Partitioning Forest Evaporation

Abstract

Evaporation, the transformation of liquid water to vapor, plays a crucial role in forested ecosystems by contributing significantly to the total evaporation through interception evaporation and transpiration. This process is critical in climate models used to forecast both immediate and long-term climatic changes. Yet, accurately measuring and partitioning evaporation in forests presents challenges due to the complex interplay of factors like canopy height and density, vegetation type, and soil characteristics. Properly segmenting total forest evaporation into its key components—interception evaporation, transpiration, and soil evaporation—is vital for enhancing hydrological and climate modeling. Simplifications in current global climate models, such as GLEAM or the EC-Earth3 used by the Royal Netherlands Meteorological Institute, often overlook the essential role of interception evaporation, focusing mainly on transpiration. This research examines a way to partition total evaporation into its fundamental segments. The study’s main objectives were to partition total evaporation into interception evaporation and transpiration, and further into canopy and forest floor interception evaporation. This was accomplished using
three methodologies: 1) Eddy Covariance (EC) systems positioned above the canopy to measure
total evaporation, with leaf wetness sensors distinguishing between wet and dry canopy states; 2) Analysis of leaf wetness data to quantify canopy interception evaporation; 3) The Bowen Ratio Energy Balance (BR-EB) method to assess overall evaporation and its split into canopy and forest floor components. Selected case days for analysis included scenarios following rain and dew events, with selection criteria based on minimum evaporation thresholds and weather conditions.
Results underscored the significant roles of transpiration and interception in total evaporation,
affected by environmental dynamics and sensor placement. Notably, sensors at higher canopy levels indicated faster drying and lower interception to transpiration ratios due to increased exposure to environmental factors. Despite employing diverse methodologies, the research did not uncover uniform patterns in evaporation partitioning, pointing to the intricate relationships between environmental conditions and canopy structure. The study pinpointed methodological constraints, such as in the assumptions related to leaf wetness sensor data, which might skew evaporation calculations. Future studies should integrate additional measuring techniques, like sap flow sensors and enhanced BR-EB methods, to improve data accuracy and deepen understanding of forest evaporation dynamics.