Figure 4.3 The average percent

No adjustment for upstream

sediment concentration

variation

Adjustments made for

upstream concentration

variation

30

10

4

2

et al. (1989) developed a method to determine the number of sampling

events required to detect a statistically significant change of a given magnitude and

range of error rates. The example provided in Figure 4.4 shows the number of

monthly samples needed to detect a 30% decrease in the total nitrogen concentration

at the Neuse River at Smithfield, North Carolina. Along the bottom of the figure the

fraction decrease is shown in parenthesis. For an error rate of 30% (0.30) the

approximate number of monthly samples required to detect a 30% decrease is 55.

.

I

I

0

0.28

0.35