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Validation

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The validation of the final merged data set is an important mechanism within the production process of the ECV SM products. The validation is conducted by project partners who are independent from the development of the ECV SM data set. Results of the validation and intercomparison studies for each of the released versions of the product are described in detail in the corresponding Product Validation and Intercomparison Report (PVIR).

Correlation of the gridded soil moisture products as compared to in-situ station observations for the full year for the US. Subdivided in consecutive 4-year periods (1997-2000, 2001-2004, 2005-2008, 2009-2012, and 2013-2016) as well as for the longest period data is available (1997-20.., end date would e.g. be 2010 for CCI v0.1, but is 2016 for e.g. CCI v4.1). Note that in this case, data is not masked for common data availability. Whiskers show the median and the IQR. Above indicated the number of stations correlations were calculated for that comply to the following criteria: at least 10% of the timeseries is not NA, p-value < 0.05, and the calculated correlation is positive. And below indicated the number of years considered.

 

Independent validation by project partners

Eidgenössische Technische Hochschule Zürich (ETH) provides an independent validation service to the ESA CCI SM project. All product releases are evaluated over 4 regions (North America, Sub-Saharan Africa, Europe and Australia) using in-situ observations from the International Soil Moisture Network (ISMN), ERA-Interim/Land and ERA5-land soil moisture reanalysis at 0.25o resolution. This evaluation uses layers 1 (0-7cm) and 2 (7-28cm) of ERA-land products to compare against the CCI SM product (which represents soil moisture in the top few cms of soil).

Results of the study show no areas of either very strong positive or very strong negative correlation. The highest and most consistent observations occur over Australia, where in-situ observations were located in the same climatic region as one another. The ESA CCI SM products correlate higher with in-situ data from 0 to 5 cm depth than those from 10 cm depth. The correlations to ERA-land shows less clear distinction in SM at different depths. Over the United States the correlation to in-situ data is consistently higher over areas of grassland than over areas of forest cover. Again, the distinction in vegetation types is less clear in the correlation results with the ERA-land analysis.

Correlation between in-situ soil moisture and CCI SM for versions v0.1, v02.2, v03.3, and v04.7, as well as ERA5-Land soil moisture layer 1 (ERA5-Land l1, 0-7 cm), for absolute soil moisture (left) and the anomalies (right). 

 

QA4SM

The Quality Assurance for Soil Moisture project (QA4SM) provides an operational online validation service for soil moisture products.

QA4SM web service is developed by TU Wien GEO and AWST GmbH with funding from the Austrian Space Applications Programme (FFG). QA4SM aims to allow its users to validate soil moisture datasets in a traceable, transparent, reproducible and standardised fashion and to download the validation results for further use, for example in publications. For future versions of the ESA CCI SM product, QA4SM will be used.

Correlation (Pearson's) between ESA CCI SM COMBINED v04.7 product and GLDAS v2.1 top layer (0-7cm) soil moisture for the period 2000-01-01 to 2019-12-31.