Re: diurnal variation in capacitance probes

[Andrew Western <a.western@civag.unimelb.edu.au>]

Andrew Western writes

G'day Everyone,

I saw a paper on this phenomenon at the AGU spring meeting that has both
theoretical and experimental results.  The authors added an advective term
to the (Fickian) vapour transport equation related to diurnal thermal
expansion/contraction of the soil air.  They showed that this advective
transport was the dominant transport vapour process and did indeed explain
the diurnal variations in soil moisture observed.  The paper reference is:

Cahill, A.T., Parlange, M.B., Prosperetti, A. and Whitaker, S., 1998.
Convectively enhanced water vapor movement at the Earth's surface. Eos,
Transactions of the American Geophysical Union, 79(17): s137.

cheers
 Andrew Western
<a.western@civag.unimelb.edu.au>
>
>Lindsay Hutley writes
>
>Dear All,
>
>In a recent posting from Martin Parks describing aspects of vapour 
>flux in dry soils, he mention diurnal oscillations in EnviroScan's 
>Sentek system. We have a number of access tubes installed to 4 m in 
>soils of the Northern Territory, Australia and I have noticed that as 
>the soil dries, there is a diurnal oscillation in output.  The 
>phases of the oscillations with depth seem to be consistent with 
>some sort of thermal response and not due to overnight recharge 
>from lower soil horizons or hydraulic lift.
>
>The oscillation appears to be large in the dry season and is greatly 
>damped during the wet season.  Colleagues using tdr equipment have 
>also noted a similar phenomenon.
>
>Has anyone any ideas on this occurrence ?
>
>Regards,
>
>Lindsay Hutley
>CRC for the Sustainable Development of Tropical Savannas
>School of Biological Sciences
>Northern Territory University
>Darwin NT 0909
>AUSTRALIA
>
>Ph  +61 8 8946 7103
>Fax +61 8 8946 7107
>
>