Soil moisture top 20 cm

Name:

soil_moisture top 20 cm

Abbreviation:

sm

Unit:

kg m^-3

Definition

The volumetric soil moisture on the top 20cm of the soil layer.

Validity

Daily mean

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

GRIB 2

Discipline	= 2
Parameter Category	= 0
Parameter Number	= 22
Type of first fixed surface	= 106	Type of second fixed surface	= 106
Scale factor of first fixed surface	= 0	Scale factor of second fixed surface	= 1
Scaled value of first fixed surface	= 0	Scaled value of second fixed surface	= 2

GRIB 2 and C-code for: Perturbed forecast, Control forecast, Perturbed re-forecast and Control re-forecast

Comments

Author	Contents
Jörg Urban	The ECMWF runs a uniform soil scheme, where the wilting point = 170 kg/m3, the field capacity = 320 kg/m3 and the saturation = 470 kg/m3 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.