Does variable rate irrigation decrease the loss of water quality contaminants from grazed dairy farming?
The irrigation of grazed dairy farms in New Zealand has been cited as a cause of the contamination of shallow groundwater with nutrients (nitrogen – N and phosphorus - P) and the faecal indicator bacteria – E. coli. Drainage from uniform spray irrigation (URI) systems can be decreased if irrigation is matched to variations in soil type. Decreasing drainage with variable rate irrigation (VRI) may also decrease contaminant losses. However, no direct measurements or comparison of contaminant losses to shallow groundwater from VRI compared to URI systems exist under grazed dairying. Over a five year period, fortnightly baseflow and periodic stormflow samples were taken from artificial drainage channels that crossed a 150-ha dairy farm, but entered and exited the farm at single points upslope and downslope of the irrigated area. These channels sat just above the water table (on average 1.1-m below the soil surface, and therefore served as good proxies for contaminant concentration in shallow groundwater. Irrigation was delivered via a centre pivot irrigator at a uniform rate (18-mm per week) from October to May for three years. Irrigation then changed to rates that varied from 3-20 mm per week according to the mean daily soil moisture deficit in the top 20-cm (detected at three locations over the farm). Under URI, median concentrations of dissolved reactive P, total P, ammoniacal-N, E. coli were not significantly different upslope and downslope of the irrigated area, whereas both nitrate-N and total N showed increases downslope (53 and 28%, respectively). Following the installation of VRI, median concentrations of all analytes except E. coli at the downslope site decreased by at least 50%. Approximately 40% less water was used under VRI than under URI, while median flow rates at the downslope site decreased by 27%. Since the application rates of N and P did not change during the trial, the greater decrease in nutrient losses compared to drainage suggests that nutrient use efficiency had also improved. Pasture production and milk solids per hectare did not decrease following the installation of VRI, providing surplus water to expand production if desired. The data suggest VRI decreases the loss of contaminants to shallow groundwater under grazed dairy farming compared to those using URI.