706
WARD, JOHNSON, AND STEWART
However, although the deposition coefficient of ‘’Cs for precipita-
tion to forage is a difficult parameter to determine for growing forage,
approximate results may be obtained by the averaging procedure
applied to the data of Table 1.
It is also possible to determine a deposition coefficient character-
istic of daily rains when the pasture '*’Cs levels and the immediate
deposition by rainfall are known. Such a coefficient was determined for
certain rains in the 1964 pasture season. By an empirical determina-
tion of a half-time for pasture activity which combines losses due to
grazing, wash-off, and dilution by new pasture growth, the pasture
activity on the day following a rain can be determined by extrapolating
the measured values after the rain. Similarly, the residual contribution
from previous rains can be determined and subtracted from this value.
This method has been applied to six rain days for the 1964 forage
growing period (Table 2) and can be expressed in a general equation:
kR = P, — Piet
where k = deposition coefficient
P,
NI
R = rain deposition to the pasture, pc/m’
P; pasture activity on the day following the rain, pc/m’
measured value of pasture activity on t days preceding the
rain, pc/m*
= 0,693/T, where Ty is the pasture-activity half-time
The deposition coefficients determined in this manner were far
from constant; however, the variability could be qualitatively linked to
a dependence on the volume of precipitation in each rain. The deposition coefficients of Table 2 were generally found to vary inversely
with the amount of rainfall. These coefficients are probably also
dependent on other factors, such as the intensity and the duration of
Table 2— CALCULATION OF DEPOSITION COEFFICIENT
FROMINDIVIDUAL RAINS AND PASTURE-FORAGE
CONCENTRATIONS
Date,
1964
Rainfall,
in.
B7Cg
deposition,
pe/m?
Extrapolated
137Cs pasture
Deposition
pe/m?
(k)
activity,
coefficient
May 7
0.21
640
532
0.83
May
May
June
June
June
0.33
1.91
0.15
0.11
G.09
480
1017
280
700
182
330
570
185
180
275
0.69
0.56
0.67
0.26
1.79
26
2&8
&
21
30
Mean
0.80