State and potential management to improve water quality in an agricultural catchment relative to a natural baseline |
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Authors: | Richard W McDowell Ton SnelderRoger Littlejohn Matt HickeyNeil Cox Doug J Booker |
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Institution: | a AgResearch, Invermay Agricultural Centre, Private Bag 50034, Mosgiel, New Zealand b National Institute of Water and Atmospheric Research, 10 Kyle St, P.O. Box 8602, Riccarton, Christchurch, New Zealand c Otago Regional Council, 70 Stafford Street, Private Bag 1954, Dunedin 9054, New Zealand |
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Abstract: | Land use change and the expansion of dairying are perceived as the cause of poor water quality in the 1881 km2 Pomahaka catchment in Otago, New Zealand. A study was conducted to determine the long-term trend at four sites, and current state in 13 sub-catchments, of water quality. Drains in 2 dairy-farmed sub-catchments were also sampled to determine their potential as a point source of stream contamination. Data highlighted an overall increase in the concentration of phosphorus (P) fractions at long-term sites. Loads of contaminants (nitrogen (N) and P fractions, sediment and Escherichia coli) were greatest in those sub-catchments with the most dairying. Baseline (without human influence) contaminant concentrations suggested that there was considerable scope for decreasing losses. At most sites, baseline concentrations were <20% of current median concentrations. Contaminant losses via drainage were recorded despite there being no rainfall that day and attributed to applying too much effluent onto wet soil. Modelling of P concentrations in one dairy-farmed sub-catchment suggested that up to 58% of P losses came from point sources, like bad effluent practice and stock access to streams. A statistical test to detect “contaminated” drainage was developed from historical data. If this test had been applied to remove contaminated drainage from samples of the two dairy-farmed sub-catchments, median contaminant concentrations and loads would have decreased by up to 58% (greater decreases were found for E. coli, ammoniacal-N and total P than other contaminants). This suggests that better uptake of strategies to mitigate contamination, such as deferred effluent irrigation (and low rate application), could decrease drainage losses from dairy-farmed land and thereby improve water quality in the Pomahaka catchment. |
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Keywords: | Phosphorus Point source Watershed |
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