Throughfall spatial variability in a neotropical forest: Have we correctly accounted for time stability?

Abstract

The complexity of rainfall-canopy interactions is likely to increase under extreme weather events. Extreme conditions may affect forest structure and change the throughfall (TF) spatial distribution over time. Mistakenly claiming the time stability of TF spatial variability can lead to misunderstanding the rainfall-canopy interactions, obscuring the relationships between weather, forests, and hydrology. Herein we rely on an unprecedented six-hydrological-year event-based dataset from the Brazilian Atlantic Forest, spanning from 2013 to 2019 (summing up 427 rainfall events) to assess: (i) the effects of a prolonged drought period on the TF time stability; and (ii) the importance of forest structure for the TF time stability. The mean relative difference and time stability index were applied for different period lengths and related to forest structure, gross rainfall amount (GR), maximum rainfall intensity, and drought occurrence to assess whether they affect the TF spatial variability. The results indicated that the throughfall spatial variability is less time stable during droughts due to a combination of more light intensity events with some extreme intensities (>20 mm h−1), which were not observed in the non-drought period. Changes in forest structure became evident after drought conditions and could not be tracked in studies with short monitoring periods. Since throughfall spatial distribution is driven by forest structure (e.g., tree density, species dominance, and biomass), such dynamics affected the time stability of the spatial variability. The time instability was even greater for GR < 10 mm because of the greater rainfall-canopy interactions prior to canopy saturation. Therefore, not accounting for forest dynamics and drought effects on the TF spatial variability lead to misinterpreting time stability.