# Actual ET and productivity

I was reading a post over at Dynamic Ecology presenting an appreciation of Michael Rosenzwieg, a Professor of Ecology and Evolutionary Biology at the University of Arizona. What caught my eye was his most cited paper which is on the correlation between AET (actual evapotranspiration and productivity).  Here is the abstract:

Actual evapotranspiration (AET) is shown to be a highly significant predictor of the net annual above-ground productivity in mature terrestrial plant communities. Communities included ranged from deserts and tundra to tropical forests. It is hypothesized that the relationship of AET to productivity is due to the fact that AET measures the simultaneous availability of water and solar energy, the most important rate-limiting resources in photosynthesis.

As a hydrologist I knew about actual evapotranspiration (evaporation plus transpiration) but hadn’t paid attention to the link with productivity.  To an ecologist, productivity refers to the rate of biomass production through photosynthesis –  where inorganic molecules, like water and carbon dioxide, are converted to organic material.  Productivity can be measured as mass per unit area per unit time e.g. g m-2 d-1.

In Australia, Actual evapotranspiration is mapped by the Bureau of Meteorology (Figure 1).  There are high values along the coast north of Brisbane, Cape York and ‘The Top End‘.  If Rosenzeig’s correlations hold, these areas are the most ecologically productive in Australia.  In Victoria the highest AET is around Warrnambool, Gippsland and particularly, a small area on the east coast near Mallacoota.  Many of the areas with highest AET are heavily forested.

Figure 1: Average annual areal actual evapotranspiration (link to source)

Rosenzweig quantified the relationship between AET and productivity:

$\mathrm{log_{10}NAAP} = (1.66 \pm 0.27) \mathrm{log_{10}AET} - (1.66 \pm 0.01)$

Where:

• NAAP is the net annual above-ground productivity in grams per square meter.
• AET is annual actual evapotranspiration in mm.

The 95% confidence intervals for the slope and intercept are provided.

Rosenzweig’s paper was published in 1968 and the relationship between AET and productivity is better understood now (e.g. Jasechko, S. et al., 2013).  But the simple relationship between AET and productivity does provide an interesting perspective on the Australian landscape.

### References

Michael L. Rosenzweig  (1968) Net Primary Productivity of Terrestrial Communities: Prediction from Climatological Data,” The American Naturalist 102, no. 923 (Jan. – Feb., 1968): 67-74. DOI: 10.1086/282523 (link).

Jasechko, S., Sharp, Z., Gibson, J., Birks, S., Yi, Y. and Fawcett, P. (2013) Terrestrial water fluxes dominated by transpiration.  Nature 496(7445):347-350 (link).

## One thought on “Actual ET and productivity”

1. emacwater

Hi Tony,

I wonder if evapotranspiration is also related to the forest productivity index, which I think is used to model carbon sequestration potential across Australia in the Nat carbon accounting toolbox… not sure..

I really enjoy your posts, thank you!

Kind regards

Emma MacKenzie