Plant Available Water Content - dimensionless

sniffadogsniffadog Member
edited April 2014 in Freshwater Models
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
Post edited by admin on

Comments

  • Hi, I am going to try to asnwer this myself, having gone through the docs with a little more care.

    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
  • PerrinePerrine Moderator, NatCap Staff
    Hi 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
  • Still slightly confused about this. If I understood well in invest manual (p 251 for the 2.6 version) says that PAWC=AWC/depth. From the FAO data I have a AWC=150mm/m and a soil depth of 100 cm, the PWAC would be 1.5, which is bigger than 1. If I do PAWC=depth/AWC, then I get numbers smaller than 1.
    Appreciate clarification.
    Thanks!
  • PerrinePerrine Moderator, NatCap Staff
    Hi Benjamin,
    The AWC value you have is the 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
  • aha... That makes sense!
    Thanks a lot.
  • MaryeMarye Member
    Hi Benjamin,
    How do you get Rest:layer;depth and AWC/PAWC from HWSD?FAO? I downloaded the data but I can't find Rest:layer;depth and AWC ( 150mm/m = 0.15m/m)

  • Hi Marye,

    am scratching my head to try and remember. n HWSD there is REF DEPTH (obstacles to roots). However you'll notice that soil scientists rarely dig below a meter or 1.5 m. So this tends to be the limit. Of course many tree roots (in particular tap roots go much much deeper).

    Ben

  • swolnyswolny Member, NatCap Staff
    Hi all -

    I have used the REF_DEPTH field in HWSD for 'depth to root restricting layer'. 

    For AWC, you can use AWC_CLASS field, turn the class into values, then divide by the soil depth to get the AWC fraction.

    Here is a collection of notes that I compiled about creating these InVEST layers from the HWSD, including example tables, which gives more details.

    ~ Stacie

  • MaryeMarye Member
    hi @swolny and sniffadog ;
    Thank you very much. I will try. 
  • MaryeMarye Member
    Hi swolny, no word to express my thanks. I solved the problem by the documents you send to me. 
    Many thanks for you.
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