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I still think of Canterbury as home. For a hundred years, farmers grew crops and ran sheep on the patchwork plains. But over the last twenty years, water has transformed much of this long-familiar landscape into bright green pasture grazed by dairy cows. Often the first sign of such change has been the felling of macrocarpa shelter belts to make way for irrigators up to a kilometre long travelling across paddocks. In parts of the North Island too, large-scale and rapid land use change has been taking place. For instance, north of Taupō, tens of thousands of hectares of pine forests have been felled and replaced by dairy farms.
These land use changes reflect the changing economics of farming. In 1982, the number of sheep in New Zealand peaked at 70 million; now there is less than half that number. Beef cattle numbers have also fallen. Changing how we use land in response to market signals is not new.
How ever we use our land, the quality of the water in our rivers and streams, lakes, estuaries and aquifers is affected. This report is focused on how current changes in land use are affecting the amounts of nitrogen and phosphorus that end up in fresh water. As explained in my 2012 report, Water quality in New Zealand: understanding the science, too much nitrogen and phosphorus in water lower its quality by causing excessive growth of weeds, slime and algae, affecting populations of insects, fish and waterbirds. On land, nitrogen and phosphorus are valuable nutrients; above certain concentrations in water, they are pollutants.
Correction: The Y axis on the Nitrogen graph in Figure 4.4 should run from 0 to 50 kgN/ha/yr. This has been corrected in the PDF available in the Download list.