Introduction

The EFAS and GloFAS soil water content is modelled in LISFLOOD in three layers representing top, medium and bottom sections of the soil (indicated as superficial, upper and lower soil in Figure 1). Further, for each LISFLOOD pixel, the soil moisture calculation is linked to land use classes by using the main fractions of forest (f), irrigation (i) and other (o). The fractions of each land use type do not necessarily add up to 1 for each pixel since they represent the soil area that is not water for each grid cell.

The LISFLOOD output soil moisture variable is volumetric soil water content, with a separate variable existing for the forest, irrigation and other land types, for the three layers of the soil separately for all three fractions. All these are defined as the ratio between the volume of water and the total volume of the soil, including all particles, vegetation, water and air.

The soil wetness index (swi) is defined as a summary variable, representing the soil moisture conditions in the so-called root zone. The root zone is the maximum depth at which plants can extract water from the soil, and as such helps with interpretation of the soil moisture, especially across different models with different discretisation of the soil. In LISFLOOD, the root zone is represented by the top two soil layers. The 1st layer has a fixed 50 mm thickness, while the 2nd layer has variable thickness across the different land fractions.

The soil wetness index is a value from 0 to 1, computed by scaling the actual root zone soil water content (a ratio between the actual soil water content volume in the root zone over the total volume of the soil) between the residual soil water content and saturation soil water content, as the lower and upper extremes of the root zone.



Figure 1. Representation of the hydrological processes in LISFLOOD with the three soil layers (from OS LISFLOOD manual at https://ec-jrc.github.io/lisflood-model/).

In the soil wetness index definition used here, the 'irrigation' fraction of the soil moisture is not directly considered and only represented as part of the 'other' soil fraction. This is an acceptable simplification as the irrigation fraction has only small contribution in general.

The soil wetness index defined here uses the residual and saturation soil water content, which is not the only possible way to define a linearly scaled ratio for soil moisture. Other flavours of definitions also exist, for example when the lower and higher limits of the residual and saturation water contents are replaced by the permanent wilting point and the field capacity. With that definition, the permanent wilting point is the minimal amount of water in the soil that the plant requires not to wilt, and as such higher than the residual water content, while the field capacity is the amount of water in the soil after excess water has drained away and the rate of downward movement has decreased, which is lower than the saturation water content used in the above definition

This is a list of LISFLOOD variables used in the calculation of the soil wetness index (root zone).

  • ff: Forest fraction of the soil (static file)
  • ff1: Other fraction of the soil (static file)
  • sdo1: Depth (thickness) of soil layer-1 over 'other' fraction (static file)
  • sdo2: Depth (thickness) of soil layer-2 over 'other' fraction (static file)
  • sdf1: Depth (thickness) of soil layer-1 over 'forest' fraction (static file)
  • sdf2: Depth (thickness) of soil layer-2 over 'forest' fraction (static file)
  • swro1: Residual water content of soil layer-1 over 'other' fraction (static file)
  • swro2: Residual water content of soil layer-2 over 'other' fraction (static file)
  • swrf1: Residual water content of soil layer-1 over 'forest' fraction (static file)
  • swrf2: Residual water content of soil layer-2 over 'forest' fraction (static file)
  • swso1: Saturation water content of soil layer-1 over 'other' fraction (static file)
  • swso2: Saturation water content of soil layer-2 over 'other' fraction (static file)
  • swsf1: Saturation water content of soil layer-1 over 'forest' fraction (static file)
  • swsf2: Saturation water content of soil layer-2 over 'forest' fraction (static file)
  • swao1: Actual water content of soil layer-1 over 'other' fraction (dynamic output)
  • swao2: Actual water content of soil layer-2 over 'other' fraction (dynamic output)
  • swaf1: Actual water content of soil layer-1 over 'forest' fraction (dynamic output)
  • swaf2: Actual water content of soil layer-2 over 'forest' fraction (dynamic output)

Please note: the residual water content is the same across both 'forest' and 'other' fractions over both soil layer-1 and layer-2.

Mean root zone soil water content for actual, residual and saturation computation

Step one

The actual (swact), the residual (swres) and saturation (swsat) soil water contents are computed over the whole root zone, as a weighted average over the two soil fractions and two soil layers:

  • swact = (ff*(swaf1*sdf1+swaf2*sdf2) + ff1*(swao1*sdo1+swao2*sdo2)) / (ff*(sdf1+sdf2)+ff1*(sdo1+sdo2))
  • swres = (ff*(swrf1*sdf1+swrf2*sdf2)  + ff1*(swro1*sdo1+swro2*sdo2))  / (ff*(sdf1+sdf2)+ff1*(sdo1+sdo2))
  • swsat = (ff*(swsf1*sdf1+swsf2*sdf2) + ff1*(swso1*sdo1+swso2*sdo2)) / (ff*(sdf1+sdf2)+ff1*(sdo1+sdo2))
Step two

The actual root zone soil water content is scaled linearly between the residual and saturated root zone water contents:

  • swi = (swact-swres) / (swsat-swres)

Availability of the soil moisture and related data

Soil moisture information is available though MARS and CDS. The soil wetness index (root zone) is provided from EFAS v5 and GloFAS v4, while the three individual soil layer water contents were provided only for EFAS v2, v3 and v4.

  • No labels