a special U-shaped aluminum sampler. The aluminum sampler was 60
cm long with an inside width of 6 cm. Two samplers were used to
sample the soil profile from 0 to 100 cm depths. These samples were
carried to a laboratory where a dual-energy gamma system was used to
determine water content and bulk density simultaneously.
 Distributions of 0 and p with soil depth are shown in Figure 13. Bulk
densities as high as 1.4 g/cm" were observed in the top 30 cm of soil
as compared to densities of approximately 1.0 g/cm3 between soil depths
of 60-100 cm. Assuming a soil particle density of 2.65 g/cm3, the bulk
densities of 1.4 and 1.0 g/cm3 represent soil porosities of 47 and 62%,
respectively. The higher densities and lower porosities in the surface soil
were due primarily to the compacting effect of heavy agricultural equip-
ment.


VII. An Uncollimated Radiation Attenuation Method for In
 Situ Determination of Soil Water Content
 A. Description of the Method
 Uncollimated gamma radiation has not been used for in situ deter-
mination of absolute water content but it has been used to determine
relative differences in soil water content (54, 79). The uncollimated
radiation method indicates relative changes in water rather precisely, but
measurements of the absolute water content may be subject to large
errors (72). When the gamma attenuation method is to be used for
measurements in situ, many difficulties may be encountered, such as
temperature effects in the detector and in the instruments, lack of colli-
mation, low activity sources, standardization, alignment of access tubes
for source and detector, and accurate values for mass attenuation coeffi-
cients. Despite these problems, this method has been useful for field
studies of drainage and evapotranspiration.
 Basically, this method consists of placing two access tubes into the
soil at 20 to 30 cm distance apart, one for the gamma radiation source
and another for the detector. When the source and detector are placed
at the same soil depth, the attenuated fraction of the gamma beam is
measured, and, by the Lambert-Beer law, changes in water content can
be determined.
 If two measurements of radiation intensity (1) of the attenuated
beam II and 12 are taken at two different times tl and t2, the difference
between 11 and 12, can be attributed to water content variation at this
soil depth. The change in water content from 01 to 02 can be expressed
according to the equation

 02 01 In3 [17]

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