All we hear in the media is that irrigation is bad for the environment and that it is intensifying agriculture, which is also assumed to be bad. These messages that irrigation is bad are usually provided with little, if any, supporting scientific data. While bad irrigation can result in excess nutrient loss, good irrigation can reduce nitrogen loss to ground water and improve soil quality. It also has the potential to improve river flows, and reduce the risk of erosion.
FAR research is providing scientific data tp show just how can irrigation benefit the environment.
Irrigation can help reduce nutrient loss
Irrigation is a very effective tool to mitigate nitrogen loss through leaching. Irrigation ensures that the plant is actively growing and, if it is growing, it is relatively easy to i) work out the nutrient requirement of the plant for each stage of growth, ii) estimate the amount of nitrogen available from the soil and iii) apply the requisite amount of nutrients, particularly nitrogen, to meet the plant’s demand.
This is demonstrated by FAR research that shows the amount of nitrogen fertiliser required to optimise yields is 7 kg/tonne of wheat produced higher in dryland wheat crops than it is in irrigated crops. The extra nitrogen applied in the dryland crop is required to ensure nitrogen is available when the plant needs it, irrespective of the weather conditions. In many situations, this extra nitrogen applied to dryland crops is not utilised, due to unfavourable soil and growing conditions, leading to the potential for it to be leached by a rainfall event during, or at the end of, the growing season. As well as this, irrigation reduces any nitrogen lost to the atmosphere through volatilisation.
Irrigation improves soil water holding capacity
Long term trials (14 years) comparing irrigation and dryland crop production on a Templeton soil at the FAR Chertsey Research Site in Mid Canterbury have shown increases in soil carbon in the irrigated treatments compared to the dryland ones, irrespective of the crop establishment practice. This soil improvement has resulted in a 3% increase in the water holding capacity of the soil, which means that at field capacity it will hold an extra 1mm of water.
Improvements in soil quality reduce N leaching
Improving soil quality through irrigation so that it has a higher water holding capacity means there is less drainage, and less drainage means less nitrogen is leached. If there are 10 rainfall events per year where the soil moisture exceeds field capacity, then, using the scenario above, that is 10mm less water that is lost in drainage. An industry led science project is using fluxmeters to measure drainage below the root zone and nitrogen concentration in drainage water. In this project, the average drainage figure from across three irrigated farms in Canterbury over two and a half years, was approximately 13mm/year and the average nitrogen loss was 6.7kg/N/ha or 0.5kgN/mm leached. Thus in the dryland scenario above, it is possible an extra 10mm of water may be lost, which could equate to a loss of a further 5kg/ha of N to the ground water. To be conservative and apply this to only half the irrigated area of cropped land in New Zealand this would reduce nitrogen loss to ground water by 3750 tonnes.
Irrigation also means it is possible to have active plant growth throughout the year, reducing the risk of wind erosion or surface sediment runoff. In the cropping situation, irrigation can be used to help establish crops if there is insufficient soil moisture for germination and growth. Earlier establishment of crops in the autumn not only reduces the risk of water or wind erosion and the subsequent loss of phosphorus in sediments, but it also means the crop has established prior to the winter. Early establishment means earlier development of an extensive root system and more plant dry matter (bigger plants), factors which result in increased plant uptake of nitrogen, which would otherwise have been susceptible to leaching, from the soil profile.
It is acknowledged that not all irrigation is benefiting the environment, but instead of constantly criticising irrigation as damaging the environment we should also be celebrating its benefits to the environment. The next step is to look at improving the poor irrigation practices and determining how we can efficiently irrigate more land in high risk areas, such as the Ruataniwha and the Wairarapa, to benefit the New Zealand economy and the environment. Alongside good scientific data that illustrates the benefits of irrigation, we also need good scientific data that demonstrates its negative impacts, so these impacts can be addressed. Using good quality scientific data to support irrigation management should result in increased stream flows, reduced water extraction from aquifers, reduced nitrogen leaching to ground water and reduced erosion, sediment and phosphate loss.