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Frequently Asked Questions

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This page provides a series of FAQs. For more details on the product, please see the Product Specification Document (PSD) and the Product User Guide (PUG). In addition, both Dorigo et al. (2017) and Gruber et al. (2019) provide a good overview of the product and algorithm.

 

Acessing and Using the Data

 

How can I download the data?

Please download the data from our FTP, after registration. For more details, see the data access and download page. This page also provides details on how to view and use the product.

What format is the data in?

The data is provided as global, daily netCDF files (i.e. one file for each day of the product). On the FTP you will find zipped as well as unzipped versions.

How do I cite the data?

A list of references which should be cited when using hte data are provided on the publications page as well as in the global attributes of the netCDF files.

Is the product available in near-real time?

The product is not available in near-real time, however, a near-real time soil moisture product can be downloaded from the Copernicus Climate Data Store. This product is based on the ESA CCI SM v03.3 algorithm.

How do I convert volumetric soil moisture to soil wetness content?

SM (%) =  SM_vol (m3m-3) / porosity_vol (m3m-3).

The ancillary porosity map provided with the Soil Moisture dataset is expressed as volumetric percent (m3m-3 * 100%). For the computation, we used the equations of Saxton and Rawls (2006) which take clay, sand, silt, and organic matter as input. Both the soil fractions and equations may cause errors, especially at the global scale. In practice, you can use any porosity dataset as input.

 

Product Characteristics

 

What products are available?

There are three products available:

  • ACTIVE - based on active scatterometer data
  • PASSIVE - based on passive radiometer data
  • COMBINED - based on a combination of the active scatterometer data and the passive radiometer data

What is the spatial and temporal coverage of the product?

The current product, version 04.5 covers the period November 1978 to the end of 2018 for the PASSIVE and COMBINED product and from January 1991 to the end of 2018 for the ACTIVE product.

What is the observation density of the product like?

The fraction of number of observations is shown in the following figure for the COMBINED product v04.5. Note that at the start of the product, there is signficantly less temporal coverage than in the later periods. This is due to to the product (COMBINED and PASSIVE) being based only on the SMMR radiometer.

 

Fractional coverage of observations for the COMBINED product (v04.5).

Why are there less observations in some areas?

For areas with dense vegetation (tropical, boreal forests), strong topography (mountains), ice cover (Greenland, Antarctica, Himalayas), a large fractional coverage of water, or extreme desert areas we are not able to make meaningful soil moisture retrievals. Hence, these data are masked in the final product.

Which products are used in the product?

The following sensors are used in the production of the COMBINED product:

Sensor coverage in the COMBINED product (v04.5)

In particular, the following datasets are used (at ESA CCI SM v04.5):

How has the data been verified and validated?

More information on the verficiation and validation of the product can be found here.

 

Other FAQs

 

Is the product the "absolute" truth?

The COMBINED dataset is rescaled to GLDAS v2.1 prior to merging. We perform this processing step to obtain a final product in absolute volumetric units [m3/m3]. Even though the original dynamics of the remote sensing observations are preserved, this step imposes the absolute values and dynamic range (min-max) of the GLDAS-Noah product on the combined product. As a consequence, the combined product CANNOT be considered an independent dataset representing absolute true soil moisture.


Why do the provided soil moisture images appear in data stripes?

Microwave images from the earth's surface are taken while the satellite is orbitting the earth in fixed paths. This is particularly apparent at the start of the data period, but as we reach the present, the spatial data density gets higher and these stripes are no longer apparent.

How should I interpret the soil moisture noise layer?

The noise layer gives you information about the random uncertainty of the retrieval and, hence, about the quality of it.

Which of the products (ACTIVE, PASSIVE or COMBINED) is most suitable for model verification?

In theory, the combined product combines the best of the active and passive products, so we would take the merged/combined product.