Regional patterns and drivers of water flows along environmental, functional and stand structure gradients in Spanish forests

Abstract. The study of the water cycle in the forest at large scales, such as countries, is challenging due to the difficulty of correctly estimating forest water flows. Hydrological models can be coupled to extensive forest data sources, such as national forest inventories, to estimate the water flow of forests over large extents, but so far the studies conducted have not analysed in detail the role of stand structure variables or the functional traits of the forest on predicted blue/green water flows. In this study, we modelled the water balance of Spanish forests using stand structure and species data from forest inventories to understand the effects of climate, stand structure, and functional groups on blue water flows. We conducted model simulations across a gradient of different climatic biomes and predominant functional trait species. We calculated both the blue water (surface runoff and drainage to groundwater) and green water (evapotranspiration). Our analysis focused on relative blue water, the ratio between blue water and total precipitation. Climatic, stand structure, and topographic variables were used to interpret the determinants of variation of relative blue water. The results showed that higher relative blue water is mainly concentrated in the wetter regions of Spain and during the autumn-winter season. Leaf Area Index (LAI) of the forest stand is the most important predictor of relative blue water, exhibiting a negative effect until it reaches a plateau at higher levels. Deciduous forests showed a greater relative blue water content than evergreen functional groups, primarily due to leaf fall during the autumn-winter season. This study highlights the significant importance in a Mediterranean region of seasonal distribution in water flow and how seasonal LAI can act as a crucial filter for excess water.