Remote Sensing Monitoring And Influencing Factors Assessment Of Winter Wheat Growth In Guanzhong Plain

A rapid and accurate understanding of the effects of climate change and management measures on crop growth can effectively address the multiple challenges posed by population growth,water scarcity and climate change.In this paper,remote sensing monitoring of winter wheat growth in the Guanzhong Plain and assessment of the impact factors were carried out by using field survey and satellite remote sensing inversion,the main findings are as follows:（1）Based on winter wheat field sampling data and MODIS remote sensing data,inversion models of winter wheat growth parameters,soil moisture and phenology were constructed.The results showed that the random forest regression model and the extra tree regression model had the highest accuracy in inverting the chlorophyll and leaf nitrogen contents of winter wheat,with R² of 0.864 and 0.859,respectively.The improved logistic function and cubic function with the introduction of vegetation index could improve the simulation accuracy of above-ground biomass and leaf area index with R² of 0.995 and 0.970,respectively,and the temperature vegetation drought index and vegetation condition albedo drought index could better invert the soil moisture of 0-5 cm（R²>0.547）.The normalized difference phenology index and threshold method could effectively invert the greening,heading and maturing periods of winter wheat with R² of 0.652,0.841 and 0.763,respectively.（2）Based on the determined growth parameters,soil moisture and phenlogy inversion models,the spatial and temporal distribution characteristics of winter wheat growth,nitrogen nutrient status and agricultural drought in the Guanzhong Plain were clarified.The results showed that the interannual variation of aboveground biomass and leaf area index of winter wheat in the study area showed an overall increasing trend,higher in Baoji and lower in eastern Xianyang and Xi’an,and chlorophyll and leaf nitrogen contents gradually decreased from south to north.From 2000 to 2020,the initial dates of greening and maturity of winter wheat were delayed at the rate of 0.54 days year^-1 and 0.09 days year^-1,respectively.The sowing-greening and heading-maturity lengths increased at a rate of 0.54days year^-1 and 0.09 days year^-1,respectively,and the greening-heading lengths decreased at a rate of-0.52 days year^-1.From 2000 to 2020,the irrigation requirements in the study area increased at 0.68 mm year^-1 and 2.07 mm year^-1 during the whole growth period and the sowing-greening period,respectively,and decreased at-1.26 mm year^-1 during the greening-heading period.The frequency of drought events in winter wheat was higher in Weinan and Xianyang,with the frequency of light drought>medium drought>heavy drought>extreme drought.N deficits（N nutrient index less than 1）existed in Xi’an,Xianyang,and Weinan during the sowing-greening and heading-maturity stages of winter wheat.（3）The effects of climate change and crop management on growth,phenology,irrigation requirements and agricultural drought index of winter wheat were clarified by using path analysis and residual trend method.The path analysis showed that precipitation,phenology and leaf nitrogen content were the main factors affecting aboveground biomass of winter wheat in the Guanzhong Plain,with total path coefficients of0.55,0.75 and 0.39,respectively.precipitation,radiation and relative humidity were the main drivers of irrigation requirements,with total path coefficients of-0.73,0.61 and-0.59,respectively.phenology was mainly affected by precipitation,radiation and wind speed,with total path coefficients of 0.44,-0.30 and-0.25,respectively.Relative humidity is the main driver of agricultural drought,with a total path coefficient of 0.32.The residual trend method analysis showed that temperature had the largest relative contribution to aboveground biomass of winter wheat,with an average absolute value of 44.3%.The relative contribution of precipitation to irrigation water requirement,phenology and agricultural drought in winter wheat was the largest,with absolute values averaging 29.2%,35.7%and 25.9%,respectively.Aboveground biomass,phenology and agricultural drought index of winter wheat were mainly influenced by crop management,with absolute relative contributions averaging 63.3%,76.2%and 78.9%,respectively.Irrigation water demand,on the other hand,was mainly influenced by climate change,with an average absolute relative contribution of 56.3%.