# Plant Available Water Content - dimensionless

sniffadog
Member

Hi, I have Plant Available Water Content from Source:

But this is expressed as cm.

How do I convert this to the dimensionless [0,1] values required by the Water Yield module ?

thanks

Benjamin

But this is expressed as cm.

How do I convert this to the dimensionless [0,1] values required by the Water Yield module ?

thanks

Benjamin

Post edited by admin on

## Comments

Given that AWC(x) = Min(Rest:layer:depth; root:depth) . PAWC

then if PAWC is in cm, the PAWC[0,1] is PAWC(cm)/Rest:layer:depth(cm)

Now the wider problem here is Rest:layer;depth, which is a very difficult variable to determine. This is often just the depth of the soil pit (100cm in the Harmonised World Soil Database).

Hope someone will pick me up if I am wrong.

Benjamin

The distinction between AWC and PAWC is not always clear in the literature so you're right to think about units carefully.

In InVEST, the AWC is the value actually available to plants, equal to the root depth (or restricting layer depth, if lower) times the proportion of water in soil (PAWC, between 0 and 1).

If your source gives the soil property PAWC in cm, there should be a note of the reference (e.g. X cm for each 100cm). This seems to be the case for the Harmonised World Soil Database: http://webarchive.iiasa.ac.at/Research/LUC/External-World-soil-database/HWSD_Documentation.pdf

(p.10, you will see some typical values in mm/100cm of soil: you can check they are consistent with your values).

Best

Appreciate clarification.

Thanks!

The AWC value you have

isthe number between 0 and 1 that represents the proportion of water in the soil: it represents 150 mm of water per meter of soil, equivalent to a proportion of 0.15.As I was saying, there is a confusion possible between the two values, so to clarify:

this value between 0 and 1 (i.e. 150mm/m =

0.15m/m) is the one that should go in the raster "PAWC" in the InVEST model.Cheers

Thanks a lot.

0.15m/m)