The EFAS soil moisture is modelled in LISFLOOD in three layers representing top, medium and bottom since EFAS v1.0. Further, for each LISFLOOD pixel, the soil moisture calculation is linked to land use classes by using fractions of forest (f), irrigation (i) and other (o). The fractions of each land use type that are used for soil moisture calculation do not necessarily add up to 1 for each pixel since they represent the soil area that is not water for each grid cell. Soil moisture data available though MARS and CDS are stored as the three individual soil layers (This is more flexible and allows the user to extract the information that is needed). The soil moisture that is stored in MARS and CDS is the volumetric soil water content, which is defined as the ratio between the volume of water and the total volume of the soil, including all particles, vegetation, water and air.
In contrast, the soil moisture layer that is shown on the EFAS-IS, is the average relative soil moisture calculated for the forest (f) and other (o) fraction for the two top layers only, which is also referred to as the soil wetness index. These values are averaged for up to 10 days before the forecast valid date of LISFLOOD simulations forced with observations fields (Water Balance) to form the final product.
For customised calculations, additional static files are provided from MARS or CDS. The soil depth available for the EFAS domain is the cumulative depth in meters from the top of the surface to the bottom of each layer.In order to calculate soil wetness index, you need the below static maps. Note that the formulas below are for the gridded fields of all variables.
Wilting point
is defined as the minimal amount of water in the soil that the plant requires not to wilt. If the soil water content decreases to this or any lower point a plant wilts and can no longer recover its turgidity when placed in a saturated atmosphere for 12 hours.
Wilting point for each layer:
Field capacity
is the amount of soil moisture or water content held in the soil after excess water has drained away and the rate of downward movement has decreased
Field capacity for each layer:
1. Calculate the individual soil depth for layer 1 and 2
From CDS you will retrieve the depth from the surface of the soil to the bottom of that layer. To get the layer thickness of the second layer, we need to subtract the top layer:
sd1 = Soil depth of layer 1
sd2 = Soil depth of layer 2 - Soil depth of layer 1
where sd1 and sd2 and the thickness of layer 1 and 2 respectively
2. Calculate the mean wilting point and field capacity as a mean across layer 1 and 2 together
Wilting point: thmin = (thmin1 * sd1 +thmin2 * sd2)/(sd1 + sd2)
Field capacity: thmax = (thmax1 * sd1 +thmax2 * sd2)/(sd1 + sd2)
3. Calculate the mean volumetric soil water content across layer 1 and 2 together and the mean forecasted volumetric soil water content (swvtot) as an average of the whole forecast horizon
The mean volumetric soil water content (swv) is calculated as:
swv = (swv1 * sd1 + swv2 * sd2)/(sd1 +sd2)
Then the mean forecasted volumetric soil water content (swvtot) is calculated as the mean swv of all forecast steps in the 10-day forecast driven by ECMWF's high-resolution forecast:
swvtot = mean(swv)
swv1 and swv2 are the volumetric soil water contents that are stored in the EFAS archive for layer 1 and 2.
4. Finally, calculate the soil wetness index as a ratio of swv between wilting point and field capacity
SM = (swvtot - thmin)/(thmax - thmin)
To visualise the soil wetness index, see the 'Plot Soil Wetness Index Map' section of Plot CEMS-Flood Data