Name:soil_moistureAbbreviation:smUnit:kg m-3
DefinitionThe volumetric soil moisture on the top 20cm of the soil layer.
Validityinstantaneous
CommentTo 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.
GRIB 2
Discipline= 2  
Parameter Category= 0  
Parameter Number= 22  
Type of first fixed surface= 106Type of second fixed surface= 106
Scale factor of first fixed surface= 0Scale factor of second fixed surface= 1
Scaled value of first fixed surface= 0Scaled value of second fixed surface= 2
GRIB 2 and C-code for: Deterministic forecast, Perturbed forecast and Control forecast

 

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.