B. Example of an In Situ Method
 Several limitations of the use of the in situ method in the field require
that the method be performed with care. Reginato and Jackson (70)
reported changes in gain of the photomultiplier-tube and resolution of
the scintillator crystal due to temperature effects in the detector. Prob-
lems of geometry, non-parallelism of the access tubes and errors were
reported by Ryhiner and Pankow (74). De Vries (21) attempted to
use partial collimation. Standardization problems have been studied by
Reginato and Van Bavel (72) and general description of this method
may be found in papers of the latter author (88 and 89). McHenry and
Gill (60) reported the use of portable equipment for soil water content
changes in a 100-cm profile as a function of rainfall, soil cover, and
profile characteristics. Rawls and Brooks (68) reported the use of this
method for measuring bulk density, and McHenry and Dendy (59) re-


 /





 o ;E 1:1 0'

 c 0 E

 O( .04 -




 tJ' -
 S0 05
 0 .02 .04 .06 .08


 Experimental Water Content
 Change, 6e (cm3/cm3)

 Figure 14. Actual difference in soil water content, AO,, versus water con-
 tent difference determined by uncollimated radiation. The measurement error
 was 0.005 cm3/cm3.
 was 0.005 cm3! cm3.