Environment Southland soil and science programme
co-ordinator George Ledgard inspects a crop of swedes at
Makarewa, near Invercargill. Photo by Allison Beckham.
Environment Southland staff are assessing the most
cost-effective way to map land planted in winter forage crops
using satellite imagery, after a pilot study last year showed
the most accurate method was also the most expensive.
The council wanted to map the extent of crops such as kale
and swedes, the use of which was known to lift levels of soil
contaminants including nitrogen, phosphorous, sediment and
E.coli, council soil and science programme co-ordinator
George Ledgard said.
Satellite mapping would help the council understand how many
hectares of winter crops were being grown and improve
nutrient loss estimates at a catchment and regional scale, he
That information would feed into the council's Water and Land
2020 project - the council's response to the National Policy
Statement for Fresh Water - which requires regional councils
to set catchment water quality limits.
Most of the contamination occurred when crops were grazed by
a high concentration of stock, he said. Grazing could also
leave ''puggy paddocks'' which during periods of heavy
rainfall could lead to sediment-laden water flowing overland
into nearby waterways.
Recent research by AgResearch showed winter forage crops
could leach more than 60kg of nitrate and 5500kg of suspended
solids per hectare per year.
The number of hectares in crops across the province in winter
crops was a ''best guess'' by the council, he said, the
estimate being 40,000-60,000ha, but he had been told the
figure could be high as 120,000ha.
''Being able to map more accurately will make our science
more robust and will give us a better understanding of the
winter forage crop landscape.''
Last year, the council hired Landcare Research to carry out a
pilot satellite imagery mapping project over about 800sq km
in the Gore/Mataura/Edendale/Hokonui Hills area.
Mr Ledgard would not disclose the cost of the project, saying
it was commercially sensitive.
Free satellite imaging such as Google Earth and LandSat was
used in the trial, as well as high-resolution satellite
imagery the council had to pay for.
All the systems could map crops but some were better. The
best results were from multiple images of the same land but
that was also the most expensive and technically challenging
''The next step in this project is to test the relative
difference in accuracy between the methods and decide on the
method most suitable for our needs.''
One of the issues was how best to map hilly land with
paddocks shaded by terrain, he said.
Mr Ledgard said mapping would only ever be used as a
''snapshot in time'' analysis and at this stage he believed
the free imaging available would be sufficient.
''We are looking for an overview, and I don't believe the
cost of using high-resolution imagery can be justified.''