Spatio-temporal Variability Of Surface Soil Moisture In The Western Songliao Plain Based On SMOS Data

As the key component of the earth’s water cycle, soil moisture has an important significance to the material and energy exchanges of terrestrial ecosystem. Soil moisture is also an important indicator of land degradation and drought monitoring. The spatio-temporal variability of surface soil moisture helps well to understand the roles of hydrosphere in climate, ecological and biological processes. Satellite remote sensing is an effective means of regional surface soil moisture real-time, dynamic estimation and monitoring. Soil moisture information derived from passive remote sensing has many advantages including shorter revisit period, higher accuracy of inversion algorithm and lower spatial resolution, thus it can suitable for rapidly monitoring soil moisture at large scale.The Western Songliao Plain located in southwest part of the Northeast China belongs to the northern ecological fragile zone,where the problems of scarce precipitation, high evaporation, lack of available surface water, uneven distribution of groundwater and water resources shortage have been existing for a long time. Therefore, to grasp the spatio-temporal characteristics of soil moisture in the Western Songliao Plain is of great significance to soil moisture management and many other fields.In this study, soil moisture data were obtained from soil moisture and ocean salinity satellite(SMOS). The methods such as spatial and temporal Root Mean Square Error(RMSE), trend anlysis and distruibution center migration model were used to analyze surface soil moisture changes for the period of 2010 ~2014 in the Western Songliao Plain. Based on the Tropical Rainfall Measuring Mission Satellite(TRMM) data, SPOT-VGT NDVI data and DEM, the correlations between soil moisture and the impact factors such as precipitation,vegetation, elevation and slope were revealed.The main results of this study are shown as follows:1. During 2010 ~2014, surface soil moisture value in the northeast the Western Songliao Plain was greater than that in the southwest on the whole. The highest value of soil moisture(0.130 m3 m-3) was found in the year of 2013. In 2011, the average soil moisture was the lowest value of 0.095m3 m-3. Surface soil moisture gradually increased from May to July and reached the maximum in July. Regional soil moisture dropped to the lowest value of 0.100 m3 m-3 in September, whereas it increased slightly to 0.110 m3 m-3 in October.2. For recent five years, significant spatial changes of surface soil moisture mainly found in some counties and banners including Fuyu, Tongliao, Daan and Naimanqi, but spatial variabilities in Linxi, Bairin Right Banner, Horqin Right Wing of Middle Banner, Bairin Left Banner, Ongniud Bannar and Chifeng were not obvious. Surface soil moisture in Songyuan, Fuyu and other places changed little over time. In different years, the significant temporal changes might be found in different regions, namely, Bairin Left Banner and Holingol in 2010, Jarud Banner and Hure Banner in 2011, Holingol City and Aohan Banner in 2012, and Tongliao, Hure Banner and Naiman Banner in 2013 and 2014, respectively.3. Average surface soil moisture during the growing season regressed over past five years for each pixel was calculated. The regression slope values indicating soil moisture change trend. During 2010~2014, surface soil moisture tended to increase in Western Songliao plain with differences in space. The greatest slope of study area was 0.12 while the lowest value was 0.02. The increasing trend of surface soil moisture in study area gradually accelerated from the south to the north. The distribution center of surface soil moisture shifted to the northeast with 30.809 km in the past five years. After 2013, soil moisture distribution center moved towards the southwest and decreased migrated distance.4. The precipitation in Western Songliao Plain presented the spatial characteristics of low values in southwest and central part and high in the southeast and the northeast. During 2010~2014, the average precipitation in growing season increased from 20 mm to 22 mm with an rising trend, The precipitation gradually increased from May to July, and decreased after July. The pattern of monthly average NDVI within the growing season approximately increased from the center to surrounding area. Over the past 5 years, the regional average NDVI value increased from 0.45 in 2010 to 0.51 in 2014 with an obvious undulating trend. NDVI increased from May to August, but decreased gradually since August.5. There was significantly positive relationship between precipitation, NDVI and surface soil moisture in the Western Songliao plain. The correlation coefficient of soil moisture and precipitation was relatively higher than that of NDVI which suggested the more obvious response of soil moisture to precipitation change in semi humid and semi-arid areas. During June ~July, abundant precipitation was significantly correlated to soil moisture. In the growing season of plants, there was strongest correlation between soil moisture and NDVI, for instance, the correlation coefficient in June 2012 was 0.472. In the region of the elevation value less than 200 m, surface soil moisture is relatively high and the average value was 0.140 m3 m-3. The slope of 89.12% of the study area was 0~10 degrees, and the average soil moisture content was 0.113 m3 m-3. The maximum surface soil moisture values were found in southern slope with the average value of 0.121 m3 m-3. The surface soil moisture decreased gradually from the south slope to the north slope. The minimum soil water value(0.108 m3 m-3) was found in north slope.