wet soil bulk density (mass of solids plus water per unit bulk volume of
 soil) results in a decrease in radiation intensity. For stable (non-swell-
 ing) soils, dry bulk density may be assumed to remain constant during
 water flow, and thus changes in wet bulk density are attributed to changes
 in volumetric water content. For soils which swell upon wetting and
 shrink upon drying, a beam of dual-energy gamma radiation or two
 parallel beams with different radiation energies should be used to simul-
 taneously determine soil dry bulk density and water content. The dual-
 energy method can be used to determine soil water content and bulk
 density in surface soils compacted by heavy agricultural tractors and
 machinery. Commercial equipment is also available for using the gamma
 attenuation method to determine water content in situ under field con-
 ditions.
 The precise and accurate determinations of soil water content or bulk
 density by the single-energy gamma attenuation method require precise
 measurements of the soil sample thickness, accurate measurements of
 radiation intensity, and precise determinations of mass attenuation co-
 efficients for soil and water. For measurements of water content under
 conditions of transient soil water flow, the incident gamma intensity
 must be high in order to permit measurements of a large number of
 photon counts in a short time. For determinations of water content, the
 minimum resolvable change in soil water content ranges from 0.002 to
 0.005 cm3/cm3 for most soils. For determinations of bulk density, the
 minimum resolvable change in soil bulk density ranges from 0.10 to
 0.20 g/cm3. For best accuracy, the thickness of the soil column or sam-
 ple must be optimized for the primary energy of the radiation. For ex-
 ample the optimum thickness of soil samples for 60 KeV photons from
 241Am range from 5-10 cm; whereas for 662 KeV photons from 137Cs
 the optimum thickness ranges from 10 to 25 cm.
 Errors associated with determinations of soil water content or bulk
density by the dual-energy method are normally greater than when the
single-energy method is used. The precision of determinations of water
content and bulk density by the dual-energy method is strongly de-
pendent upon the accuracy with which the mass attenuation coefficients
are determined for each photon energy. Thus the dual-energy method
should only be used to determine water content under conditions where
the bulk density changes with the soil water content such as occurs dur-
ing water infiltration into initially dry soils with high contents of mont-
morillonitic clay minerals.
 Although the in situ gamma attenuation method for determining
changes in water content does not utilize collimation, changes in water
content can be detected as small as 0.01 cm"/cm' with an error of about
0.005 cm3/cm3 when correctly used.