ecCodes definition

Name:Soil moisture top 20 cmAbbreviation:sm20Unit:kg m-3paramId:228086

S2S details

DefinitionThe volumetric soil moisture on the top 20cm of the soil layer.
ValidityDaily averaged
Comment

Computed from instantaneous values at 00. 06, 12 and 18Z - Sea points should be set as missing values

To interprete soil moisture and to compare different models the Soil Wetness Index (SWI) is used:

SWI (soil_moisture - wilting_point) / (field_capacity - wilting_point).

Field capacity corresponds to the maximum value that the model can sustain without precipitation for more than a few hours. The vegetated fraction of the soil evaporates at the maximum rate for soil moisture larger than field capacity and stops evaporating below the permanent wilting point, with a continuous variation between the two extremes.
All Partners are asked to provide the wilting point and the field capacity for each grid point.Computed from instantaneous values

WMO GRIB2 definition

Parameter
Discipline2 Land surface products 
Parameter Category0 Vegetation/biomass
Parameter Number22 Soil Moisture
Level
Type of first fixed surface106Depth below land surface m
Scale factor of first fixed surface0
Scaled value of first fixed surface00
Type of second fixed surface106
Depth below land surface m
Scale factor of second fixed surface1

Scaled value of second fixed surface2
0.2 metre


Comments:

AuthorContents
Jörg UrbanThe ECMWF runs a uniform soil scheme, where the wilting point 170 kg/m**3, the field capacity 320 kg/m**3 and the saturation 470 kg/m**3 are globaly constant. Please note, using the GRIB 2 unit the three values read wilting point 0.17, field capacity 0.32 and saturation 0.47.

ECMWFs has four soil layers: layer 1 from 0cm to 7cm, layer 2 from 7cm to 28cm, layer 3 from 28cm to 1m and layer 4 from 1m to 2.89m. The parameter soil moisture is then computed:

sm ( 7 * Theta_1 + 13 * Theta_2 )/20,

where Theta_i are the averaged volumetric soil moistures on the i-th level.

Joint Meeting of CT-MTDCF/ET-DRC, Montreal, 8-12 May 2006: accepted parameter number 22.