converting SM measurements

[None]

Hello,
I'm analyzing soil moisture (SM) data output from a earth system
climate model. In the model, SM units are kg/m^2. As the model assumes
an uniform soil depth of 1m, this could also be kg/m^3. I need to get
this into % of dry weight and will need to assume some average dry soil
density value. What would this be?

The value certainly varies, but I could use a range. The model output
also contains vegetation cover. If there exists a relationship between
density and type of vegetation cover I could "sophisticat" my estimate
a little.

Thanks!
Alvaro

[Robert Lascano]

3 Feb 2005

 

Alvaro:

 

The relationship between soil water content when expressed gravimetrically (kg/kg or g/g) and on a volume basis (m3/m3 or cm3/cm3) is as follows:

 

VWC = GWC x (BD/WD), where

 

volumetric water content (VWC, m3/m3 or cm3/cm3) is equal to the gravimetric water content (GWC, kg/kg or g/g) multiplied by the soil bulk density (BD, kg/m3 or g/cm3) and WD is water density, assumed to be 1000 kg/m3 or 1 g/cm3. If you use g/cm3 then VWC = GWC x BD.

 

The value of BD in soils changes as a function of depth and is a highly variable number near the soil surface due to changes in tillage, rain impact, etc. It is a value that can vary from 1.0 to 1.8 g/cm3 for most agricultural soils and depends on soil texture, structure and other properties. There is no relationship between BD and vegetation. Furthermore. in these relationships simplifications normally lead to large uncertainty and associated errors.

 

I hope this helps.

 

Robert J. Lascano

 

Robert J. Lascano
Professor - Soil Physics
Texas Agricultural Experiment Station
Texas A&M University - USDA - ARS
3810 4th Street
Lubbock, TX 79415
Voice (806) 723-5238
Fax (806) 723-5272
Secretary (806) 746-6101
e-mail: r-la...@tamu.edu
e-mail: rlas...@lbk.ars.usda.gov

[Warren, Jeff]

Some forest soils can have BD < 0.3 g/cm3

 

Another consideration is soil organic matter, including roots.  Generally, more soil organic matter = lower bulk density.

 

While BD may not be directly related to vegetation, land use history (which can be related to changes in vegetation) does have a significant impact on BD - which should further complicate your analysis.  Good luck.

 

Jeff

 

____________________________________

 

Jeffrey M Warren

Ecosystem Processes

USDA Forest Service, PNW Research Station

3200 SW Jefferson Way

Corvallis, OR  97331

(541) 758-8766

____________________________________

---

From: Robert Lascano
Sent: Thu 2/3/2005 11:13 AM
To: sow...@googlegroups.com
Subject: Re: converting SM measurements

[Martin Hodnett]

Hi Alvaro

Soil moisture is expressed more usefully in volumetric
terms - which you have already - so no need for the
BD. This can vary from 0.9 to 1.6 gm/cc. The changes
in water content (eg seasonal range) will be more
useful as an indicator of behaviour than the absolute
water content.

Cheers

Martin Hodnett
VU Amsterdam





___________________________________________________________
ALL-NEW Yahoo! Messenger - all new features - even more fun! http://uk.messenger.yahoo.com