Figure 1

Brian Levine

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Erosion leading to sedimentation in surface water may deliver sediment-bound nutrients that contribute to eutrophication. Loss of native vegetation driven by land use changes have accelerated the naturally high rates of erosion in New Zealand and increased sedimentation in streams and lakes. Water quality in Lake Rotorua, in the Bay of Plenty region, New Zealand, has declined since the 1960’s due to land use developments contributing to nutrient loading and eutrophication. Anthropogenic P loads delivered to the lake are 17-19 t P y-1, 71-79% of which are sediment-bound. Lake sediments release an estimated 48% of the total annual P load. Detainment bunds (DBs) were first implemented in the Lake Rotorua catchment in 2010 as a strategy to address P losses from pastoral agriculture. Detainment bund are 1.5-2 m high earthen stormwater retention structures, constructed on productive pasture across the flow path of targeted low-order ephemeral streams. The current DB design protocol recommends a minimum pond volume of 120 m3 ha-1 of contributing catchment with a maximum pond storage capacity of 10,000 m3. No previous study has investigated the ability of DBs to decrease annual suspended sediment (SS) loads leaving pastoral catchments. Annual SS yields delivered to 2 DBs, with 20 ha and 55 ha catchments, during this 12-month study, were 109 kg SS ha-1 and 28 kg SS ha-1, respectively. Results suggest that the DB strategy decreased annual SS loads discharged from the DB catchments by 1280 kg (59%) and 789 kg (51%) as a result of the bunds’ ability to impede stormflow and facilitate soil infiltration and sedimentation. The results of this study highlight the DB strategy’s ability to consistently decrease SS loads leaving pastures in runoff, even during rare, high magnitude storm events, and suggests DBs are likely able to reduce P loading in Lake Rotorua.