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Pedro,
You are correct. This does work pretty well. For your example the equation
would be
25 x Theta(0-25) - 10 x Theta(0-10)
Theta(10-25) = -------------------------------------------------------
15
where Theta(10-25) is the volumetric water content of the 10 to 25-cm depth
range, Theta(0-25) is the vol. water content of the surface to 25-cm depth
range, and Theta(0-10) is the vol. water content of the surface to 10-cm
depth range. You are converting the vol. water contents to depths of water
for each depth range, subtracting the depths to get the depth of water for
the 10 to 25-cm depth range, and then re-computing the volumetric water
content for that depth range by dividing by its depth, 15 cm.
But, note that the calculated water content, Theta(10-25), has twice the
error rate of the two water contents used to calculate it, plus any error
associated with spatial variability between the locations of measurement of
Theta(0-25) and Theta(0-10). For longer term averages, the combined error
rates may sum to zero, but for any single measurement they should be added
(these error rates are usually small enough to be ignored). The error due
to spatial variability can be high. That error can be controlled by making
multiple sets of measurements.
We have verified that this works here at Bushland, but have not bothered to
publish it. There are plenty of early publications. Topp and his colleagues
published on this in the 80s.
Steve Evett
At 06:13 PM 3/7/2000 +0000, you wrote:
>NOTE: To get off this list, send email to majordomo@aqua.ccwr.ac.za
>with the body of the message containing the line:
>unsubscribe sowacs
>
>Pedro Flombaum writes
>
>Hi!
>
>I mesured with a TDR 2 lenght profundities (10 and 25 cm) to establish
>the soil moisture volumetric constant. What I want to know if I can
>somehow substrat the short value to the long one, so I could have an
>idea of what is happenig at the lowest portion of the soil.
>
>If I not wrong the TDR integrate the moisture over the wave guide, so
>the substrat must be relative to the length of the guide. I suppose
>that the equation should be something like:
>
> Lab*Oab - La*Oa
> ------------------ = Ob
> Lb
>
>Where La is the shortest lenght, Lab is the longest and Lb is the
>diference betwen them. Oa is the volumetric moisture constant from
>the shortest wave guide, Oab is the volumestric constant from the
>longest wave guide and Ob is the volumetric constant I am serching
>for.
>
>Perhaps you could contact me with someone working in this subjet or
>recomend me a paper,
>
>I would really apreciate your information,
>
>Thanks in advance,
>
>Lic. Pedro Flombaum
>
>
>
___________________
Steve Evett, Soil Scientist
USDA-ARS, 2300 Experiment Staton Rd., Bushland, TX 79012 USA
806-356-5775, FAX: 806-356-5750
srevett@ag.gov, http://www.cprl.ars.usda.gov/programs/