vapor pressure deficits coincided with low values of?.Results indicated that using monthly averaged?instead of a constant value for the Priestley-Taylor model can improve the simulation accuracy of the ET_c of the tea field.