Bright, J.C., McIndoe, I. and Birendra, K.C., 2019. Reducing nutrient losses through improving irrigation efficiency. In: Nutrient loss mitigations for compliance in agriculture. (Eds L.D. Currie and C.L. Christensen). http://flrc.massey.ac.nz/publications.html. Occasional Report No. 32. Fertilizer and Lime Research Centre, Massey University, Palmerston North, New Zealand. 12 pages. 1 REDUCING NUTRIENT LOSSES THROUGH IMPROVING IRRIGATION EFFICIENCY John C Bright, Ian McIndoe and KC Birendra Aqualinc Research Ltd. PO Box 20462, Bishopdale, Christchurch 8543. Email: j.bright@aqualinc.co.nz Abstract A desk-top study using data from twelve representative dairy farms located in Canterbury with well-established computer models showed that changing irrigation practices to make more efficient use of both irrigation water and summer-time rainfall reduced N-loss to water by between 4% and 58% (average of 27%). These reductions in N-loss to water were achieved without significantly reducing modelled average annual pasture production. To achieve this degree of N-loss reduction, changes were made to the irrigation rule used to determine when to irrigate and how much water to apply so that the soil is allowed to dry out more in the shoulders of the irrigation season and reduce the risk of rainfall-induced drainage throughout the season. To apply the new rule it is essential that:  Soil water content is routinely measured using reputable soil moisture monitoring equipment.  The irrigation application system can be adjusted to apply relatively small amounts of water.  The irrigation application system has a relatively short return period.  The irrigation water supply is very reliable. Introduction The National Policy Statement for Freshwater Management 2017 (NPS-FM) requires all regional councils and unitary authorities to set limits on all forms of water use to achieve aquatic ecosystem and human health objectives. If current use, or allocations, in aggregate exceed the relevant limit they must be reduced to comply with the limit within a set time frame. In some catchments, or freshwater management units, water use has already been judged to be over-allocated in terms of total nutrient loss to water. Examples include the Selwyn-Waihora and Hinds Zones in Canterbury. Farmers in over allocated areas are having to reduce nutrient loss to water by a prescribed percentage of their baseline loss to water. In some areas this is now a regulatory requirement. In other areas voluntary reductions are being encouraged through education, training and commercial or peer pressure. Nitrate leaching to groundwater is a significant pathway for N-loss to both groundwater and surface water in intensively farmed areas, particularly if they are irrigated.