SMAP Level-4 soil moisture data

[EarthdataForumContributor]

Why do the soil moisture values in the SMAP Level-4 data vary from what I expect in a particular region?

[NSIDCx - amyygilliland]

There are a few reasons that the soil moisture data values in SMAP Level-4 data products may vary from what you expect in a particular region. The first step a data user should take in investigating apparently anomalous values is to look at the rich quality information and other data flags contained in SMAP data files and associated QA files. A user should also consider all of the ancillary data that are used in generating SMAP products. For the SMAP Level-4 data in particular, the soil parameter inputs are based on texture composites from the Harmonized World Soil Database version 1.21 (HWSD1.21) and the State Soil Geographic (STATSGO2) project. These inputs and the soil hydraulic parameters derived from them may not be perfect and regional expertise might well reveal the SMAP soil moisture values as a poor reflection of conditions on the ground. While the model may have difficulty estimating absolute values of soil moisture in some cases, the SMAP Level-4 soil moisture data should still display representative temporal variations. If a data user has a good reason to doubt the magnitude of soil moisture variations, he or she can rescale the product based on local knowledge.

Kind regards,
Amy

[asniyori]

Hello Good Afternoon!!!

I am using SMAP L4 from GEE to downscale surface and root zone soil moisture. The unit is represented in 'volume fraction'. However, I am facing a challenge because some of articles are using the unit as a volumetric content (cm3/cm3) where as others are considering it as a percentage value if multiplied by 100. What is the exact unit of measurement? What if my insitu data is in %, how can I compromise or convert? Is it a direct conversion or it requires some parameters such as porosity or bulk density value?

Thank you

[NSIDCx - amyygilliland]

Dear asniyori,

Thank you for contacting the NSIDC DAAC. The units vary for each parameter in SMAP L4 data products. More information on the units for each parameter is available here: https://nsidc.org/sites/default/files/documents/technical-reference/reichle1438.pdf

What is the in-situ parameter you are comparing to SMAP data? If the parameters are the same, it will be a direct conversion.

Kind regards,
Amy

[asniyori]

Thank you. Now, I got the exact unit of measurement of SMAP L4 data. My target is downscaling surface soil moisture (SM_surface) and Root Zone soil moisture (RZ_SM). However, I have very limited in-situ data which is measured in % which needs to be converted in volumetric soil conten (m3/m3). Is this conversion direct? My second question, is there any alternatives for validation in case of data scarce region where there is no insitu data?

[NSIDCx - amyygilliland]

Hi asniyori,

Are you using in-situ data from the NSIDC? If so, please let me know which data set and I can provide guidance on the conversion. You can explore the SMAP validation data sets here: https://nsidc.org/data/smap/validation-data