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Table 2:
The removal rates of sediment and P-pollutants by the multiple
structures system during different types of run-off events
System input
System
output
Removal
rate (%)
SRH* SRV SRO SRC Total
Continuous run-off events
TSS (kg) 2784.3 821.1 16.2 89.3 3710.9 1235.2 66.7
TP (kg) 0.127 0.812 0.062 0.101 1.102 0.433 60.7
TDP (kg) 0.047 0.300 0.023 0.037 0.407 0.210 48.4
DRP (kg) 0.037 0.235 0.018 0.029 0.319 0.181 43.3
Discontinuous run-off events
TSS (kg) 2249.2 2582.6 59.3 232.2 5123.3 20.6 99.6
TP (kg) 0.218 1.908 0.097 0.259 2.482 0.014 99.4
TDP (kg) 0.081 0.706 0.036 0.096 0.918 0.005 99.5
DRP (kg) 0.063 0.551 0.028 0.075 0.717 0.004 99.4
*SRH, surface run-off from hilly land; SRV, surface run-off from village; SRO, surface
run-off from orchard; SRC, surface run-off from cropland.
SEDIMENT AND P-POLLUTANTS REMOVAL CONTRAST
Removal Efficiency of the Multiple Structures System
The loads of sediment and P-pollutants exported from the outlet of the watershed
were reduced to a great degree after gradual removal by each buffer/detention
structure upslope to downslope. But the removal effectiveness was different due
to the different hydrological characteristics of continuous and discontinuous runoff.
For example, during continuous run-off events, the total input loads of TSS,
TP, TDP, DRP by surface run-off were 3710.9 kg, 1.102 kg and 0.407 kg, 0.319 kg,
respectively. Their exports from the system were 1235.2 kg, 0.433 kg, 0.210 kg
and 0.181 kg, respectively (Table 2). The removal rates were 66.7%, 60.7%, 48.4%
and 43.3%, respectively. Comparatively, during discontinuous run-off events, the
total input loads of TSS, TP, TDP, DRP by surface run-off were 5123.3 kg, 2.482 kg,
0.918 kg and 0.717 kg, respectively, with total export loads only of 20.6 kg, 0.014 kg,
0.005 kg, 0.004 kg, respectively. Their removal rates all exceeded 99%. These high
removal rates of pollutants mainly resulted from the intermittent transport process of
polluted run-off.
Removal Efficiency of Single Structure
Due to differences in physical characteristics, spatial location in the watershed, main
removal mechanisms and degree of disturbance by humans among these structures,
removal efficiency of each single structure was different too. During continuous runoff
events, due to the huge storage capacity and long detention time of dry ponds,
they had the highest removal rate of TSS, TP, TDP and DRP, of which were 34.6%,
34.3%, 20.5% and 17.3%, respectively. For dissolved forms of P-pollutants (TDP,
DRP), a grassed filter strip had higher removal rates (Table 3). This mainly contributed
to the higher infiltration rate of water resulting from dense roots of vegetation and
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