Sediment, phosphorus, and nitrogen in channelised surface run‐off from a New Zealand pastoral catchment

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Abstract
Surface run‐off derived from 94 m (20°) and 230 m (15°) long hillslopes in a pastoral catchment was collected along a 15.8 m total intercepting face between 1 July 1983 and 31 March 1985; run‐off subsamples were analysed chemically. Surface run‐off occurred on 49 occasions, these “events” ranging from 0.2 to 115 m3. About 50% of the total run‐off occurred in six events. In 20 months the eight individual 1.5–2.4 m long collectors intercepted 4–333 m3 of water. This uneven spatial run‐off pattern, the measured flow rates, and total event volumes indicated that extensive areas on both hillslopes produced run‐off, and that extreme channelisation occurred during movement downslope. Mean flow weighted run‐off concentrations were for total phosphorus (TP) 1053 mg/m3, total dissolved phosphorus (TDP) 300 mg/m3, Kjeldahl nitrogen (TKN) 5898 mg/m3, dissolved Kjeldahl nitrogen (DKN) 1413 mg/m3, NO3‐N 69 mg/m3, suspended sediments (SS) 472 g/m3, and volatile suspended sediment (VSS) 115 g/m3. Estimated catchment surface run‐off yields in 1984 were: TP 0.75 kg/ha, TDP 0.15 kg/ha, TKN 4.43 kg/ha, DKN 0.74 kg/ha, NO3‐N 0.03 kg/ha, SS 374 kg/ha, and VSS 90 kg/ha. Virtually all phosphorus and nitrogen was transported downslope in winter and spring. Controlling these run‐off nutrient inputs to the catchment stream is briefly discussed.