counting, precision of mass attenuation coefficients used for soil and
water, heterogeneity of the soil sample, and proper instrumentation for
measurements of radiation intensity.
 The theory presented in Appendices A, B and C shows that high
count rates are needed for measurement of either 0 or p, but a high in-
tensity beam causes Compton scattering and dead time problems. For
determinations of water content in soil undergoing no or slow flow,
count rates of approximately 200,000 cpm for lo and I maintained be-
tween 80,000 and 20,000 cpm are usually sufficient. For rapid soil water
movement however, a higher intensity beam is needed in order to de-
crease the actual time of counting. Measurements of intensity may be
performed by preset time or preset count modes and the statistical treat-
ment differs for each one as indicated by Shackel (76).
 The precision of 0 or p measurements depends upon the mass attenua-
tion coefficient (x) measurement precision. As reported in section V,
many factors affect /-measurements and measurement precision of I is
very important when a 60 KeV energy is used. In this case, mass at-
tenuation coefficient values should be checked for each sample collected.
 Heterogeneity of soil samples tends to decrease the accuracy for mea-
surements of p or 0. Size and shape of soil particles, water retention
properties, chemical composition, porosity, compacted layers, and other
factors affect measurement errors. Effects of heterogeneity upon deter-
minations of p or 0 have been minimized by using special geometries
such as a collimated beam with a large cross section (58); or collima-
tion in a slit (63); or a cylindrical column with rotating movement
(84) of the soil column.
 Under optimum conditions, experimental errors in the determination
of soil water content can be close to the minimal theoretical error at-
tributable only to the statistical fluctuation of radioactive disintegrations.
Errors as small as 0.005 cm3/cm-3 have been obtained for water con-
tent measurements, but errors 10 times greater than this value also have
been reported.


 B. Example of Water Content Determination
 Elzeftawy, Mansell and Selim (23) used the single-energy gamma at-
tenuation method (g,=0.0772 cm2/g and L,,=0.0837 cm2/g for 662
KeV photons from 137Cs) to determine distributions of soil water content
in 100 cm long columns of Lakeland sand during the initial phase of
water infiltration. Resulting distributions of water content are shown in
Figure 6 for a steady water application rate (2 cm/hr) into soil with ini-
tial water contents of 0.20, 0.11 and 0.01 cm'/cm3. Experimentally de-
termined values of 0 were similar to those calculated from a numerical
solution of the continuity equation for water. For a given period of in-
 24