Effectiveness of reuse dams on irrigated dairy farms in reducing discharge of water and nutrients to regional drainage systems

This paper explores the production and environmental risks associated with current reuse dam practices within the Shepparton Irrigation Region (SIR). Runoff from grazed border-check irrigated pastures has elevated nitrogen (N), phosphorus (P), salt (electrical conductivity, EC) and sediment levels due to direct washing or mobilisation of applied fertilisers, plants, animal excreta and soil during each irrigation event. Goulburn-Broken catchment managers encourage installation of reuse dams on dairy farms to intercept irrigation runoff and reduce irrigation-induced discharge of water and accompanying nutrients to regional drainage systems/receiving water bodies. Reuse dams are typically a below ground sump that collects surface runoff delivered by farm drains, with the water then pumped into farm delivery channels for use in other irrigations. Water supplied to farms is metered but reuse data are not collected. A preliminary study monitored reuse dam operation on 16 dairy farms for three week periods between December 2013 and May 2014. Data collected included dam water levels, evaporation losses, rainfall, groundwater and surface water delivery to the farm. The study found many reuse dams were operated in ways that deviated from current recommended practice, potentially increasing productive and environmental risk on and off farms (Finger and Kumaran 2014). To assess potential risks from current reuse dam practices, fifteen reuse dams in the SIR were monitored for five week periods between November 2014 and mid May 2015. In addition to variables measured in 2013/14, weekly water samples were collected from reuse systems and analysed for N, P, pH and EC. Depth-volume relationships for each dam were also determined. The observed reuse dam characteristics, hydrological performance, and nutrient and salinity concentration distributions for the monitored reuse dams are summarised. These attributes were then used to indicate whether the water contained in reuse dams exhibits qualities that pose risks to the environment or productive land if not managed appropriately.