Soil moisture is one of the Essential Climate Variables (ECVs) as defined by the Global Climate Observing System (GCOS) of the World Meteorological Organisation (WMO). Soil moisture influences the land atmosphere interactions on both weather and climate timescales. Long-term carbon storage and release in soil is strongly influenced by soil moisture — only a healthy and adequately moist soil can act as carbon sink in the strategies for greenhouse gases (GHG) reduction and adaptation to climate change impacts.

Soils are a cross-cutting theme within the European Green Deal (EGD), as the sectors of water management, agriculture, forestry, and biodiversity are inherently interdependent. Soil quality and soil moisture play a key role in the future EGD policies, namely in the future Common Agricultural Policies unified under the Farm to Fork Strategy, policies for environmental protection (Biodiversity Strategy for 2030) and the climate change action (The European Climate Law).

Soil moisture measurements at point scales, performed by practical users in agriculture and hydrology (e.g., farmers, agronomists) or by scientists dealing with soil moisture as an ECV, are not immediately representative of the soil moisture at the larger scales that are relevant for practical applications. Point scale measurements use physical tactile sensors which are invasive and subject to local issues. To overcome this, complex sampling designs and interpolation methods can be implemented, however uncertainties need to be improved and practical calibration guidelines developed.

Remote observation of the Earth can be used for real-time and continuous assessment of soil moisture on the kilometre-scale, however, intermediate scale soil moisture methods such as cosmic-ray neutron sensing (CRNS) are needed so that gap from point scale to remote sensing can be bridged. The necessary hardware and data processing tools need to be harmonised and reliable calibration, validation and characterisation methods developed.

There is also a need to set out appropriate validation practices for the deep-sensing CRNS method, including fusion of supporting soil data, moisture profiles, and vegetation information, and to harmonise the different methods across scales, in a holistic and yet metrologically traceable approach. Furthermore, there is a need for 'the next logical step', i.e., for performing the data fusion of the multi-scale soil moisture measurements to generate high-quality, temporally, and spatially consistent soil moisture information, useful for land surface sciences and applications, such as climate observations, weather forecasting, hydrology, and agriculture.

The overall aim of this project is to develop novel and traceable methods and establish a metrological infrastructure for soil moisture measurements covering lateral scales ranging from the decimetre to kilometre. 

The specific objectives of the project are:

1. To develop metrological framework, including primary and secondary transfer standards, to ensure SI-traceable point-scale soil moisture measurements with uncertainties of 5 % under laboratory conditions. To develop metrological framework for validation of existing cosmic-ray neutron sensing (CRNS) devices, currently available in the market, under laboratory conditions.
2. To develop new validation practices for cosmic-ray neutron sensing (CRNS) methodology for use in outdoor conditions. This includes the application and validation of neutron transport models used to interpret CRNS detector signals specific to the soil moisture measurand, and the standardisation of the CRNS on-field calibration procedure for soil moisture assessment on lateral scales ranging from 10² m to 10³ m and to depths of up to 1 metre.
3. To investigate the constraints and accuracy of soil moisture measurement methodologies using intercomparison campaigns on local and remote sensing. In addition, to develop procedures, summarized in good practice guides, to overcome (i) temporal and spatial differences regarding the sensing domains of soil moisture measurement methods and (ii) the influence of other state variables such as air humidity and soil temperature affecting the measurements.
4. To develop a multi-scale metrological system and metrologically traceable methods for soil moisture monitoring, covering lateral scales ranging from 10^-1 m to 10³ m and to depths of up to 1 metre and temporal scales ranging from hours to days, to assess the soil moisture with traceable relative uncertainty of 20 % or better. This includes the development of a cross-disciplinary harmonisation system on the medium sub-kilometre-scale and the establishment of (i) metrological traceability of soil moisture measurements using point-scale sensors (from Objective 1) and satellite measurement techniques and (ii) fit for purpose modelling. In addition, to develop techniques to support the harmonisation of soil moisture assessment.
5. To cooperate with user communities to define design criteria for emerging and future hydrological and meteorological/climatological soil moisture networks using the combination of point-, intermediate- and large-scale methods. To cooperate with the European Metrology Network for Climate and Ocean Observation (EMN COO) and relevant international organisations (e.g., WMO) to facilitate the dissemination of the project outputs.

The time frame of the EPM project 21GRD08 SoMMet is October 2022 – September 2025 (duration of 36 months).

Further details on the metrology community and the granting authority can be found on the following websites:

• EURAMET e.V.
• European Partnership on Metrology (EPM)
• EPM Participant portal
• EURAMET website dedicated to 21GRD08 SoMMet