Determining Factors
Precipitation
The primary meteorological characteristics of a rainfall event that influence stemflow are:
- Rainfall continuity – the more frequent and extended are the gaps during the event where no rainfall occurs, the higher the likelihood that potential stemflow volume is lost to evapotranspiration; this is also governed by air temperature, relative humidity and most significantly, wind speed
- Rainfall intensity – the amount of total stemflow is diminished when the amount of rain in a given period surpasses the capacity of the flow paths
- Rain angle – stemflow generally starts earlier when rainfall is more horizontal; this is more of a determinant in an open forest with a lesser degree of crown closure
Species
The species of the tree affects the amount of timing and stemflow. The particular morphological characteristics that are key factors are:
- Crown size – stemflow potential is greater as crown size relative to the diameter at breast height increases
- Leaf shape/orientation – leaves which are concave and elevated horizontally above the petiole are able to contribute to stemflow
- Branch angle – stemflow potential heightens as the angle of the branches and twigs increases
- Flow path obstructions – abnormalities on the flow path, such as detached pieces of bark or scars, on the underside of the branch can divert water from stemflow and become a component in throughfall
- Bark – stemflow is affected by the degree of absorptive ability and smoothness of the bark alongside the branch and stem
Stand Characteristics
In addition to the effects of individual tree species, the overall structure of the forest stand also influences the amount of stemflow that will ultimately occur, these factors are:
- Species composition – the total stemflow for the stand is determined by the contributions of individuals and their species-specific traits
- Stand density – morphological characteristics such as branch angle and thickness are largely determined by the amount of density of competing trees in the stand
- Canopy structure – individuals located in the understory in a stand with multiple vertically-stratified stories will have a lessened amount of total stemflow due to the interception of dominant and codominant individuals
Other
- Seasonality – in the case of deciduous or mixed forests, stemflow rates are slightly higher in the dormant season when no leaves are present and evapotranspiration is reduced; this effect becomes more pronounced as the stem diameter increases
- Diurnality – variations in branch weight influence the amount of stemflow; branches are heavier in the morning (with dew) and lighter in the afternoon
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