IRRIGATED PUBLIC OPEN SPACES
Using a lawn irrigation trial to find a balanced fertilisation/
watering approach for use in the wet-dry tropics
R Hasan, C Fairfield
Publication Date (Web): 10 May 2017
Irrigation consumes more water in Darwin than in many other cities in Australia. Darwin is the only Australian capital city not to have been subjected to water restrictions, and people in Darwin use around 2.1 times more water than other Australians (454 kL per property per annum; Australian average, 213 kL per property per annum (PWC, 2013)).
Inefficient irrigation wastes water and energy. Over-irrigation of invasive, non-native grasses - species not ideally suited to northern Australia in the first place - causes demand amounting to approximately 70% of the average domestic water consumption.
A vicious cycle
Permeable, nutrient-poor soils, suffer increased leaching of nutrients as a result of over-irrigation. Propagules and seeds are conveyed to drains and watercourses by over-irrigation, where they out-compete native species and adversely impact local ecosystems in swales, creeks, and coastal estuaries. Over-irrigation drives increased mowing frequencies, giving rise to municipal solid waste in the form of excessive amounts of grass clippings. Added chemical fertilisers drive growth, necessitating more frequent mowing, and causing leaching of excess nitrate load to drains and receiving waters. Increased mowing leads to increased soil erosion, exacerbating the nutrient loss and necessitating further irrigation and fertilisation in a vicious circle designed to keep the grass aesthetically appealing, regardless of the actual harm to the environment.
The treatment of water to a potable standard consumes significant energy, and to circulate this while over-irrigating lawns and public open spaces adds to the embedded energy burden of irrigation. The energy required to fertilise, mow, irrigate, re-fertilise and re-mow is only affordable in a financial sense.
A site north east of Darwin’s CBD was divided into plots of 3m by 3m and subjected to different regimes.
Three levels of 25:5:8.8 (N:P:K) fertiliser were applied (none, 50% and 100%), and two levels of irrigation were applied (11mm/week and 22mm/week). The dominant species present were Pangola grass, Gamba grass and Bahia grass. The soil was a nutrient-poor, gravelly clay loam with a high infiltration rate and a poor water-holding capacity. From the middle of July, the grass was regrown and after eight weeks it was cut and the biomass yield measured.
Is the grass greener?
There were two key findings from this study. The first is that less irrigation still delivers acceptable growth rates among the grass species studied. The second is that zero fertiliser use (with irrigation) reduced biomass yield by 50%.
Based on where over-irrigation falls in the water-energy nexus means that this issue will need to be addressed more broadly across northern Australia now and into the future.
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