I. Introduction
 Measurements of soil water content, 0, and bulk density, p, with time
and depth in the soil profile are valuable inputs to water management of
agricultural soils for the production of crops. From water content dis-
tribution data, soil water flow, water uptake by plant roots (transpira-
tion), and soil water retention can be estimated for periods of redis-
tribution following a given rainfall or irrigation event. Direct methods
(31) such as gravimetric determination, thermoelectric, neutron scatter-
ing, gamma ray attenuation, and electrical resistance as well as indirect
methods such as tensiometers have been used to measure soil water con-
tent. The gravimetric or weighing procedure is usually considered as
the standard for comparison with other methods for water content mea-
surements. Although the neutron scattering technique is widely used to
provide in situ determinations of soil water content, this method has
certain inherent disadvantages which can be overcome by the gamma at-
tenuation method. For example, a severe restriction of the neutron
method is that the volume of soil which is measured for water content
increases as the soil becomes drier; whereas the gamma attenuation
method measures the water content of a soil volume which is inde-
pendent of the soil water content.
 The gamma-ray attenuation technique is one of the most important
of the direct methods for the determination of water content of soils.
Attenuation of gamma radiation during transmission through soil has
been used for nearly three decades as a method for determining volu-
metric water content and dry bulk density. Although this method is best
suited for laboratory determinations in soil columns, it has also been
used for in situ measurements under field conditions. The gamma at-
tenuation method offers many advantages for determining soil water con-
tent since rapid, nondestructive determinations are provided for small
volumes of soil.
 For stable soils which do not swell upon wetting with water or shrink
upon drying, bulk density may be assumed to remain constant during
water flow through the soil, and thus changes in measured intensity of
transmitted radiation may be attributed to change in water content.
However, for unstable soils, the bulk deniit\ is .ubject to changes dur-
ing the time that water flows through the soil and cannot be assumed to
be a constant. Either a single beam of dJual-.en-ri\ gamma photons or
two separate beams with greatly different energies can be used to simul-
taneously determine water content and bulk density in these unstable
soils.