Recent water cycle changes in Spanish forests are driven by stand structure more than climatic changes

The water cycle in forests of many regions is being impacted by climatic changes, often including a decrease in precipitation and an increase in temperature, leading to an increase in green water (evapotranspiration) and a decrease in blue water (runoff + drainage). Additionally, forest expansion and development are prevailing processes in many rural areas due to the abandonment of traditional land uses. Stand leaf area growth further amplifies green water and reduces blue water availability. However, the interaction between climate change and stand structure changes is not well understood at large scales. We modeled Spanish forest water cycle using national inventories (1990–2020), analyzing climate and forest structure trends at plot level. Using three inventory surveys, we assessed green and blue water changes across forest types and compared managed vs. unmanaged forests. Results show green water increased and blue water decreased over time. Leaf area index (LAI) growth (trees and shrubs) had a stronger effect on green water than climate change. These factors, along with recent precipitation declines (2010–2020), also significantly reduced blue water. Basal area reduction improved blue water yield, but only in the short- mid-term, as stand LAI tended to recover over time. This study demonstrates that changes in stand structure can be as important, if not more so, than climatic changes in influencing the water cycle at the regional level. Moreover, our results support the idea that effective basal area reduction can enhance blue water production, but only if basal reduction practices are consistently maintained.