Estimating root zone soil moisture at distant sites using MODIS NDVI and EVI in a semi-arid region

This study investigated the potential of Normalized Difference Vegetation Index (NDVI) and Enhanced Vegetation Index (EVI) to estimate root zone soil moisture at native in situ measured sites, and at increasingly distant sites within the same climatic setting. In situ data were obtained from Soil Climate Analysis Network (SCAN) sites near the Texas-New Mexico border area, and NDVI and EVI products from the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor on board the Terra satellite. Results show that soil moisture values at the same depth are highly correlated ( r = 0.53 to 0.85) at distant sites as far as 150 Km from the native site, and NDVI and EVI are highly correlated at each site (r = 0.95 to 0.98). Raw time series has higher mean correlations than deseasonalized time series at every depth. Deseasonalized time series using NDVI and EVI are significantly correlated with soil moisture at distant sites (NDVI r = 0.35 to 0.73). The correlation reaches maximum value when the Vegetation Index (VI) lags soil moisture by 5 to 10 days. NDVI has a slightly higher correlation with soil moisture than EVI. Values of r decrease with distance from the native site. Regression analysis was also conducted using deseasonalized NDVI and deseasonalized soil moisture time series with a 5 day time lag of NDVI. The model estimated soil moisture at all depths, with adjusted R2 ranging from 0.44 to 0.59. Overall, deseasonalized NDVI values produce consistent results, and show that NDVI can estimate root zone soil moisture at distant sites in the study area.