# Clarification on converting ERA5-Land volumetric\_soil\_water\_layer\_1 (0–7 cm, m³ m⁻³) to mrsos (kg m⁻²), and top-layer depth for cmip6

**URL:** <https://forum.ecmwf.int/t/clarification-on-converting-era5-land-volumetric-soil-water-layer-1-0-7-cm-m-m-to-mrsos-kg-m-and-top-layer-depth-for-cmip6/14032>\
**Category:** C3S - Datasets and Usage\
**Tags:** cmip6, era5-land\
**Created:** [15 August 2025 08:39 UTC](https://forum.ecmwf.int/t/clarification-on-converting-era5-land-volumetric-soil-water-layer-1-0-7-cm-m-m-to-mrsos-kg-m-and-top-layer-depth-for-cmip6/14032 "2025-08-15T08:39:49Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![Chen\_Jintina](https://forum.ecmwf.int/letter_avatar_proxy/v4/letter/c/d9b06d/32.png) [@Chen\_Jintina](https://forum.ecmwf.int/u/Chen_Jintina)\
**Post date:** [15 August 2025 08:39 UTC](https://forum.ecmwf.int/t/clarification-on-converting-era5-land-volumetric-soil-water-layer-1-0-7-cm-m-m-to-mrsos-kg-m-and-top-layer-depth-for-cmip6/14032/1 "2025-08-15T08:39:49Z")

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Dear ECMWF support team,

I’m comparing soil moisture between **ERA5-Land** and **CMIP6** at the same timestamps and find discrepancies up to **~50%** when converting ERA5-Land’s top-layer volumetric soil water to the CMIP-style `mrsos` (mass content of water in soil, kg m⁻²). I’d appreciate your confirmation on the correct unit conversion and related definitions.

**Data/variables:**

- **CMIP6:** `moisture_in_upper_portion_of_soil_column` (kg m⁻²), i.e. `mrsos`.

- **ERA5-Land:** `volumetric_soil_water_layer_1` (0–7 cm) in m³ m⁻³.

reference: [Gridded data underpinning the Copernicus Interactive Climate Atlas: Description of the datasets and variables - Copernicus Knowledge Base - ECMWF Confluence Wiki](https://confluence.ecmwf.int/display/CKB/Gridded+data+underpinning+the+Copernicus+Interactive+Climate+Atlas%3A+Description+of+the+datasets+and+variables#GriddeddataunderpinningtheCopernicusInteractiveClimateAtlas:Descriptionofthedatasetsandvariables-Variablesandindices)

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**Author:** ![Chris\_Moncsko](https://forum.ecmwf.int/letter_avatar_proxy/v4/letter/c/a698b9/32.png) [@Chris\_Moncsko](https://forum.ecmwf.int/u/Chris_Moncsko)\
**Post date:** [22 August 2025 13:26 UTC](https://forum.ecmwf.int/t/clarification-on-converting-era5-land-volumetric-soil-water-layer-1-0-7-cm-m-m-to-mrsos-kg-m-and-top-layer-depth-for-cmip6/14032/2 "2025-08-22T13:26:20Z")

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For clarity, are you using a conversion of `mrsos(0-10)` = `swvl1()`_`(10/7)/1000 (kg m-2)`? or `mrsos` = `swvl1`_`* 100`? The former comes from Section 2.4 “Harmonisation of variables across catalogues” of your reference link, however it seems wrong to me and I believe it should be the latter.  
`mrsos` uses 10cm depth, while `swvl1` uses 7cm, so you either extend the `swvl1` values for another 3cm to match the depth of `mrsos` or you reduce the `mrsos` values by 30% to match the `swvl1` depth.

Also, `mrsos` is the mass of water in all phases states, while `swvl1` seems unspecified and could be the liquid phase equivalent volumetric density. Perhaps the mass density is significantly off in locations where surface temperatures are cold and there is a significant amount of frozen water in the top 7cm, thus reducing the mass density of the water component of the soil? Are the `mrsos` values always _lower_ than expected?

Edit: My supposition is invalid. `swvl1` explicitly includes both liquid and solid phases, as stated in [this post](https://forum.ecmwf.int/t/does-volumetric-soil-water-layer-1-in-era5-land-products-represent-the-total-water-content-including-liquid-and-ice/1638). Perhaps a strong vertical gradient in the moisture can shift the 0-7 and 0-10cm averages by a lot? They’re also products from entirely different models and grids, so there is bound to be _some_ difference (50% is not yet an order of magnitude)