Agriculture Reference
In-Depth Information
The surface water quality characteristics NO
3
-N and COD
Mn
have
been considered as two of the four water pollutant load control indexes in
China. Furthermore, the relationship between water quality characteristics
was tested at the signifi cance levels of p < 0.01 and 0.05, which showed
that NO
3
-N had a high positive correlation with TN, TP, Hg, Cd and Pb,
and CODMn had a positive correlation with Hg, Cd and Pb (Table 2).
Therefore, we focused on the two conventional water quality charac-
teristics NO
3
-N and COD
Mn
, which not only can reduce the complexity of
the study, but also is of great signifi cance for water resources management.
1.2.3 MULTI-SCALE WATERSHED DELINEATION
1.2.3.1 BASIC WATERSHED UNITS
A watershed is the up-slope area contributing flow to a given location.
Such a feature is also variously referred to as a catchment or basin, and
comprises a part of a hierarchy in that any given watershed is generally
part of a larger watershed [19]. A digital elevation model (DEM) with 30
m × 30 m grid resolution was adopted to create basic watershed units for
the Beijing mountainous area using the hydrologic functions in the Ar-
cView extensions. The minimum number of cells for a stream network in
the hydrologic functions is very important for the watershed delineation.
Many studies have shown that when the minimum number of cells was
smaller, the extracted stream networks were denser, and the created basin
areas were smaller. The stream network extracted by hydrologic functions
should be similar to that existing in Nature, and each of the monitoring
sites should be located at the pour point of different basic watershed units
for the purpose of this study. Therefore, the thresholds of 15,000, 12,000,
10,000, 8,000 and 5,000 were chosen as the minimum number of cells
for a stream network during the test of basic watershed unit creation. The
results indicated that a few monitoring sites were located in the same basic
watershed unit when the thresholds was bigger than 10,000, and the stream
networks extracted were far denser than natural stream network when the
threshold was less than 10,000. Ultimately, the thresholds of 10,000, i.e.,
