Daily thermopluviometric grids for the Basque Country since 1970: Steps toward a spatial dataset for climate monitoring.

In this study, we present a new gridded dataset of minimum and maximum air temperature (EuskalKlim-Tgrid00) and total precipitation (EuskalKlim-Pgrid00) for the Basque Country with a resolution of 1 km, going back to 1970 with a daily temporal resolution.

The methodology integrates data from both long-term manual weather stations and a dense network of automatic weather stations belonging to different governmental organisations. The time series has undergone extensive quality control and homogenisation.

The complex topography of a mountainous region such as the Basque Country has a strong influence on atmospheric processes. These relationships, which may be regionalised and non-linear (i.e., thermal inversion layers), can be captured by data-driven modelling using geomorphometric indices derived from Digital Elevation Models (DEM) and other explanatory covariates. In this project, Ensemble Machine Learning (EML) was used to fit models and generate spatial predictions in the same way that multivariate geostatistical methods are used to generate interpolations. This task takes the form of a regression problem where the ground-based measurements are the dependent variable and static covariates (i.e., geomorphometric indices) together with dynamic covariates (ERA5 land reanalysis, CMSAF SIS) are the predictor variables. We used Recursive Feature Elimination (RFE) to reduce model complexity and to identify the most important variables. For model evaluation, we used 5-fold cross-validation and different metrics to evaluate the ML models.

To demonstrate the convincing performance of our spatial dataset, we also compared it with the gridded thermopluviometric dataset developed in the framework of the Basque Climate Change Strategy KLIMA2050, with a spatial resolution of 1 km and covering the period 1971-2016, which can be considered as independent dataset using different modelling methods.

The new series are already being used, for example in the first report on the state of the climate in the Basque Country (Euskalmet-IHOBE, 2025). These indicate that the Basque Country has been warming at a rate of about 0.3°C per decade since 1970. The significant long-term warming trend means that most years are now warmer than almost all those observed during the 20th century. The decade 2014-2023 was 1.1°C warmer than the period 1971-2000. However, there are no clear trends in annual and seasonal precipitation, nor in derived climate indices such as wet and dry days and extreme precipitation events.