Dynamic Simulation Of Evapotranspiration And Estimation Of Crop Coefficient In Typical Area Of Horqin Sandy Land

It is of great significance to study the variation and driving factors of evapotranspiration and improve the accuracy of evapotranspiration estimation for the exploration of hydrological cycle in arid and semi-arid areas.This paper takes the typical geomorphic types of Horqin Sandy land-meadow,semi-mobile dune,semi-fixed dune,corn and rice as research objects.Based on the meteorological data of 2013-2018 measured by the meteorological environment monitoring system,the reference crop evapotranspiration(ET0) at different time scales was estimated by FAO 56 P-M model.And then,based on 3 statistical methods to comprehensively reveal the influencing factors of ET0.Based on the fluxes data measured by the Eddy conariance system of meadow and dune to analyzes the variation of actual evapotranspiration(ETa)in different precipitation types and the responses to meteorological factors.On this basis,the crop coefficient(Kc)of meadow and dune was calculated by ET0 and ETa.Finally,constructe a Kc inversion model based on Normalized Difference Vegetation Index(NDVI),Soil-Adjusted Vegetation Index(SAVI),Modified Soil-Adjusted Vegetation Index(MSAVI),Enhanced Vegetation Index(EVI)and Ratio Vegetation Index(RVI),Leaf Area Index(LAI)and Soil Moisture Content(SM).The main conclusions are as follows:(1)ET0 of different geomorphic types reached its maximum in 2017.The seasonal variation is summer>spring>autumn>winter.The monthly variation is parabolic,and the overall performance from April to July is meadow>rice>corn>semi-fixed dune>semi-mobile dune.The relative contribution rate method shows that the ET0 of meadow and maize is mainly affected by the saturated water vapor pressure difference,the relative contribution rate is 25.884%and 25.479%,and the ET0 of other experimental areas is greatly affected by temperature.The partial correlation analysis shows that the net radiation and saturated water pressure difference have a significant positive correlation with ET0(P<0.01),and the range of partial correlation coefficient was 0.575-0.927.The path analysis method indicates that the saturated water pressure difference is the primary factor affecting ET0,and the maximum path coefficient is 0.648.The comprehensive analysis shows that the key factors affecting ET0 are saturated water pressure difference and net radiation;temperature indirectly affects ET0 through the key factors,while wind speed has little effect on ET0.(2)The influence of precipitation on ETa is delayed in meadow,and the dune ETa is consistent with the change in precipitation.Correlation analysis shows:The ETa values of meadow and dune are not related to wind speed.ETa values in the dry and wet years of meadow were not significantly correlated with precipitation,but were highly correlated with temperature,saturated water pressure difference and net radiation(p<0.01).The correlation between ETa and 4 meteorological factors in different precipitation years of dune was extremely significant(P<0.01).ETa in dry years is mainly affected by precipitation with a correlation coefficient of 0.580 and in wet years is mainly affected by temperature.(3)The univariate linear regression model shows that Kc-LAI fits best in the meadow,followed by NDVI,The Kc value simulated by the optimal model is close to the measured value.Kc and MSAVI fit best in the dune,and the estimated Kc value has a good correlation with the measured value.In the polynomial regression model,Kc-LAI model is still the optimal of the the meadow,and the fitting effect of the simulated value of Kc and the measured value is improved.The Kc-MSAVI fitting of dune is optimal,but R2 decreases slightly,and the simulated value of Kc is well fitted to the measured value.For the stepwise regression model,the model of Kc-ND VI and SM and MS AVI had the best effect in meadow,while the model of Kc-MSAVI and SM had the best fit in dune.The addition of soil moisture content improved the model accuracy.Using the Kc simulated by the stepwise regression equation to estimate ETa,the results show that the simulated values of ETa in meadows and dune during 2017 and 2018 were close to the measured values,and the model has good applicability.