By Bob Wilcock
About 40% of the land area of New Zealand is in some form of agriculture. Sheep and beef farming are the most extensive (33%) followed by dairy farming at 6%, and the remainder being horticulture and cropping. Based on a large number of comparative land use studies we have a good understanding of how agriculture affects water quality and know that about 97% of the nutrient loads entering our freshwaters are from diffuse sources, in contrast with point-sources such as pipes and wastewater discharges.
The cumulative effects of more than one of these contaminant may be greater than the sum of their individual parts. For example, elevated levels of N and P may stimulate vigorous plant growth that results in high pH levels during late afternoon and thereby exacerbate the toxicity of ammonia to fish and aquatic insects.
In general, the order for yields from greatest to least, is as follows:
N: flat > rolling ~ easy ~ steep land
P: steep > easy ~ rolling > flat land
Sediment: steep > easy > rolling ~ flat land
The major source of E. coli in most farming systems is via overland flow from ruminant faeces and this is likely to be greatest on steeper land, although this is not the case where large herds of cattle are allowed direct access to waterways.
Estimates have been made using the SPARROW (SPAtially Referenced Regressions On Watershed attributes) model to estimate their relative contributions of nitrogen (N) and phosphorus (P) to freshwaters and to the coasts of New Zealand. Dairying and sheep+beef farming each contribute 30-40% of N entering freshwaters and the coast, with forests contributing most of the remainder. About 50% of P entering freshwaters and the coast is in sediment, about 20% from sheep+beef farming and 10% from dairying.
A broad suite of mitigation measures is available to farmers and offers some hope that increased production need not be accompanied by water quality degradation, so long as they are widely adopted (PDF; Waikato Regional Council’s ‘menus of practices’).
Dr Bob Wilcock is a NIWA Principal Scientist and Programme Leader – Causes and Effects of Water Quality Degradation
Elliott, A.H.; Alexander, R.B.; Schwarz, G.E.; Shankar, U.; Sukias, J.P.S.; McBride, G.B. (2005). Estimation of nutrient sources and transport for New Zealand using the hybrid mechanistic-statistical model SPARROW. Journal of Hydrology (NZ) 44: 1–27. (Abstract and PDF)
McDowell, R.W.; Wilcock, R.J. (2008). Water quality and the effects of different pastoral animals. New Zealand Veterinary Journal 56(6): 289-296. (Abstract)