Campbell Scientific HydroSense® Soil Water Content System (CS620, CD620) - Manual (Page 25)

Appendix B. How Many Soil Water Content Measurements Are Enough
N is the number of water content measurements and L is the acceptable range
as defined by the user. For example, the acceptable water content range might
be
±2.5% so L would be 5.0.
Equation 4 relies on a good estimate of the population standard deviation. A
useful approach using these statistical methods is to establish an estimate of the
standard deviation by taking several water content measurements, then use that
standard deviation estimate and the associated t value in [4]. The range is
specified by the user and the calculated N value defines the number of
measurements. The same s and t values are used, but the number of
measurements can be determined for several different ranges.
N
t s
L
= =
⋅ ⋅
=
4 4 2 201 174
5 0
2 35
2 2
2
2 2
2
( . ) ( . )
.
.
The calculation of N using the sample standard deviation shows that 3
measurements are required to give a 90% probability that the sample mean
gives the desired
±2.5%.
B-3