Delineating Iberian pyroregions using agglomerative clustering of fire regime descriptors

Wildfire activity in the Iberian Peninsula (581,353 km²) is highly heterogeneous due to strong gradients in climate, topography, vegetation, disturbances, land use, and management. This spatial variability challenges fire modeling, risk assessment, and fuel reduction strategies across contrasting regions. Previous efforts to map fire regimes succesfully used clustering of historical or remote-sensed fire data. However, the resulting zones were often large and spatially fragmented, rendering them challenging to integrate into landscape scale stochastic wildfire modeling.To address this, we delineated pyroregions, defined as spatial units with generally homogeneous fire regime conditions, to support subsequent fire weather characterization and the definition of modeling domains for stochastic wildfire simulations. Our objective was to generate contiguous spatial units that exhibit both similar historical fire incidence and consistent fire-weather and topographic characteristics. To achieve this, we populated subwatersheds (obtained from HydroBASINS; n = 4,409; mean area 13,391 ha) with contemporary fire regime descriptors derived from burned area and ignition records –sourced from national (AGIF for Portugal and EGIF for Spain) and European (EFFIS) databases– complemented with fuel moisture (Camprubí et al., 2022; 10.5281/zenodo.6784663) and weather data (ERA5-Land reanalysis data). Descriptors included annual ignition density, annual and summer burned area, wind direction distributions, and fuel moisture content for live woody and fine fuels in the period 2001-2024. Pyroregions were obtained via spatially constrained agglomerative clustering with Ward linkage, enforcing contiguity using a Queen connectivity matrix, which ensured that merges occurred only between adjacent subwatersheds. Following a two-step aggregation scheme, we first delineated 16 broad pyroregions representing major wildfire-regime zones and then partitioned them into 78 similarly sized subareas (pyromes; mean area 7,570 km²) for modeling applications. Finally, boundaries were refined to reduce sharp transitions associated with subwatershed geometry and to produce smoother contours. The resulting map captured transboundary similarities and contrasts in fire regimes and revealed clear structure, including altitudinal gradients and a marked Atlantic to Mediterranean contrast, with large contiguous regions over the inner mesetas and major depressions, and a near continuous coastal belt.