83
_3
p : density of water [g*cm ]
w = 1.00 [g-cnT3]
Combining equations (3-1) and (3-2) yields a relationship for
volumetric water content in terms of the count ratio and the dry bulk
density of the soil.
9 = A + B-ln(CR) + C-/>dry (3-3)
It was conceivable that a calibration curve might be necessary for
each of the lysimeters and for various ranges of depth. Therefore,
linear regressions of the forms in Equations (3-1) and (3-3) were
determined for each lysimeter and for each of the depths. Statistical
testing revealed that a single regression could be used for all depths
and for both lysimeters for both the wet density and volumetric water
content. The final form of the calibration equation for determination
of volumetric water content was
9 = 1.52 0.33"In(CR) 0.89-pdry (3-4)
The coefficient of variation (R^) for the regression shown in
equation (3-4) was 0.888. Measured volumetric water content when
plotted against the water content estimated by the calibration
(equation 3-4) should lie about a line with a slope of unity and an
intercept of zero (Figure 3-4). Examination of Figure (3-4) indicated
that relatively large errors could be associated with the exact
estimate of volumetric water content. Therefore, the gamma probe