Re: hygrometers

[John Manwaring]

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> Do you know of any hygrometer, relative humidity,
> mechanical psychrometer data which relates to soil
> moisture?
> -Ross Bolton

In situ soil psychrometers and psychrometer based sample 
chambers have been used for many years to acquire soil moisture 
data (as water potential or soil suction).  They can monitor 
changes in soil moisture vapor pressure either via wet bulb 
temperature depression (psychrometric) or dew point temperature 
depression (hygrometric).  Since these parameters are highly 
affected by temperature, soil temperature data is also collected by 
these sensors.

If you're looking for references, a good starting point is the book 
"Methods of Soil Analysis", which has a thermocouple 
psychrometry chapter by Rawlins and Campbell (1986).  A revised 
edition is in press now, with an updated chapter by Andraski and 
Scanlon; hopefully, this will be out shortly.  If you need other 
references or specifics, I would be happy to provide them.

--John Manwaring