Developing a desertification index integrating intra-annual dynamics of vegetation cover and soil moisture

Desertification is a key expression of dryland ecosystem degradation, affecting land productivity and ecological sustainability. Reliable assessment requires indicators that capture both vegetation status and water limitation, yet the seasonal coupling between vegetation cover (VC) and soil moisture (SM) remains insufficiently understood. This study analyzed VC–SM interactions in Inner Mongolia, northern China, from 2001 to 2025. SM was estimated using the temperature vegetation drought index derived from MODIS-NDVI and MODIS-LST data. Temporal trend analysis and time-lag cross-correlation were applied to characterize seasonal VC–SM dynamics across land-cover types, and four desertification indices were developed. Intra-annual SM fluctuations closely tracked vegetation phenology. Increased SM promoted vegetation growth, while vegetation development enhanced water consumption and slowed SM accumulation. The difference between growing-season maximum VC and minimum SM was strongly correlated with peak VC (r = 0.97, p < 0.01), indicating its ability to reflect soil productivity and vegetation–water coupling strength. VC–SM coupling varied among land-cover types, with stronger and longer-lagged responses in forest and cultivated lands than in grasslands and unused lands. Among the four indices, the Moisture Vegetation Normalized Difference Desertification Index (MV-NDDI) showed comparatively stable performance in capturing VC–SM relationships and distinguishing desert from non-desert areas, although the reported correlations were modest in magnitude. These findings suggest that MV-NDDI may provide a useful indicator for desertification monitoring in water-limited dryland ecosystems.