Woodward water-balance model

Woodward uses a water- and energy-balance model to predict vegetation physiognomy and for various locations around the world.  Woodward's model estimates available water (AVW) and actual evapotranspiration (AE) to estimate the maximum LAI which can be supported on a site during a typical year (Woodward 1987); other climate information is used to predict the physiognomy of the vegetation which should be present.


The model uses mean monthly temperature (MMT), Absolute minimum temperature (MMINT), monthly temperature range (MTR), monthly precipitation (MPR), monthly solar radiation (MRAD) and monthly relative humidity (MRH) -- to derive the predictions of AVW, evapotranspiration (ET) and physiognomy.


The water balance can be summed up as follows:



The net radiation balance can be summed up like this:



Many of the above factors area a function of leaf area index (LAI), such as light attenuation through the canopy, which in turn affects photosynthesis and transpiration rates.  Interception (and conversely throughfall) are a function of leaf area index.  Evapotranspiration rate is also a function of LAI.


Woodward's model takes the climate information mentioned above and predicts AE (he assumes AE equals potential evapotranspiration) and AVW for odd-numbered values of LAI ranging from 1 to 9.  As long as AVW exceeds AE (on an annual basis), the value of LAI can be supported.  If AE exceeds AVW, however, the value of LAI cannot be supported.


Annual absolute minimum temperature (AMINT) is the first variable to consider for the prediction of physiognomy, followed by AVW and the distribution of AVW throughout the year.  A summary of expected physiognomy based on AMINT in given in the table below:


Temperature

Phenomenon

Physiognomy

> 15C

Temperature not limiting

Broad-leaved evergreen when rainfall adequate

-1 to 15C

Chilling

Broad-leaved evergreen when rainfall adequate

-15 to 0C

Freezing and supercooling

Broad-leaved evergreen

-40 to -15C

Freezing and supercooling

Broad-leaved evergreen

< -40C

Freezing and supercooling

Evergreen and deciduous needle-leaved (coniferous)


AVW influences physiognomy by its influence on maximum LAI which can be supported on a site.  Typical values of LAI for major vegetation units (biomes) are given in the following table (data from Asner 2003).


Vegetation type

LAI

Crops

0.2-8.7

Desert

0.6-2.8

Forest, boreal deciduous broadleaf

0.6-4.0

Forest, boreal evergreen needleleaf

0.5-6.2

Forest, borea/temperate deciduous needleleaf

0.5-8.5

Forest, temperate deciduous broadleaf

1.1-8.8

Forest, temperate evergreen needleleaf

0.8-11.6

Forest, temperate evergreen broadleaf

0.01-15.0

Forest, tropical deciduous broadleaf

0.6-8.9

Forest, tropical evergreen broadleaf

1.5-8.0

Grasslands

0.3-5.0

Plantations

1.6-18.0

Shrublands

0.4-4.5

Tundra

0.2-5.3

Wetlands

2.5-8.4


Finally, the seasonal distribution of AVW must be considered.  There are many drought-deciduous forests which support higher values of LAI than Woodward's model would predict because of abundant rainfall during the growing season.  You must interpret these patterns carefully (the necessary information IS provided in the output).


Woodward's model also predicts extremely high LAI for high-latitude regions which in fact have very low LAI.  This is a result of the fact that AE is quite low, which leads to a water surplus in Woodward's model, but growing seasons are very short and there is not enough energy for much plant growth.  You can control for this by evaluating the temperature criteria listed in Table 1 before carrying out any further analyses.


Using AMINT and annual precipitation (APR) to predict vegetation type, Woodward's water-balance yields the following results (from Woodward and Williams 1987):


AMINT range

Annual precipitation

Vegetation type

≥ 0C

≥ 600 mm

Evergreen or deciduous broad-leaved forest

≥ 0C

≥ 600 mm

Deciduous broad-leaved forest

≥ 0C

< 600 mm

Grassland/shrub

≥ -15C, < 0C

≥ 600 mm

Frost-resistant, evergreen broad-leaved forest

≥ -15C, < 0C

< 600 mm

Grassland/shrub

≥ -40C, < -15C

≥ 600 mm

Deciduous broad-leaved forest

≥ -40C, < -15C

< 600 mm

Grassland/shrub

< -40C

≥ 400 mm

Evergreen or deciduous needle-leaved (conifer) forest

< -40C

< 400 mm

Tundra (grassland/shrub)


Also, if AMINT is < -40C, these additional criteria apply:



Likewise, if AMINT ≥ 0C, these additional criteria apply:

Copyright 2003-2011, David M. Lawrence

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