QA4SM - An Online Validation Service for EO Soil Moisture Data Users and Producers

Abstract

Earth Observation (EO) from space allows us to create global Soil Moisture (SM) records to understand the effect of changes in global water availability on the environment. Many different EO satellites and retrieval models - for example ESA’s SMOS mission was designed to measure SM over land - collect large amounts of data every day. Therefore the need for rigorous, automated quality assessment procedures stands out among requirements by both the developers and users of EO SM data. While validation standards have been agreed upon in several scientific publications, their application often varies between independent studies; for instance in terms of the defined reference measurements, validation metrics and in the presentation of the results. Together with the complexity of processing large data volumes for global validation efforts, this calls for unified tools to perform this task and provide standardized quality assessments. The Quality Assurance for Soil Moisture (QA4SM, qa4sm.eu) platform bridges the gap between Analysis Ready Data (ARD) production and validation.

QA4SM is built on a powerful computing environment, providing a virtual space where users can freely perform validations of the SM data included in the database. Available data range from single satellite missions (e.g. SMOS, SMAP) or state-of-the-art multi-sensor products (e.g. the European Space Agency Climate Change Initiative, ESA CCI SM) to model or reanalysis data sets (e.g ERA5). In addition, insitu SM measurements provided by the International Soil Moisture Network (ISMN, ismn.earth) are included and regularly updated to allow global comparisons against up to 5 decades of continuous ground measurements. Every validation is personalized by the user through a simple interface to apply spatial and temporal constraints or make use of advanced validation techniques, such as random error characterization of a data set through Triple Collocation Analysis (TCA). All methods included in the core algorithm of QA4SM are based on the best practices and requirements agreed upon by the Global Climate Observing System and the Committee on Earth Observation Satellites. The outcome of each validation, including graphical outputs and validation metric scores, can be stored, further processed or included in scientific studies and reports thanks to the use of traceable Digital Object Identifiers (DOIs).

Recently, QA4SM has seen an involvement in the ESA commissioned Fiducial Reference Measurements for Soil Moisture (FRM4SM) project. The goal is to verify and adapt newly designed FRM protocols and make use of FRM compliant reference data to better characterize errors in satellite SM measurements with QA4SM. Platform developments include product intercomparisons, deeper-level analysis of the results (e.g. using land cover and climate classification) and more precise uncertainty estimation. However, in the interest of meeting the AR objectives of operability and immediacy, the most anticipated capability will consist of the possibility for users to validate their own data sets. While ARD become a new normal in EO, QA4SM sets up to be the reference AR tool for validation across the users and producers community.

Here, we demonstrate how the QA4SM online validation platform operates, what new data and features were recently added and give an outlook on planned developments.

QA4SM was created with support of the Austrian Space Application Program. From 2021 on the service development is also supported by the European Space Agency under the FRM4SM project.