Spatial And Temporal Dynamics Of Soil Relative Moisture In Songnen Plain From 2000 To 2015

Soil moisture is one of the important indicators for monitoring land productivity.Soil moisture level is closely related to the growth and development of crops and vegetation.Moreover,it plays a decisive role in the ecological landscape structure and spatial pattern distribution.In recent years,As one of China's major commodity grain bases,Songnen Plain has been aggravated by the impact of global climate change and human disturbance had seriously led to the reduction of river and lake areas and the intensification of soil salinization in the Songnen Plain,resulting in significant changes in hydrological processes.The comprehensive situation of farmland and their trend of soil drought is severe,which destroyed the structure and function of ecosystems and threatening National food security.At the same time,the lack of long sequence and large-scale soil moisture data makes it difficult to monitor soil moisture continuously in time and space.It is of great significance to accurately obtain time series soil moisture data and monitor the drought situation of farmland.This work will not only ensure the national food security,but also maintain social and economic sustainability.In view of this,time-series MODIS images were selected to calculate the monthly temperature vegetation dryness index(TVDI)of Songnen Plain during 2000 —2015.There was a strong correlation between the TVDI value and soil moisture,which signified that soil related humidity can be inversed by TVDI value and metereological situ soil relative moisture.The spatial dataset for the surface soil moisture during the crop growing season(from April to October)was built with a 1 km cell size.The temporal and spatial variation characteristics and dynamic trends of soil moisture were analyzed by ensemble empirical mode decomposition(EEMD)and empirical orthogonal function(EOF).The results were as follows:(1)Based on MODIS images with good time series and soil relative humidity data measured by meteorological stations in the study area,the method of retrieving soil relative humidity by temperature-vegetation drought index was selected to obtain soil relative humidity data in Songnen Plain from 2000 to 2015.Combining the field measured soil relative humidity data in late April 2015 with the corresponding time meteorological station data,the RMSE is 14.53,which proves that the method of soilrelative humidity inversion using temperature-vegetation drought index is applicable in Songnen Plain.(2)The result showed that soil moisture in Songnen Plain was gradually arid from northeast to southwest in terms of spacial distribution.Moreover,the result of the ensemble empirical mode decomposition function analysis illustrated that the residual value of soil relative humidity in the past 16 years increased slowly from 47.277 to58.480,indicating that the soil moisture in Songnen Plain increased year by year.At the same time,the relative humidity of soil in different regions showed differentiations.The regions with serious soil water shortage often occurred in the southwest of the plain,and the regions with high frequency of moderate water shortage are concentrated in the middle of the plain.The seasonal variation of soil relative humidity was also observed in April and October.The relative humidity of soil revealed a decreasing trend from2000 to 2015,while the relative humidity of soil increased gradually with the passage of time from May to September.(3)The inversion data were analyzed by the empirical orthogonal function decomposition method,There were three main spatial modes for soil relative humidity in Songnen Plain.The first eigenvector(accounting for 40.89% of the total variance)and its time coefficient indicated that soil moisture tends to became wetter gradually;the second eigenvector(accounting for 15.72% of the total variance)and its time coefficient indicated that soil moisture in the north of Songnen Plain has increased after2011,while the relative humidity in the south showed the opposite change characteristics,and the change intensity was weaker than that in the north.The third eigenvector(accounting for 10.69% of the total variance)and its time coefficient showed that the change trend of soil relative humidity in the northwest and southeast of Songnen Plain is distinct after 2002.The change trend of soil relative humidity in the west was turning drying and then wetting,while that in the East was turning wetting and then drying.