Why soil moisture metrology?
Water and soil are vital resources that are increasingly affected by climate change and environmental degradation. Soil moisture is a key component of the terrestrial water cycle and influences agriculture, forestry, groundwater recharge, weather and climate, and greenhouse-gas exchange.
Many soil moisture observation systems operate at different spatial scales, but their measurements need to be harmonised.
The overall objective of SoMMet was to develop novel metrological tools and establish a metrological foundation for soil moisture measurements across spatial scales ranging from decimetres to kilometres. This foundation supports the traceability and harmonisation of different soil moisture measurement methods.
Why the project was needed
Soil moisture is one of the Essential Climate Variables (ECVs) identified by the Global Climate Observing System (GCOS) of the World Meteorological Organization (WMO). It influences land-atmosphere interactions over weather and climate timescales. Soil moisture also strongly affects the storage and release of carbon in soils.
Soil is a cross-cutting theme of the European Green Deal (EGD) because water management, agriculture, forestry and biodiversity are closely interdependent. Soil quality and soil moisture are relevant to several EU policy frameworks, including the Common Agricultural Policy, the Farm to Fork Strategy, the EU Biodiversity Strategy for 2030 and the European Climate Law.
Point-scale soil moisture measurements are not necessarily representative of the larger spatial scales relevant to many applications. In situ sensors measure only a small volume of soil and can be influenced by local conditions. Systematic sampling and interpolation can improve spatial representativeness, but robust uncertainty evaluation and practical calibration guidance are also required.
Earth observation enables regular, large-area assessments of soil moisture at kilometre scales. Intermediate-scale methods such as cosmic-ray neutron sensing (CRNS) can bridge the gap between point-scale measurements and remote sensing. This requires harmonised hardware and data-processing tools, together with reliable calibration, validation and characterisation methods.
SoMMet addressed these needs by developing validation practices for CRNS and methods for integrating supporting soil data, moisture profiles and vegetation information. It also investigated how measurements across different scales could be combined in a metrologically traceable way to generate high-quality soil moisture information that is consistent in space and time.