Generally, SMAP soil moisture products are given in volumetric units (cm3/cm3). But, Google Earth Engine provides SMAP ssm and susm in mm (https://developers.google.com/earth-engine/datasets/catalog/NASA_USDA_HSL_SMAP10KM_soil_moisture).
It's clear that this is the height of the water column. But, the data info does not specify the height of the soil column considered in calculating (or converting) ssm/susm into mm. So, I'm not sure how to convert GEE's SMAP soil moisture products into cm3/cm3. Have they considered 5cm soil column for ssm and 100cm column for susm?
Appreciate any help with this.
These documents explain that it isn't as simple as you might hope. The second sentence in the first document is the following, and sums up why "surface soil moisture" is in units of mm and not volume/volume (emphasis added):
"The surface soil moisture is assumed to hold a maximum of one inch (or 25-mm) of available water, which means the top-layer soil depth is dependent on the soil texture."
https://gimms.gsfc.nasa.gov/SMOS/SMAP/Surface_Soil_Moisture_SMAP.pdf
https://gimms.gsfc.nasa.gov/SMOS/SMAP/Sub_SurfaceSoil_Moisture_SMAP.pdf
https://gimms.gsfc.nasa.gov/SMOS/SMAP/SoilMoisture_Profile_SMAP.pdf
Thus, the thickness of the soil layers is variable in this product. The idea is to be a measure of plant sustaining condition of the soil, not a total water supply.
All that said, The surface soil moisture value times a surface area (e.g. a square meter or square kilometer of land surface) would give a volume of water for the surface layer (of variable thickness). The subsurface soil moisture value times a surface area will similarly give a volume of water. This approach will not be giving you the volume of soil that is holding this volume of water.
The changes in soil moisture over time give a sense of evapotranspiration rates which can then be used to extrapolate soil moisture adequacy for plant/crop growth at various life stages. Precipitation (or irrigation), also measured in mm, replaces water lost through evapotranspiration.
Matthew Mabey Thanks for the explanation. This explains this dataset as an assimilated dataset developed with the help of SMAP L3 data, but not the original SMAP SM product as the dataset name implies. Looks like we can't get the original SMAP SM products in volumetric units in GEE, and there's no direct way of converting.
Indishe Senanayake Hello, Dr. Indishe Senanayake.I have a similar problem when collecting global soil moisture content data: how to convert the MM unit in SMAP SSM into the mass percentage of moisture content?
Could you give me some advice?
Thank you sincerely!
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