New Zealand Journal of Agricultural Research abstracts
Minimising surface water pollution resulting from farm-dairy
effluent application to mole-pipe drained soils. II. The contribution
of preferential flow of effluent to whole-farm pollutant losses in
subsurface drainage from a West Otago dairy farm
R. M. Monaghan
AgResearch Ltd
Invermay Agricultural Centre
Private Bag 50 034
Mosgiel, New Zealand
L. C. Smith
AgResearch Ltd
Woodlands Research Centre
RD 1
Invercargill, New Zealand
Abstract To evaluate the role of artificial
drainage systems in the transfer of nutrients and faecal organisms from
soil to waterways, mole-pipe drainage flows were monitored from two
large (27 × 40 m), hydrologically isolated field plots
that were part of a long-term dairy pasture in West Otago, New Zealand.
One plot was grazed only whilst the other plot was spray irrigated with
untreated dairy-shed effluent on seven occasions, shortly following
grazing events in spring, summer or autumn. Monitoring throughout the
4-year study showed that relatively large amounts of ammonium-N,
total-P, and Escherichia coli (E. coli) bacteria were
transported through the artificial drainage system via direct drainage
of effluent following three of the seven effluent irrigation events.
These effluent drainage events occurred when the maximum depth of
effluent application exceeded the soil water deficit measured in the
0–45-cm layer. A pronounced non-uniform pattern of effluent application
was observed, with areas to the outside of the irrigator run
effectively receiving double the average application depth, and at an
instantaneous rate greater than 100 mm h–1. Although
the volumes of effluent transported in drainage flow were relatively
small, the concentrations and loads of ammonium-N, total-P, and E.
coli bacteria in the resulting drainage were large. Based on the
measured irrigation uniformities and soil water balance calculations, a
simple model of effluent flow through mole-pipe drained soils was
developed. Model outputs indicated that two key management strategies
could avoid or reduce the transport of pollutants from these
effluent-treated soils, specifically (i) increasing irrigator
groundspeed and (ii) storing effluent when soil conditions are wet.
Failure to implement these management practices is likely to result in
the delivery of large amounts of pollutants to surface waterways when
flows in the receiving surface waters are low and temperatures
relatively high.
Keywords effluent irrigation; mole-pipe drainage;
preferential flow; ammonium-N; total-P; E. coli bacteria
A04041; Received 30 April 2004; accepted 15 September 2004; Online
publication date 15 December 2004
New Zealand Journal of Agricultural Research, 2004, Vol. 47:
417–428
0028–8233/04/4704–0417 © The Royal Society of New Zealand 2004
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