Abstract: Soil moisture plays a significant role in agricultural monitoring and yield estimation. Monitoring soil moisture by remote sensing has such merits as fast effectiveness, strong dynamic contrast and large monitoring areas. Meanwhile, TVDI integrates the reaction of both vegetation growth state and land surface temperature to soil moisture, thus satisfying the demand of this study. The VIIRS, sensor carried by Suomi NPP, as the successor of MODIS/AVHRR, has not been used in soil moisture monitoring in domestic yet. VIIRS sensor data EDR daily surface reflectance product(GIGTO-VI1~5BO)was studied in this paper and the normalized differential vegetation index(NDVI) of the 125th Regiment of the 7th Agricultural Shi in Xinjiang from June 29th to July 10th, 2015 was calculated. In addition, temperature vegetation dryness index(TVDI) was optimized and dry and wet edges were extracted to form characteristic space based on the EDR daily land surface temperature product (GMTCO-VLSTO) so as to calculate TVDI result. In the TVDI feature space, when NDVI is less than 0.1, TVDI cannot response soil moisture condition with no vegetation coverage; when 0.1 < NDVI < 0.7, Tsmax and Tsmin value decrease and increase with the increase of the vegetation coverage, respectively; and when 0.7 < NDVI, the sensitivity of TVDI reduced, owning to the saturated NDVI. The measured ground soil moisture content data in the same period is adopted for verification and this model is promoted to the economic belt regions in the whole north slope of Tianshan Mountain. The study results prove that:(1) The correlation index R² between TVDI result and measured ground data is 0.846; the uncorrelated two-tailed test value between them is smaller than 0.001. The soil moisture model built has sound reaction to water content in unsaturated farmland soil;(2) The characteristic space built shows excellent performance and the dry and wet edges are superior in conforming to theoretical triangular space distribution with better fitting result compared with the previously studied AVHRR/MODIS data; (3) The large breadth and fine-time resolution ratio of VIIRS data further improves timeliness of data processing in large area monitoring; only with 1-4 images can all areas in Xinjiang be completely covered for daily monitoring. This paper pioneers in China in the application of VIIRS sensor data product to realize soil moisture monitoring. This sensor, as an upgrade of AVHRR/MODIS sensor, has been optimized in performance, wave band setting and product format, thus rendering it with better performance. MODIS sensor is designed to serve until 2020s, thus the status of the VIIRS products will rise in the future. Therefore, the monitoring on soil water content in Xinjiang fields could be further strengthened with the application of VIIRS-TVDI to monitor soil moisture and is of certain practical value, which will fill a vacancy in the use of VIIRS data.

Key words: soil moisture, VIIRS, TVDI, Xinjiang