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The leaf area index is a non-dimensional number representing the square metres of leaf area per square metre of the earth's surface.  It is as it determines the degree of evapotranspiration and, for example, how much rainwater will be intercepted by the vegetative canopy, rather than falling to the ground.

In IFS the leaf area index varies only with climatology, month by month.  Separation of HTESSEL tiles into high and low vegetation gives a more accurate seasonal variability of vegetation.  Anomalous weather (e.g. winds stripping trees of leaf), widespread fire damage (e.g. major change of albedo), etc have no effect.

The Leaf Area Index for each grid box is computed as follows:

Total leaf area index = (Leaf area index of high vegetation) x (cover in grid box of high vegetation) +  (Leaf area index of low vegetation) x (cover in grid box of low vegetation).

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