Insights into the canopy-rainfall interactions: new experiences from a long-term Neotropical forest monitoring

Abstract

One of the most important processes in the hydrology of the critical zone is the rainfall interception by forest canopies. The canopy-rainfall interactions drive the water and nutrient cycles by redistributing rainfall in both time and space. This defines the fate of many hydrological processes, such as soil water dynamics, evapotranspiration, streamflow, spatiotemporal pattern of nutrients, and groundwater recharge. Although the canopy-rainfall interactions are well-known in many forests worldwide, there is still a knowledge gap in the effects of extreme weather (e.g., droughts) on these interactions. In this regard, the present study aims to improve the understanding regarding rainfall partitioning in a Neotropical forest during a prolonged drought. Rainfall partitioning starts with the canopy intercepting the rainfall and splitting it into stemflow and throughfall. Throughfall and stemflow is the amount of water that reaches the floor, known as net rainfall. A parcel of the intercepted water returns to the atmosphere by evaporation during and after the event. The canopy evaporation and throughfall are the most significant part of the rainfall partitioning, summing up to 99.5% in some tropical forests. Therefore, they are the subject of the present study. Physical models mimic reality and are key tools to advance the knowledge of complex physical processes such as rainfall interception. The Liu and Gash models have presented adequate performances to model the rainfall interception in different climates and forests. However, they had never been applied to drought conditions. The Liu model overcame the Gash model in the Neotropical forest because it better represents the stratified canopy. In non-drought periods, solar radiation and the energy stored in biomass and the air inside the forest are responsible for canopy evaporation. Besides the abovementioned energy sources, the energy advection from surrounding areas plays a more important role and increases canopy evaporation during droughts. Another important consideration is the spatial distribution of throughfall and how it behaves during droughts. The time stability index was considered to assess the behavior of the spatial variability of throughfall over time to highlight the likely influence of forest and weather dynamics on it. Misinterpretation of time stability of throughfall was observed in prior studies because the changes in forest structure and weather patterns had not been considered. Biomass, the dominance of some species, and tree occupation are forest characteristics driving the spatial distribution and time stability of throughfall. These structures change due to ecological succession or regenerating from a disturbance (e.g., droughts), which modify the spatial distribution of throughfall. Moreover, maximum rainfall intensities are different in drought periods, changing the canopy’s saturation/unsaturation processes, and therefore the time stability. In this sense, droughts modify the canopy-rainfall interactions by enhancing canopy evaporation and changing the spatial distribution of throughfall over time.