Selection of Water Quality Monitoring Stations - Water Quality Management: Assessment and Interpretation - page 6

specify the appropriate sampling sites. To pinpoint the locations more precisely,
micro location and representative sampling considerations must be followed.
The

first approach systematically locates the sampling sites so as to divide the
river network into sections which are equal in respect to the number of contributing
tributaries. The first stem involves stream ordering, where each exterior tributary or

link contributing to the main stem of the river, (one which has no other tributaries or
one with a certain minimum mean rst order.

Ordering is carried out along the entire river such that a section of the river formed by
the intersection of two upstream tributaries will have an order described as the sum
of the orders of the intersecting streams. At the mount of the river, the magnitude
(order) of the

flow) is considered to be of the

final river section will be equal to the number of all contributing
exterior tributaries.
Next, the river is divided into hierarchical sampling reach by de

ning centroids
for each reach. The major centroid which divides the basin into two equal parts is
found by dividing the magnitude of the final stretch of the river by two. Accord-

ingly, the major centroid where a first hierarchy station is to be placed is located in

that link whose magnitude is closest to:

M ij ¼½ ð N 0 þ 1 Þ= 2 ð 2 : 1 Þ

where M ij = the first hierarchical location; with M = the magnitude (order) of the link,

i = the hierarchical level of the station to be placed on that link, and j = the order of
that station within the ith hierarchical level (e.g., M 11 = the first station at the first
hierarchical level and M 12 = the second station of the same hierarchy). N 0 = the total
number of exterior tributaries at the most downstream point of the basin where
station M 11 is placed M 12 (Or the stream number closest to it) divides the total
catchment area into two equal parts for which new centroids may be determined.
In the above procedure, it must be noted that a link determined at a given
hierarchy does not necessarily have the value of M ij because a link of that number
may not exist. In this case, the link closest in magnitude is chosen as the centroid
when this link is speci ed a sampling location is placed at its downstream junction.

Although Sanders et al. ( 1983 ) located the station M ij at the downstream point of
the reach that has the corresponding stream order number, it may be allocated to
any site along that reach, considering such local factors as accessibility of the site or
the presence of a stream gauging station also; note here that the squared brackets in
Eq. ( 2.1 ) indicate a truncation of the enclosed value to an integer value.
As noted above, M 12 divides the total basin into two equal parts where new
centroids may be determined for the upstream part, the

first station with the second
hierarchical order is found by which is the magnitude of the link that divides the
region upstream of M 12 into two equal areas with respect to their drainage density.

M 21 ¼½ M 12 þ 1
2

ð 2 : 2 Þ