Effects of orifice-weir outlet on hydrology and water quality of a drained forested watershed

Orifice-weir structures at ditch outlets are proposed to reduce peak drainage rates during high flows and to store water during the growing season in poorly drained managed pine plantations. Two coastal watersheds, one conventionally drained (DI) and another with an orifice-weir outlet (D3), were monitored to examine the effects of this orifice treatment on drainage outflows and nutrient exports from drained pine plantations in eastern North Carolina. Five years (1995-1999) of measured hydrologic data showed that the daily water table elevation on D3 was 7 cm higher on average, but was 13.5 cm higher during wet periods compared to conventional drainage. The peak drainage rates from D3 were substantially dampened by the orifice-weir. Accordingly, average annual outflow was reduced by 18%. The reduction in outflow was as much as 34% in 1995. Taking the characteristic differences observed in concentrations between these two watersheds during the pretreatment phase into consideration, the measured average annual TKN concentration in the watershed with the orifice appeared to be higher, and total P and sediment lower than expected for conventional drainage. Despite the reduction of flow in all 5 yr, the measured exports of NO3-N, TKN, and total N increased in the first 3 yr (except for TKN in 1995) and decreased in 1998 and 1999 with no significant effects because of the orifice-weir treatment. However, on an average annual basis, total sediment and total P export from D3 were reduced by 54% and 30%, respectively. These results showed that an orifice-weir at the drainage outlet can be used to reduce peak rates, annual drainage outflows, total P and sediment export. The orifice-weir outlet did not have an effect on the export of nitrogen components as happens when controlled drainage with a raised weir is used.