Water

Water use

Irrigation is increasingly used as a means to increase agricultural productivity in New Zealand. ⁴¹⁰⁷ Almost all horticultural production requires irrigation. In 2017, the total consented irrigated land is estimated to be in the order of 1.1 million hectares. The consented irrigated area within the Canterbury region accounts for nearly two thirds of this area. ⁴¹⁰⁸   The growth of irrigation in certain areas of the country is due to a desire to sustain and intensify farming enterprises in areas which are relatively water-poor. For example, the east coast of the South Island receives relatively little rainfall, because the location of the Southern Alps intercepts the prevailing weather, causing precipitation to fall before it reaches the region.

Issues arise where under low flow conditions peak demand (mainly for irrigation), cannot be met by current abstraction methods or sustained using the current methods of use. Methods to store surface water from lakes and rivers need to be found if the resource managers are to keep up with demand and to ensure that the environmental impacts on water quality of low flow levels are mitigated and environmental limits set under the National Policy Statement for Freshwater Management are met. However, suitable places to store water are few and far between, as they generally require the flooding of land. This is often land which has other important uses and environmental values. The horticulture sector in Gisbourne is currently investigating alternatives to surface-based storage and is investing in Managed Aquifer Recharge as an alternative storage method. The impact of intensification on water quality must also be considered. A solution which addresses both water demand and avoids low flows in rivers addressing both water quality and quantity issues and futureproofing against climate change is a real challenge for this generation to overcome.

Water quality

Whilst pollution from point sources has been decreasing over the last decades, non-point source leaching of nutrients has been identified as an issue. ⁴⁶⁵⁶ The National River Water Quality Network indicated that in respect of the period from 1989 to 2007, there was a general trend towards improvement in visual clarity of water, but levels of nutrients such as phosphorus and nitrogen were increasing in some catchments. Expansion and intensification of agricultural land use has caused increases in nutrients in waterways through leaching and runoff in some areas. ⁴¹⁰⁹

Agricultural has been identified as a source of increasing nitrogen in waterways. As native vegetation is turned into pasture, wetlands drained and stream bank vegetation removed, the land becomes less able to absorb water. As a result more nutrients are introduced into waterways from surface run off, delivering sediment, organic matter (effluent) and nutrients that have been deposited on the land. Leaching of contaminants through the soil profile further increases the nutrient loadings in water systems.

The issues relating to nitrogen especially have created an increased awareness of the effects that agricultural activity can have on waterways and how it is dealt with at a national level under the RMA.

Nutrient pollution

Increase nutrients in waterways, predominantly in the form of nitrogen and phosphorus, is one of the most significant byproducts of agricultural activity. The nutrients can come from a number of sources including fertiliser which is used in horticulture. Permanent crops use relatively low levels of nitrogen and phosphorus based fertilisers. However vegetation production systems often require more due to the nature of the plants in the systems. For example leafy green crops tend to have inefficient and small root systems; requiring greater fertiliser inputs. Areas like Horowhenua and Pukekohe tend to use increased fertiliser in the months of October, November and December to meet market demand. 

Horticultural and arable land uses make up less than two per cent of the country’s land area, a tiny portion compared with other agricultural land uses. Reports show that roughly five per cent of New Zealand's land area is suitable for soil-based horticulture in total meaning there are limits to the potential size of the nitrogen and phosphorous footprint. Land suitable for high-value horticulture is also under significant threat from rural-residential and residential subdivision in some key parts of New Zealand, limiting opportunities for expansion of the sector in traditional ways. 

Most of the land used for horticulture is flat or slightly undulating, which means that it does not produce large amounts of runoff. The slope of the land is a key driver for erosion and sediment discharges. The one place in New Zealand where cultivation occurs on rolling land is Pukekohe; and in Pukekohe there are substantive engineered erosion mitigation solutions in place. However, there is still the potential for runoff of sediment and phosphrous (in soil) into surface water during heavy rain or for nitrate to leach through the soil profile into groundwater. Mitigations for soil and phosphorus discharges potentially increase the drainage of nitrogen below the root zone. {im:4110}}

Runoff is more likely to occur when soils are cultivated close to the water’s edge, without an appropriate series of mitigations in place. This can have marked effects on freshwater quality in some areas. On modest slopes soil loss is often an issue in heavy rainfall events, mitigation such as silt traps, earth bunds, deep ripping, paddock contouring, cover crops and riparian planting are ways to mitigate sediment entering waterways from cropping land. 

In working with growers, Horticulture New Zealand have produced a Code of Practice for Nutrient Management and Erosion Sediment Management Guidelines to reduce nutrient leaching and ensure industry best practice.  There is also a national network of fluxmeters measuring discharges in real-time to better quantify the nutrient footprint and explore options to reduce it.

Sedimentation

Although most forest in New Zealand was removed between 800 and 80 years ago (in two waves with initial Maori impacts followed by extensive impacts of European settlement), the effects of these changes in the landscape continue to be felt, as the hills and riparian areas continue to be used for agricultural production.

The removal of hill and riparian forests for agricultural use increases the amount of rainfall running off the land into streams and rivers. The tree canopy in a mid-latitude forest typically intercepts and then evaporates away up to one third of the rain that falls on that canopy, effectively reducing the rain that reaches the ground. Canopy interception losses are a function of effective leaf area and canopy roughness, and in west coast locations with frequent rain, can be significantly greater than water losses to transpiration (water drawn up through the tree roots via photosynthesis). There is little difference in canopy interception losses between native and exotic forests, tall manuka and kanuka or dense infestations of gorse or broom, provided that a closed canopy exists.

But such interception losses are far greater than that occurring under tussock, short pasture grasses or crops. In contrast to the highly compacted soils often found under pastoral or arable agricultural systems, forest soils have much higher infiltration capacities and seldom generate overland flow or surface run-off. When forest is replaced by short-rooted pasture grass, increased amounts of rainfall are allowed to run off the land. In the absence of tree roots to hold the soil together, greater amounts of sediment made up of inorganic matter (rock and soil) and organic matter (mostly animal faeces), are washed from the land into groundwater, streams, rivers and wetlands.

Horticultural farmers who rely on cultivation have learned over time not to leave land in fallow because it is during the resting periods that cover crops can be utilised to increase the carbon content of soil. Common cover crops grown include mustard, oats, rye grass and arable crops. In cultivated vegetable cropping systems, there is a significant reliance on rotation to protect soil health. It is a system that has endured for thousands of years. It has resulted in a high proportion of commercial vegetable cropping being conducted on shared or leased land. As a result, many of the mitigations for sediment control are designed to use on short-term rotation land.

Last updated at 6:09PM on February 1, 2018