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TECHNICAL REPORTS

Surface Water Quality

Sources of Sediment and Phosphorus in Stream Flow of a Highly Productive Dairy Farmed Catchment

^a AgResearch Ltd, Invermay Agricultural Centre, Private Bag 50034, Mosgiel, New Zealand
^b National Institute of Water and Atmospheric Research, P.O. Box 11 115, Hamilton, New Zealand

^* Corresponding author (richard.mcdowell{at}agresearch.co.nz)

Received for publication September 3, 2006. Both sediment and phosphorus (P) are important contaminants for surface water quality. Knowing the main sources of sediment and P loss within agricultural catchments enables mitigation practices to be better targeted. With this in mind seasonal loads of suspended sediment (SS), dissolved reactive P (DRP), total P (TP), and bioavailable P (BAP) were measured in a low gradient stream draining an intensively farmed New Zealand dairying catchment. Integrating in situ samplers were deployed to collect samples and the results merged with continuous flow data to calculate seasonal loads during 2005 through 2006. Flow rate, SS, and TP concentrations peaked in winter-spring and were lowest in summer-autumn. Concentrations of BAP in trapped sediment were greatest in autumn, contrasting with winter and spring when greater amounts of sediment were trapped, but with lower P enrichment. Analysis of ¹³⁷Cs and mixing model output showed that a major source of sediment and associated P in winter and spring was stream banks. Possible causes for this include trampling and destabilization by stock, channel straightening and sediment removal, and removal of riparian trees that stabilize banks. Modelling indicated that overland flow probably from topsoil (but could include sediment from lanes) contributed most sediment during summer and autumn. Remediation aimed at decreasing particulate P inputs to streams should focus on riparian protection measures, such as permanent stock exclusion and planting with shrubs and trees, ensuring runoff from lanes is minimized, and decreasing Olsen P to nearer optimum agronomic levels.

Abbreviations: OC, organic carbon • DRP, dissolved reactive P • BAP, bioavailable P • LSD, least significant difference • msl, meters above sea level • PP, particulate P • TP, total P • SS, suspended sediment