Opportunities and challenges in using catchment-scale storage estimates from cosmic ray neutron sensors for rainfall-runoff modelling,

Abstract. Climate change increases the occurrence and severity of droughts due to increasing temperatures, altered circulation patterns, and reduced snow occurrence. While Europe has suffered from drought events in the last decade unlike ever seen since the beginning of weather recordings, harmonized long-term datasets across the continent are needed to monitor change and support predictions. Here we present soil moisture data from 66 cosmic-ray neutron sensors (CRNSs) in Europe (COSMOS-Europe for short) covering recent drought events. The CRNS sites are distributed across Europe and cover all major land use types and climate zones in Europe. The raw neutron count data from the CRNS stations were provided by 24 research institutions and processed using state-of-the-art methods. The harmonized processing included correction of the raw neutron counts and a harmonized methodology for the conversion into soil moisture based on available in situ information. In addition, the uncertainty estimate is provided with the dataset, information that is particularly useful for remote sensing and modeling applications. This paper presents the current spatiotemporal coverage of CRNS stations in Europe and describes the protocols for data processing from raw measurements to consistent soil moisture products. The data of the presented COSMOS-Europe network open up a manifold of potential applications for environmental research, such as remote sensing data validation, trend analysis, or model assimilation. The dataset could be of particular importance for the analysis of extreme climatic events at the continental scale. Due its timely relevance in the scope of climate change in the recent years, we demonstrate this potential application with a brief analysis on the spatiotemporal soil moisture variability. The dataset, entitled “Dataset of COSMOS-Europe: A European network of Cosmic-Ray Neutron Soil Moisture Sensors”, is shared via Forschungszentrum Jülich: https://doi.org/10.34731/x9s3-kr48 (Bogena and Ney, 2021).

Tracer studies have been key to unravelling catchment hydrological processes, yet most insights have been gained in environments with relatively low human impact.We investigated the spatial variability of stream isotopes and water ages to infer dominant flow paths in a~10-km 2 nested catchment in a disturbed, predominantly agricultural environment in Scotland. We collected long-term (>5 years) stable isotope data of precipitation, artificial drainage, and four streams with varying soil and land use types in their catchment areas. Using a gamma model, Mean Transit Times (MTTs) were then estimated in order to understand the spatial variability of controls on water ages. Despite contrasting catchment characteristics, we found that MTTs in the streams were generally very similar and short (<1 year). MTTs of water in artificial drains were even shorter, ranging between 1 to 10 months for a typical field drain and <0.5 to 1 month for a country road drain. At the catchment scale, lack of heterogeneity in the response could be explained by the extensive artificial surface and subsurface drainage, "short-circuiting" younger water to the streams during storms. Under such conditions, additional intense disturbance associated with highway construction during the study period had no major effect on the stream isotope dynamics. Supplementary short-term (~14 months) sampling of mobile soil water in dominant soil-land use units also revealed that agricultural practices (ploughing of poorly draining soils and soil compaction due to grazing on freely draining soils) resulted in subtle MTT variations in soil water in the upper profile. Overall, the isotope dynamics and inferred MTTs suggest that the evolution of stream water ages in such a complex human-influenced environment are largely related to near-surface soil processes and the dominant soil management practices. This has direct implications for understanding and managing flood risk and contaminant transport in such environments. K E Y W O R D Sagricultural catchment, Anthropocene hydrology, artificial drainage, gamma model, land use management, soil water isotopes, transit times, water isotopes