Irrigation is good for farming and for the environment. This message needs to be made loud and clear. So what are three key good things about irrigation for the environment?
1. Irrigation reduces nutrient loss
Irrigation is a very effective tool to mitigate N loss through leaching. Irrigation ensures that the plant is actively growing, and if it is growing, it is relatively easy to work out the nutrient requirement of the plant for each stage of growth and apply the amount of nutrients, particularly nitrogen, to meet the plant demand. This means there is no need to apply extra N, as all N applied is available to the actively growing plant. This is apparent from FAR research which shows 7 kg/tonne of wheat less nitrogen is required to optimise yield in irrigated wheat than in dryland wheat crops. The extra N applied in the dryland crop is required to ensure N is available when the plant needs it, irrespective of the weather conditions. In many situations, the extra N in dryland crops is not utilised as it is not available to the plant due to unfavourable soil conditions. This means there is the potential for it to be leached by a rainfall event during the growing season or as excess N at the end of the season.
2. Irrigation improves soil water holding capacity and reduces irrigation demand
Long term trials (14 years) comparing irrigation and dryland crop production on a Templeton soil at the FAR Chertsey Research site have shown increases in soil carbon in the irrigated treatments compared to the dryland, 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. This may not sound like much but, if there were 10 rainfall events which returned the soil to field capacity this would supply an extra 10mm to the crop which would then not be needed from irrigation. While this will result in savings for farmers of say $25 per ha, the major saving is in the volume of water applied. There are approximately 130,000 ha of irrigated arable land in New Zealand and if each of these hectares can store an extra mm of water, due to increased water holding capacity, then each rainfall event will save 1.2 billion litres of water.
3. 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 that less N 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. The average drainage figure from a fluxmeter project in the Canterbury region, across three farms over three years, was approximately 13mm/year and the average N loss was 6.7kg/N/ha or 0.5kgN/mm leached. Thus in the dryland scenario it is possible an extra 10mm of may be lost, which could equate to a loss of a further 5kg/ha of N to the ground water.
These are just some of the potential impacts of irrigation to improve the environment. Irrigation also means it is possible to have active plant growth throughout the year, thus reducing the risk of wind erosion or surface sediment runoff.
Irrigation can have huge benefits to the environment. These are the very things that irrigation is often claimed, with no limited scientific evidence, to cause. We need to celebrate that irrigation can increase stream flows reduce water extraction from aquifers and reduce N leaching to ground water.