FCover is the percentage of plant material that covers the soil surface when observed from straight above. Monitoring fCover on agricultural fields from the early season onwards presents an indication of the rate of crop development and vigor. Increasing fCover signifies development of leaf area or above-ground biomass. A relationships exists between fCover, NDVI and LAI (Li et al., 2015; Richter et al., 2012; Shang et al., 2014). In most cases, however, this relationship is highly non-linear.
FCover also provides a measure of the susceptibility of the soils in a farm field to erosion. Crops with high fCover at early developmental stages intercept incident radiation and rainfall better, thereby increasing soil shading and decreasing soil evaporation. This can also be used to estimate irrigation requirements.
FCover can be estimated from digital images taken of a field from a vertical position looking downwards. The device used can be a digital camera or smartphone. The picture is interpreted into “crop/plant area” and “soil area” from which fCover is estimated. See here for more information.
Establishing a robust relationship between fCover measured in-situ and NDVI can allow the determination of spatial distribution of fCover in an area or farm field. Within the STARS project, a strong relationship between the two variables was established for a number of crop types.
Figure 4.5 Relationship between NDVI and fCover for important crop types in Mali (Source: STARS Mali Team)
In Bangladesh, the Weighted Difference Vegetation Index (WDVI) (Clevers, 1989) was used to calculate fCover. It is linearly related to crop water use (Rajan et al., 2010), which is a main driver for the irrigation scheduling tool being developed for STARS. WDVI is calculated as follows:
WDVI = NIR - (C × Red)
Where: NIR = total measured NIR reflectance; Red = total measured red reflectance; C = slope of the (soil-specific) soil line, or ratio between NIR and red reflectance of soil. A map showing the percentage of ground cover for a wheat crop in Bangladesh is shown in Figure 4.6.
Figure 4.6 A map showing percentage of ground cover (fCover) for wheat in Bangladesh (Source: STARS CIMMYT team).