Analysis of relationships among vegetation condition indices and multiple-time scale SPI of grassland in growing season

Drought is one of the important environmental disasters in various parts of the world which has an impact on many aspects of society, and it is the most complex but least understood of all natural hazards. The present study is an attempt to improve our understanding of characteristics and relationships between vegetation condition and meteorological factors that cause drought. Vegetation Condition Index (VCI) and phenology metric called Percent of Average Seasonal Greenness (PASG) which are based on 10 years (1999-2008) time-series SPOT VGT-S NDVI were used to monitor grass growing condition. Multiple-time scale (1-month, 2-month, 3-month, 6-month, 9-month and 12-month) Standardized Precipitation Index (SPI) was used to detect drought. By analyzing the changes of correlation coefficients among VCI, PASG and SPIs, the 3-month SPI was found to have the best correlation with VCI and PASG, but the relationship varies significantly during different periods of the grass growing season. Correlation coefficients between VCI, PASG vs. SPIs reached 0.695 and 0.725 respectively at the last ten days of September and the first ten days of October. Compared with PASG, VCI shows more sensitive to precipitation on the whole, but the results of grass condition monitoring caused by drought is more stable using PASG than VCI. Both of them have a certain complementary for each other.