Nutrient losses associated with irrigation, intensification and management of land use: A study of large scale irrigation in North Otago, New Zealand

Irrigation of pasture enables the intensification of land use, but can also result in increased losses of nitrogen (N) and phosphorus (P). In 2006 an irrigation scheme was introduced into the Kakanui River and Waiareka Creek catchments in North Otago, New Zealand, which has intensified land use, especially dairying. Supplementation of the Waiareka Creek by direct discharge of 'clean' irrigation water from a nearby River is practiced to raise the minimum flow. This supplementation is hypothesized to dilute N and P losses associated with increased land use intensification and irrigation return flow. Farm losses of N and P before irrigation were then used as a reference to judge in the Kakanui River, and compare against dilution in the Waiareka Creek, the effectiveness of best management practices (BMPs) to improve water quality in 2010 and 2020. Data for N and P fractions from three sites since the mid 1990s were analysed, and flow adjustments for direct discharge to the Waiareka Creek made. Similar concentrations of N and P fractions in Waiareka Creek before and after irrigation began suggest the current minimum flow of 100Â LÂ s-1 is insufficient to improve the nutrient status of the Creek, but does dilute recent intensification, which without dilution would have increased concentrations by 30-400%. In the lower Kakanui catchment, direct discharge does not occur and N and P concentrations increased, while little change occurred in the upper Kakanui catchment. Within each catchment, N and P losses from sheep and beef farms and dairy farms (with and without BMPs) were modelled for 2010 and 2020 and compared against that estimated in 2000. This showed that although substantial decreases could be made by adopting BMPs, the predicted increase in N and P losses (up to 200% by 2020) would require either more rigorous use of existing strategies or additional strategies to improve water quality, over and above dilution which is restricted by a need to minimise the risk of flooding.