Forest water balance model
- Silva Joint Research Unit
Precipitation interception refers to water, in liquid or solid form, that is intercepted by tree crowns, both leaves and wood bodies, before it falls to reach the soil. During this process of precipitation, water will enter into the ecosystem by several paths :
Note : We have chosen not to address snow interception, a phenomenon which is to date not fully understood and supported by surprisingly few published studies, nor do we address fog interception (production of liquid that will fall from the canopy to reach the soil).
Interception cannot be directly measured but rather is calculated as being the difference between incident
rainfall and throughfall, by using the following formula:
In = Pi – Ps - Pt, in which In represents the interception, Pi the incident rainfall, Ps the rainfall which reaches the soil through the canopy and Pt, the stemflow.
Above : A system for collecting rain flowing down the trunks (photo UR BEF, INRA-Nancy).
Opposite : Measuring rain that reaches the ground by using a network of rain gauges for direct reading.
Rainfall interception is interpreted by its physical aspects and depends on the intensity of the rainfall, on the leaf development (leaf area index, see the page Phenology and Leaf Area index) and on climatic conditions. During and after a rainy period, the evaporation rate will depend on the energy arriving at the crowns that can be measured by potential evapotranspiration (PET, see the page Meteorology).
With the start of rainfall, the canopy begins to accumulate water on its interception surfaces. When these surfaces start to saturate, water then flows downwards. This defines the saturation capacity of a canopy, which is the maximum amount of water that can be stored. Typically this capacity is between 1 and 2 mm: it is higher for coniferous with high leaf area index and lower in broad-leaved stands.
Rainfall interception is spatially a highly variable phenomenon and is linked to heterogeneity in the canopy structure. Measurements show that, just following rainfall, some rain gauges placed below the canopy capture very little water, while others receive more than the incidental rainfall! This phenomenon is known as the funnel which is rainfall that has been locally concentrated by preferential flows in the crowns. Aussenac (1970) also showed that throughfall depends on the distance from the canopy to the trunk.
Rainfall interception is highly variable and is dependent on species, management techniques, climate and season variability.
However, we can provide a few ranges. In temperate climates, rainfall interception varies from 15 to 40% of the incident rainfall.
Overall, conifers intercept more rainfall (In representing between 30 and 40%) than deciduous trees (In representing between 15 and 30%)
for two main reasons: conifer stands generally show a higher leaf area index and, with the exception of larch, they bear leaves throughout the year.
Thus, in the same climate conditions, evergreen stands are often characterized by lower water availability in the soil than deciduous forests.
Additionally, stemflow (Pt) is highly dependent on tree species. Two extreme examples are beech, which has smooth bark allowing Pt to reach or exceed 10% of the incident rainfall, as opposed to species with coarse bark, such as oak, where the Pt represents only about 1 to 2% of rainfall. When stemflow is relatively low, this water flows remains in the soil at the base of the tree.
Mean values of rainfall interception as % of incident rainfall measured in RENECOFOR stands in the French forest service observation network.
Aussenac G et Boulangeat C (1980) Interception des précipitations et évapotranspiration réelle dans des peuplements de feuillus (Fagus sylvatica L.) et de résineux (Pseudotsuga menziesii (Mirb) Franco). Annales des sciences forestières, 37 (2), 91-107.
Ulrich E, Lelong N, Lanier M, Schneider (1995) Interception des pluies en forêt : facteurs déterminants. Interprétation des mesures réalisées dans le sous-réseau CATAENAT de RENECOFOR. ONF, Bulletin technique n°30.
Llorens P (1997) Rainfall interception by a Pinus sylvestris forest patch overgrown in a Mediterranean mountainous abandoned area .2. Assessment of the applicability of Gash's analytical model. Journal of Hydrology 199: 3-4.
Loustau D, Berbigier P, Granier A, Elhadjmoussa F (1992) Interception loss, throughfall and stemflow in a maritime pine stand. 1.Variability of throughfall and stemflow beneath the pine canopy. The HartX-Synthesis: an experimental approach to water and carbon exchange of a Scots pine plantation. Journal of Hydrology 138: 449-467.