Soil moisture top 20 cm

 

Name:soil_moisture top 20 cmAbbreviation:sm20Unit:kg m-3
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

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

Parameter
Discipline= 2 Land surface products 
Parameter Category= 0 Vegetation/biomass
Parameter Number= 22 Soil Moisture
Level
Type of first fixed surface= 106 Specified height level above ground m
Scale factor of first fixed surface= 0 
Scaled value of first fixed surface= 00
Type of second fixed surface= 106
Missing
Scale factor of second fixed surface= 1
 
Scaled value of second fixed surface= 2
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.