Conclusion

The results of this current study found that DBs located on pastures in the Lake Rotorua catchment attenuated 789 kg SS at the Hauraki site, and 1280 kg SS at the Awahou site, accounting for 51% and 59% of the annual inflow SS loads, respectively. These findings demonstrate that DBs are an effective strategy available to pastoral farmers attempting to mitigate their contribution to Lake Rotorua eutrophication, since and the majority of P transported from the catchment is sediment-bound and has the potential to become biologically available.
Large portions of the annual SS yields attenuated by the DBs occurred during rare, high runoff magnitude events, which delivered the greatest surface runoff and SS yields to the bunds during this study. Identifying the ability of DBs to effectively decrease SS yields during these rare, high magnitude storm events is important since large but less frequent runoff events have been observed to contribute significant sediment loads to streams in the northwest portion of the Lake Rotorua catchment where this study was conducted.
The annual SS yield treatment efficiencies observed in this present study were related to changes in SS concentrations through sedimentation, and the portion of runoff infiltrating the soil in the ponding area. Greater SS yields were discharged from the DBs with greater discharged runoff volumes, which emphasises the importance of optimising DB design to maximise the amount of runoff infiltrating the soil. Still, the impoundment of runoff generally decreased event SS concentrations, suggesting that DBs may effectively decrease SS loads where soil infiltration rates, and pond storage to catchment area ratios, are not as high as those this present study.
While this study found DBs consistently decreased SS yield discharged from the DBs, identifying methods to improve concentration treatment efficiencies, such as integrating flocculants, or allowing the bottommost layer of the pond to infiltrate the soil rather than be released, would improve yield treatment efficiencies. Also, cost:benefit analyses should be conducted to determine whether removing the pond drainage mechanism (i.e.. riser/outlet valve/discharge pipe unit) would be beneficial, keeping in mind this might affect pasture productivity. Longer-term studies should also be conducted on more DBs in the Lake Rotorua catchment to further understand the potential of the strategy to effectively mitigate pastoral farming’s impact to surface water quality. Studies should also investigate the cause of declining soil infiltration rates in the ponding area and methods for maintaining or rehabilitating infiltration rates in order to maintain SS yield treatment efficiencies over the life of the DB.