"Yannis" schreef in bericht
...
it's an index of dryness actually, calculated by the formula:

I = P/(10+T) for the annual de Martone index, where P stands for annual
precipitation and T fot the annual mean temperature, or
I'=12P'(10+T') for the monthly de Martone index, where P' is the monthly
precipitation and T' the monthly mean temperature.

It is said that de Martone index values lower than 20 account for the
necessity of watering the soil. The de Martone index is one of the many
that have been internationally proposed for 'counting' how dry or wet an
area is. It is easy to use and calculate, as all weather stations provide
with temperature and precipitation data.

Thank you for explaning, Yannis. I have been puzzling with some ppn-figures
of the past; and indeed its a good and easy indicator to estimate either
waterdeficiency or water-surplus!
But only as long as the index has some value 0. When ppn is zero; the
index is zero and tells nothing about the waterdeficiency.

Here's the currently (so-called) "water-surplus" map of the Netherlands of
the period april 1 till now -- only the blue spots give a watersurplus --
the rest is coloured with waterdeficiencies.
http://www.knmi.nl/voorl/kd/produkt/...overschot.html

The values at the map are calculated as the ppn minus the evaporation at
several sites across the country.

Oh, well, we're not the only place in Europe with a lack of water.

Wijke