NPP Soil Water Content

Once a month soil water content measurements are made
at 10 depths (where possible) at each of 10 access tubes at
each of the 15 LTER-II sites using a neutron probe (Campbell
Model 503DR Hydroprobe). Measurements are taken at 30cm,
60cm, 90cm, 120cm, 150cm, 180cm, 210cm, 240cm, 270cm, and
300cm when possible or to the greatest depth it was possible
to install the access tubing before hitting impenetrable
caliche. If fewer depths were measured, the missing depths
have a zero in the raw data set of count values. These are
changed to "." in converted water content data set.
Calculated water content values equal to less than zero are
changed to zero in converted water content data set.
Converted water content values are a volume/volume
relationship and represent cm3 water/cm3 soil.

Data from neutron probe data logger is dumped to disk. Raw
count data is then converted to water content using a
Fortran program called WC2.FOR (water content for LTER-II).
Water content data is then sorted by i.d. number using a
LOTUS 123 macro found in WC2_SORT.WQ1 which creates final
version of water content data set for the month in a
separate file. Two files per month are saved: raw count
data (mmddyy-2.RAW; ex. 121189-2.RAW where -2 indicates
LTER-II NPP site neutron probe readings) and calculated/
sorted soil water content data appended to data file
containing that year's data (NPPSWCyy.DAT; example
NPPSWC89.DAT contains NPP soil water content data for 1989).

Regression equation was derived by Mahlia Nash, and after
datalogger upgrade was added to hydroprobe, by David
Hudson, both working for Dr. Peter Wierenga (as of 1994 at
university at Tucson (Hudson's phone # is 602-621-3236).

See PROBE.HIS file for probe history and regressions used
for different data periods when different probes were used.
The hydroprobe used whenever possible is Hydroprobe Model
CPN503DR (Campbell Pacific Nuclear, Pacheco, CA) with data
logger. This probe has a 50mCi241 Am-Be source and a
hydrogen detector. Neutrons encountering hydrogen become
thermalized. The detector totals returning thermalized
neutrons over a 16 second sample time which is the raw count
value displayed. The raw count value is then substituted
into the proper regression equation for cm3 of water per cm3
of soil.