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Acronyms
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General definitions
Active (soil moisture) retrieval: the process of modelling soil moisture from radar (scatterometer and synthetic aperture radar) measurements. The measurand of active microwave remote sensing systems is called “backscatter”.
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Table 1: List of Soil Moisture Products
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Data Product | Data Record | Temporal Aggregation |
|---|---|---|
ACTIVE | CDR | Daily |
Dekadal | ||
Monthly | ||
ICDR | Daily | |
Dekadal | ||
Monthly | ||
PASSIVE | CDR | Daily |
Dekadal | ||
Monthly | ||
ICDR | Daily | |
Dekadal | ||
Monthly | ||
COMBINED | CDR | Daily |
Dekadal | ||
Monthly | ||
ICDR | Daily | |
Dekadal | ||
Monthly |
User Requirements
The aim of C3S Soil Moisture is to provide data that meets the accuracy requirements set by GCOS-200 [RD.1]3, while staying in line with community requirements on data coverage, format, provision system and metadata.
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Table 2: Summary of C3S ECV Soil Moisture requirements showing target requirements.
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Requirement | Target | Source |
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Product Specification | ||
Variable of interest | Surface Soil Moisture | GCOS-200 |
Unit | Volumetric (m³/m³) | GCOS-200, CMUG, [RD.10] |
Product aggregation | L2 single sensor and L3 merged products | CMUG |
Spatial resolution | 1-25 km | GCOS-200 |
Record length | >30-35 years | CMUG |
Frequency | Daily | GCOS-200 |
Product accuracy | 0.04 m³/m³ | GCOS-200 |
Product stability | 0.01 m³/m³/y | GCOS-200 |
Quality flags | Should be provided with observations | Gruber et al. (2020) |
Uncertainty | Estimates should be provided for each observation | CMUG |
Format Specification | ||
Product spatial coverage | Global | CMUG |
Product update frequency | Regular updates <1 month, resp. | CMUG |
Product format | Daily images, Monthly mean images | CMUG, C3S |
Grid definition | 0.25° | CMUG |
Projection or reference system | Projection: Geographic lat/lon Reference system: WGS84 | CMUG |
Data format | NetCDF | CMUG |
Data distribution system | FTP, Web access, WMS, WCF, WFS, OpenDAP | CMUG |
Metadata standards | CF, obs4mips | CMUG |
Quality standards | QA4ECV | [RD.10], Gruber et al. (2020) |
Gap Analysis
This section provides a Gap Analysis for the soil moisture product. The purpose of this section is to describe the opportunities, or obstacles, to the improvement in quality and fitness-for-purpose of the Soil Moisture CDR. In this section we address the data availability from existing space-based observing systems; development of processing algorithms; methods for estimating uncertainties; scientific research needs; and opportunities for exploiting the new generation of Sentinel satellites.
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Table 4: Historical, current and envisaged radiometers suitable for soil moisture retrievals
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Satellite Sensor | Provider | Operation | Used freq. | Extra information |
SMMR | NASA | 1978-1987 | 18.7 GHz | Scanning Multichannel Microwave Radiometer data is used for the earliest periods of the C3S soil moisture record. |
|---|---|---|---|---|
SSMI, SSMIS | NASA, DoD | Since 1991 | 18.7 GHz | Onboard satellites from the Defence Meteorological Satellite Program (DMSP), however with the latest satellite DMSP-F19 failing and only F16, F17 and F18 available but functioning past their expected lifetime, continuation is currently at risk. Also 18.7 GHz is not the preferred frequency of operation for soil moisture retrievals. |
AMSRE | NASA | 2002-2011 | 6.9, 7.3, 10.7 GHz | Onboard the Aqua satellite. Predecessor mission to AMSR-2. |
WindSat | NRL, AFRL, DoD | Since 2003 | 6.6, 10.7 GHz | Onboard the Coriolis satellite of the US military. Access to raw data (brightness temperature data) is therefore restricted. C3S soil moisture uses measurements between 2007 and 2012 only. |
SMOS MIRAS | ESA | Since 2009 | 1.4 GHz | First L-band mission for soil moisture retrievals. Functioning properly but the design life was three years with a goal of five years. Part of the current CDR and ICDR. |
AMSR2 | JAXA | Since 2012 | 6.9, 7.3, 10.7 GHz | Based on the AMSR-E sensor on the AQUA mission. AMSR2 is a sensor on the GCOM-W1 satellite. Still functioning properly, follow up is expected in 2023 with the launch of GCOM-W2. After that, GCOM-W3 is still uncertain and under discussion. Soil moisture derived from AMSR2 is part of the current CDR and ICDR. |
GMI | NASA | Since 2014 | 10.7 GHz | Part of the Global Precipitation Mission (GPM) satellite. Coverage only between 65°N and 65°S. Lower frequencies than that of GMI are preferred for soil moisture retrievals. This sensor is included in the current version of ESA CCI SM (v07.1) and therefore planned for inclusion into C3S soil moisture as an operational sensor. |
SMAP | NASA | Since 2015 | 1.4 GHz | Latest L-band mission specifically designed for soil moisture retrievals. Although the radar failed shortly after launch, the radiometer is functioning well. A SMAP based soil moisture product is integrated since C3S soil moisture CDR/ICDR v3. In the first instance, the lifetime expectancy of the mission was 3 years. |
FengYun-3B FengYun-3C FengYun-3D | CAS CAS CAS | 2011-2021 Since 2013 Since 2019 | 10.7, 18.7 GHz 10.7, 18.7 GHz 10.7, 18.7 GHz | This series of meteorological satellites is launched by the Chinese space agency. Soil Moisture information is derived from X- and Ku-band measurements (only the more reliable X-band data is used in C3S soil moisture). New FengYun satellites are launched regularly but are at the moment only applicable for inclusion in the C3S soil moisture CDR, not the ICDR, due to NRT access restrictions. |
AMSR3 | JAXA | 2023-2030 | 6.9, 7.3, 10.25, 10.7 GHz | Follow-up mission to AMSR2 with similar capabilities. Measures additional X-band frequency compared to predecessor. |
FengYun-3F FengYun-3G FengYun-3H FengYun-3I | CAS CAS CAS CAS | 2022-2028 2023-2027 2023-2029 2026-2034 | 10.7, 18.7 GHz 10.7, 18.7 GHz 10.7, 18.7 GHz 10.7, 18.7 GHz | Currently planned Chinese Academy of Sciences (CAS) missions that can potentially be included in C3S soil moisture. |
MWI | EUMETSAT | 2024 | 18.7 GHz | Microwave Imager similar to SSMIS on board the MetOp-SG B satellites. First satellite to be launched in 2024. |
CIMR | ESA | ~2028-2033 (under development) | L-, C-, X-, Ka-, Ku-bands | “The Copernicus Imaging Microwave Radiometer (CIMR) mission is currently being developed as a High Priority Copernicus Mission. Its characteristics go beyond what previous microwave radiometers (e.g. AMSR series, SMAP and SMOS) provide, and therefore allow for entirely new approaches to the estimation of bio-geophysical products from brightness temperature observations. Most notably, CIMR channels […] are very well fit for the simultaneous retrieval of soil moisture and vegetation properties” (Piles et al., 2021) |
WCOM, FPIR and PMI | CAS | Undefined | 6.6 – 150 GHz | The payload of the Water Cycle Observation Mission (WCOM) satellite includes an L-S-C tri-frequency Full-polarized Interferometric synthetic aperture microwave radiometer (FPIR) and a Polarized Microwave radiometric Imager (PMI) covering 6.6 to 150 GHz. This wide range of simultaneous observations will provide a unique tool for research on soil moisture retrieval algorithms. The future accessibility of the data outside of China is however uncertain. |
Combined
Due to the wide range of satellites (both active and passive) currently available and in development for the upcoming decade, and the flexibility of the system as explained by the merging strategy in the C3S ATBD [RD.4] (Chapter “Merging strategy”), there is a negligible risk concerning the extension of the COMBINED product into the future. Furthermore, the quality that has been achieved is expected to be maintained or improved during the upcoming years through a set of initiatives described in the ATBD CCI [RD.7] such as the successful integration of FengYun, GPM and ASCAT-C, the inclusion of daytime observations and various other algorithmic updates.
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Table 5: Summary of C3S Soil Moisture requirements proposed by the consortium (shown in Table 2), specifications of the current C3S products, and whether the requirements are met.
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Requirement | Target | C3S Soil Moisture Products | Comment | Status |
Product Specification | ||||
Parameter of interest | Surface Soil Moisture (SSM) | Surface Soil Moisture | In addition to Surface Soil Moisture, GCOS provides requirements for Root-zone soil moisture, which is currently not included in C3S Soil Moisture. | Achieved |
|---|---|---|---|---|
Unit | Volumetric (m³/m³) | Volumetric [m³/m³] passive merged product, combined active +passive merged product; [% of saturation] active merged product | Conversion between volumetric units and % saturation is possible using soil porosity information. | Achieved |
Product aggregation | L2 single sensor and L3 merged products | L3 merged active, merged passive, and combined active + passive products | C3S Soil Moisture aims to provide merged products only. | Achieved |
Spatial resolution | 1-25 km | 0.25° (~25 km) | C3S Soil moisture is provided on a regular lat/lon grid. Pixel size in kilometres therefore varies with latitude. | Achieved |
Record length | >30-35 years | >43 years (1978/11 - present) | Not strictly required by CMUG. CMUG only states, that datasets of that length cover a period long enough for climate monitoring. | Achieved |
Revisit time | Daily | Daily | CMUG is highlighting the added value of sub-daily observations for special process studies, but also state that monthly observations are sufficient for some applications (e.g. trend monitoring). | Achieved |
Product accuracy | 0.04 m³/m³ | Variable (0.04-0.10 m³/m³), depending on land cover and climate (current assessment for various climates, land covers and texture classes based on in-situ data shows accuracy to be < 0.1 m³/m³) | Relative to (in situ) reference data. Based on estimates of unbiased root-mean-square-difference (see Gruber et al. (2020) and [RD.10]). | Approached |
Product stability | 0.01 m³/m³/y | 0.01 m³/m³/y | No formal guidelines exist yet on how to best validate the stability of merged soil moisture products over time. | Achieved, but no formal guidelines followed |
Quality flags | Should be provided with observations | Quality flags provided: Frozen soils, dense vegetation, no convergence in retrieval, physical bounds exceeded, weights of measurements below threshold, all datasets unreliable, barren ground | C3S soil moisture is not provided when quality flags are raised (flagging of deserts as an exception). Most, flags are therefore only informational. This is to simplify using the data. | Achieved |
Uncertainty | Daily estimate, per pixel | Daily estimate, per pixel | Uncertainty estimates are derived from triple collocation and gap filled using vegetation density information. | Achieved |
Format Specification | ||||
Product spatial coverage | Global | Global | Only land points, Antarctica excluded, permanent gaps for tropical forests. | Achieved |
Product update frequency | Monthly to annual | 10-20 days (ICDR), and 12 monthly (CDR) | 10-daily chunks are processed with a 10 day delay (ICDR). Monthly averages only computed for completed months. | Achieved |
Product format | Daily images, Monthly mean images | Daily images, dekadal (10-day) mean, monthly mean images | No threshold for minimum number of observations per dekad / month is set. | Achieved |
Grid definition | 0.25° | 0.25° | Regular sampled grid in latitude and longitude dimension. | Achieved |
Projection or reference system | Projection: Geographic lat/lon Reference system: WGS84 | Projection: Geographic lat/lon Reference system: WGS84 | Achieved | |
Data format | NetCDF | NetCDF 4 | Each time stamp (day/dakad/month) is provided as an individual file. | Achieved |
Data distribution system | FTP, WMS, WCF, WFS, OpenDAP | Data is distributed through the Climate Data Store (CDS) at https://cds.climate.copernicus.eu/ | ||
Programmatic access via CDS API possible (see https://cds.climate.copernicus.eu/ | ||||
Achieved | ||||
Metadata standards | CF, obs4mips | NetCDF Climate and Forecast (CF 1.7) Metadata Conventions; ISO 19115, obs4mips (distributed separately through ESGF) | Achieved | |
Quality standards | QA4ECV | QA4ECV and QA4SM standards and best practices implemented and verified. | Following best practice guidelines (Gruber et al. (2020) and [RD.10]). | Achieved |
Computation of accuracy (and stability) metrics requires the use of independent reference data at the moment. In situ measurements of soil moisture are harmonised and distributed by the International Soil Moisture Network4. However, it is known that accuracy assessment of satellite measurements using in situ data is affected by the uneven distribution of in situ data and the presence of representativeness errors, which inflate the differences between the satellite and ground measurements (Dorigo et al., 2021). It is also expected that the accuracy of soil moisture retrieval varies, depending on factors such as vegetation density or surface geometry (summarised as differences in land cover). While GCOS-200 targets are expressed as single values, the accuracy goals of C3S Soil Moisture are therefore evaluated separately for different land cover classes and are expected to vary between 0.04 and 0.1 m3/m3. Higher accuracy is expected on homogeneous surfaces (e.g. crop- and grasslands) while larger discrepancies are expected for densely vegetated and mountainous regions and urban areas.
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