ported an adaptation for inmasiuring sediment densities. When correctly
used, the in situ method can detect changes in water content as small as
1% (0.01 cm3/cm3) and with a theoretical error of about 0.5%.
 Figure 14 shows the results of a laboratory experiment to observe the
 s.ensii\ii\ of this method for determining a change in water content.
 A sandy soil was placed in a large container and its water content was
 changed by addition of known volumes of water, homogeneously dis-
 tributed in the soil. The change in water content was measured by the
 g.lmiinm attenuation method (A\) and these results were compared with
 the actual water content change (AB,) determined gravimetrically. The
 error in this determination was 0.005 cm3/cm3. These results show that
 the in situ method can be used to determine changes in soil water con-
 tent under field conditions.


 IX. Other Gamma Radiation Methods for Determining Soil
 Water Content and Bulk Density
 Some other gamma radiation methods for determining soil water con-
tent are Jeserving of mention, despite the fact that they are not attenua-
tion methods. The combined method for determining water content by
neutron moderation and density by gamma cateilinL (2, 30, 44) is
frequently" uved in field research in\ o1\ ing soil moisture-plant-atmosphere
rel:tiornhilp such as those by Denmead and Shaw (20) and Maertens
et al. (56). Recently, two other methods for deiermnining water content
by gamma radiation were introduced, the neutron .critai.ion, and the
gamma photoneutron methods. The neutron activation method is based
upon bombardment of hydrogen ('H) atoms with neutrons (n) to pro-
duce deuterium (2H) and 2.225 MeV gamma rays (y) according to the
equation
 iH + n 2H + y [18]
By measuring this 2.225 MeV peak of the gamma radiation, the amount
of hydrogen, and thus the water content, can be determined dirc.tl\.
Parker et al. (67) and Metzger et al. (61) reported studies of this
neutron activation methodology for determining water content in soils
of the Moon and Mars.
 The gamma phlooneutron method (12), is based upon the bombard-
ment of deuterium (QH) nuclei in water with high energy (greater than
2.225 MeV) gamma rays (y) to produce li\drogen atoms ('H) and
neutrons (n) according to the equation
 H + y-- ) IH + n [19]
The production of photoneutrons is proportional to the deuterium con-
tent in water. The natural abundance of deuterium is 0.015% but, in
 36