A total of 421 cost-shared water control structures have been installed in the
Pamlico basin at a total cost of 3,843 or
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,244 per unit (on average). The NC
estimated that as of July 1989 more than 2,500
control structures had been installed to provide controlled drainage on
approximately 150,000 acres (60,704 ha) in eastern North Carolina. Based on
these data; an average water control structure serves about 24 hectares. This is
fairly close to the North Carolina SCS estimate of 0.013 unit/acre, which converts
to 31 hectares per water control
(USDA Soil Conservation Service,
1991). As a conservative estimate, we have used 24 hectares/unit for our
calculations.
Table 2-13. Water Control Structures Cost Data
4,615
3% of base
10,104 ha
I
Assuming 24 hectares
per structure.
Evans et al. (1991) summarize the nutrient transport reductions from 14 studies of
controlled drainage in eastern North Carolina. Representing 125 site-years of
data, these studies give some of the most detailed effectiveness data for any
practice in the Tar-Pamlico basin. From the results of these studies, it is apparent
that controlled drainage can significantly reduce nutrient load to the stream. The
reduction in nutrient load is attributable to decreased outflow, increased crop
uptake (as evidenced through increased yields), denitrification, and sedimentation
(phosphorus). Table 2-14 presents the average results of these studies, modified
from Evans et al. (1991).
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