4.2.2. FCover

FCover is the percentage of plant material that covers the soil surface when observed from above and at nadir. Monitoring fCover on agricultural fields from early season gives an indication of the rate of crop development and vigor. Increasing fCover signifies development of leaf area or above-ground biomass. There is a relationship 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 soils on an agricultural field to erosion. Crop types with high fCover at early developmental stages better intercept incident radiation and rainfall, thereby increasing soil shading and decreasing soil evaporation. It can also be used to estimate irrigation requirements.

FCover can be estimated from digital images taken of a field from a vertical position. The device used can be a digital camera or smartphone. The picture is interpreted into “crop/plant area” and “soil area” from which coverage is estimated. See here for more information.

Establishing a robust relationship between in-situ measured fCover and NDVI can allow a determination of the spatial distribution of fCover of an area or agricultural field. In various projects, strong relationships between the two variables are often established for a number of crop types.

Figure 4.5 relationship between NDVI and fCover for major crop types in Mali (Source:  ICRISAT)
 

In Bangladesh, the Weighted Difference Vegetation Index (WDVI) (Clevers, 1989) was used to calculate ground cover (GC) or fCover. It is linearly related to crop water use (Rajan et al., 2010), which is a main driver for an irrigation scheduling tool that we developed. 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 percent ground cover for a wheat crop in Bangladesh is shown in Figure 4.6.

  

Figure 4.6 A map showing percent ground cover for a wheat crop in Bangladesh.