Relationship between Soil Moisture and Vegetation Activity in the Mongolian Steppe

Drought has become widespread throughout the Northern Hemisphere since the mid-1950s, affecting the Mongolian steppe and pastureland used for livestock. Given this background, we investigated the relationship between modeled root-zone soil moisture (W(m)) and vegetation activity based on Normalized Difference Vegetation Index (NDVI) data for the Mongolian steppe during the period 1982-2005. In general, interannual change in NDVI coincided with that in W(m). NDVI showed a stronger correlation with W(m) (r = 0.91) than with precipitation (P) (r = 0.65). A strong positive correlation was found between seasonal changes in NDVI and above-ground biomass (r = 0.94). A comparison between years with high and low NDVI(max) revealed that the significant difference in P led to a significant time-lagged (about a half month) difference in W(m) and finally to that in NDVI with time lags of about one month. In addition, NDVI(max) value of a given year was correlated with the W(m) value for the current year (r(2) = 0.53), and was more strongly correlated with the combination of the current year W(m) and the preceding year NDVI(max) of (r(2) = 0.55). This result suggests that on the interannual basis, the vegetation activity is primarily controlled by the current year soil moisture and slightly affected by underground structures stored in the root system.