Study On Evapotranspiration Estimate Over Orchards By Remote Sensing Based On Modified METRIC Model

Fruit tree,as a kind of crop with higher economic value and more functions,is a model of efficient agriculture.In recent years,the planted area of fruit trees has gradually expanded in many areas.However,due to the large water demand of fruit trees,the irrigation water has increased sharply,which makes the severe contradiction between supply and demand in water-poor areas.Therefore,it is very important for orchard managers and water resource planners to grasp the evapotranspiration information of orchards accurately and adopt reasonable and efficient irrigation strategies.The study objects are the almond orchard and three pistachio orchards and the study area is located in the South San Joaquin Valley,California.The remote sensing approach is applied to estimate evapotranspiration over the orchards and the study data are the Landsat-5,7,8 satellite imagery data,wheather data from the California Irrigation Management Information System(CIMIS)and measurements from the tower fluxes which are located in the two types of orchards.Firstly,the sensitivity of the intermediate parameters of Mapping Evapotranspiration at High Resolution with Internalized Calibration(METRIC)model to the daily evapotranspiration of orchards during the whole growing season is analyzed,and the METRIC model is localized corrected.Secondly,the reference evapotranspiration which is used to internally calibrate METRIC model is optimized.Then,an automatic selection method of endmember pixels in METRIC model is proposed,and the automatic METRIC model is used to estimate evapotranspiration over two types of orchards.Finally,the spatial and temporal characteristics of evapotranspiration are analyzed,and the study period is from 2009 to 2012 in the almond orchard and is from 2015 to 2016 in three pistachio orchards.The main research results obtained are as follows:(1)The study of sensitivity of the METRIC model intermediate parameters to daily evapotranspiration over two types of orchards showed that the net radiation flux had the greatest influence on the daily evapotranspiration of orchards during the whole growing season.When the net radiation flux changed by 5%,the average daily evapotranspiration of almond orchard changed by 0.42 mm/day,and Control orchard to High-S orchard of pistachio orchards changed by 0.30mm/day,0.24mm/day and 0.15 mm/day.Sensible heat flux and near-suface temperature gradient had the second effect on the daily evapotranspiration of orchards.However,the aerodynamic resistance in the METRIC model had almost no effect on the daily evapotranspiration of orchards.(2)When three groups of reference evapotranspiration were used to internally calibrate METRIC model during estimating daily evapotranspiration of orchards,the METRIC model with the internal calibration of ETr-PM reference evapotranspiration calculated by standard Penman-Monteith of ASCE-EWRI over the crop surface of 0.5m tall had the worst accuracy,followed by METRIC model with the calibration of ETo-PM which was also calculated by standard Penman-Monteith of ASCE-EWRI over the crop surface of 0.12m tall,and the METRIC model with ETo-CIMIS which was calculated by modified Penman equation with a wind fuction developed at the University of Calfornia,Davis over the crop surface of 0.12m tall had the best accuracy.After internally calibrating by ETo-CIMIS reference evapotranspiration,the accuracies of daily evapotranspiration over the orchards were as follows:the determination coefficient,root mean square error,bias and mean relative difference in the almond orchard during the growing season were 0.89,0.63mm/day,-0.35mm/day and 7.87%;and the determination coefficient,root mean square error,bias and mean relative difference in three pistachio orchards were 0.57,1.41mm/day,-0.74mm/day and 21.60%,respectively.(3)When the threshold points of NDVI and surface temperature of "cold" pixels are 95%and 5%,and those of NDVI and surface temperature of "hot" pixels are 5%and 95%,respectively,the endmember pixels in METRIC model can be selected reasonably and accurately during the automatic selection.The accuracies of daily evapotranspiration over two types of orchards estimated by automated METRIC model are both higher.During Landsat satellite overpassing dates,the determination coefficient,root mean square error,bias and mean relative difference of daily evapotranspiration in the almond orchard were 0.87,0.53mm/day,0.03mm/day and 7.96%,respectively;the determination coefficient,root mean square error,bias and mean relative difference of daily evaportranspiration in three pistachio orchards were 0.58,1.30mm/day,-0.43mm/day and 20.61%,respectively.After using cubic spline interpolation to interpolate daily evapotranspiration on satellite overpassing dates to continuously daily evapotranspiration,the determination coefficient,root mean square errors and mean relative difference of continuously daily evapotranspiration in the almond orchard were 0.85,0.56mm/day and 8.00%,and the determination coefficient,root mean square errors and mean relative difference of continuously daily evapotranspiration in three pistachio orchards were 0.56,1.28mm/day and 23.58%.(4)The continuously daily evapotranspiration estimated by automated METRIC model can capture the seasonal variation of orchard evapotranspiration,that is,the daily evapotranspiration of orchards increased gradually in the early growth stage,reached the maximum in the middle growth stage,and then decreased gradually.At the same time,the automated METRIC model can also show the spatial characteristics of orchard evapotranspiration.Among them,the spatial distribution of almond orchard was relatively uniform.For three pistachio orchards,the highest and more uniform evapotranspiration was in the Control orchard,followed by Med-S orchard,and the lower evapotranspiration was in the High-S orchard,and the spatial difference of orchard evapotranspiration was significant in the High-S orchard.