Improved estimations of dry vegetation cover and mass using MODIS data at a dust hotspot in the Gobi Desert

Monitoring of dry vegetation is crucial in environmental management such as assessments of wind erosion risk, livestock forage, and fire risk. In arid and semi-arid regions, wind erosion has severe environmental and socioeconomic consequences. As one of the most important influencing factors for wind erosion and the dust emission it causes, vegetation fraction cover affects threshold friction velocity for sand saltation, which is determined by roughness density. However, there has been little research into estimating the coverage and mass of dry (non-photosynthetic) vegetation. In our study area at Tsogt-Ovoo (TsO) in the northern Gobi Desert, Mongolia, we investigated the suitability of MODIS data to estimate dry vegetation cover and mass. We compared observed dry vegetation cover (VCd_obs) and mass (Md_obs) with values estimated from the Normalized Difference Vegetation Index (NDVI) and six other potential Short Wavelength InfraRed (SWIR) dry vegetation indices. Of the seven vegetation indices, the Soil Tillage Index (STI), which is calculated from the ratio of SWIR bands 6 and 7, provided good estimates of VCd_obs and Md_obs. To verify those estimates, we obtained roughness lengths and threshold friction velocities from field observations of sand saltation, wind speed, and wind direction at stony and non-stony sites in our study area. We compared roughness length (z0) and threshold friction velocity (ut) for sand saltation with dry vegetation cover determined from STI. At both sites, the observed roughness length and threshold friction velocity increased with increasing dry vegetation cover estimated from STI. Based on their relationships with STI, we predicted values of z0 and ut in the absence of dry vegetation that were in accordance with published data. The threshold wind speeds at 10 m height derived from ut at both the stony and non-stony sites were reasonably close to the value that is commonly used in dust models. Thus, we verified that STI is a suitable index for estimation of dry vegetation cover and mass in the Gobi Desert and, potentially, in other arid and semi-arid regions. The STI-based monitoring of dry vegetation will improve wind erosion risk assessment.