Water availability has a significant impact on vegetation activity. Together with solar radiation and temperature, it influenced the spatial distribution of ecosystems. Soil moisture is a good indicator of water availability. Due to the difficulty of observing surface soil moisture on a large scale, the influence of soil moisture on vegetation dynamics on regional scale needs further investigation. In this study, we are trying to answer two questions:
(1) Is there a significant relationship between soil moisture and vegetation activity in China in recent years?
(2) Whether the time lag is ubiquitous in the response of vegetation activity to the change of soil moisture, and what is the spatial distribution?
In order to answer these questions, we utilized a remote-sensing based approach. The daily soil moisture data were derived from SMAP soil moisture products, which have a 9km spatial resolution, from 13/04/2015 to 12/04/2017. Considering the different performance of EVI and NDVI in high vegetation coverage areas, both MODIS 1km monthly NDVI and EVI data were engaged to represent vegetation activity. Cross correlation analysis was used to study the relationship between soil moisture and NDVI/EVI, and to detect the existence of time lag.
Results/Conclusions
(1) During the period April,2015~April,2017, most area in China shows positive correlation between soil moisture and vegetation indices (rEVI: 0.45~0.99; rNDVI: 0.43~0.99);
(2) Most area in China shows positive correlation between soil moisture and vegetation index without time lag (EVI: 80.3%; NDVI: 81.6%);
(3) Some pixels in semi-arid & arid area in China show positive correlation with one-month’s time lag (EVI: 7.4%; NDVI: 8.1%).
The results suggest that vegetation in arid and semi-arid areas of China may have unique mechanism to adapt to water deficit. This study would enhance our knowledge about vegetation-water relationship.