QUADICA v2: Expanded large-sample water quality data for Germany offer new opportunities for cross-catchment analyses

Large-sample hydrology aims to identify general spatial and temporal patterns and their exceptions across diverse catchments and to infer underlying processes by linking observed responses to hydroclimatic, biogeochemical, and anthropogenic drivers. While large-sample datasets for water quantity are now well established, similarly comprehensive resources for water quality have remained limited.

QUADICA (water QUAlity, DIscharge and Catchment Attributes) addresses this gap for Germany. Here, we present QUADICA v2, an extended large-sample water quality dataset covering 1386 catchments. The update expands temporal coverage to 2020, adds ecologically relevant water quality variables (including water temperature, oxygen, and chlorophyll a), and introduces long-term time series of nitrogen and phosphorus inputs from both diffuse and point sources. By linking QUADICA with CAMELS-DE, the number of stations with concurrent water quality and discharge data is effectively doubled (now 637 stations).

Beyond extending data availability, QUADICA v2 enables new analyses of driver–response relationships, network-topological patterns, and ecological impact studies across gradients of climate, land use, and pollution pressure. The dataset supports comparative large-sample studies, data-driven and machine-learning approaches, and the calibration and evaluation of process-based water quality models, providing a ground for understanding and management of freshwater systems.