Given the backdrop of the Government’s announcement in May 2018 on the introduction of dairy cow stocking rate limits to reduce nitrates entering our waterways, it is timely to share the science-based research WWA has been undertaking to assist in decision making.
Our focus has been on mitigation of nitrogen entering waterways rather than stopping nitrogen leaching from the land per se. This recognises that not all land is equal with respect to its vulnerability and environmental impact of nitrogen leaching. Nitrogen reduction occurs under environmental conditions that are deprived of oxygen and/or earth materials with high organic carbon context. Wetlands and deep aquifers are just two examples of naturally occurring systems where denitrification occurs (albeit through different mechanisms).
Our research investigated deep aquifers and focussed on identifying the nitrogen risk to the environment of different land parcels within the landscape mosaic by tracking the movement and attenuation of constituents in groundwater from sub-soil drainage to discharge at sinks (drains, streams, rivers, lakes or wetlands).
We use the analogy of the
“Long Game of Golf” to explain the concept, where the longer the game continues, the greater the chance you have of being rained upon. It is the same with groundwater flow, the longer and deeper the flow path, the greater the probability of denitrification occurring.
Using three-dimensional groundwater simulation models, we can generate land vulnerability maps on a grid basis across the landscape that identify the risk profile of land parcels in particular parts of the catchment (see example below). Low vulnerability areas are where there is significant natural denitrification occurring, that is, a significant proportion of the nitrogen leached from these areas will be denitrified before it reaches the waterways. The maps enable farm-scale catchment management decisions to be made, such as “what is the stock carrying capacity of various paddocks within my farm” and “if I was to retire high-risk paddocks from intensive grazing, could I achieve at least a similar economic outcome by offsetting stock in low-risk areas”?
This research is data and time intensive and has been occurring in earnest since the release of The National Policy Statement for Freshwater Management in 2014. Hopefully Government is aware of studies like this (and others) and will engage with scientists to inform policy.