The Normanby River and Endeavour River are two river systems flowing into the Great Barrier Reef lagoon. The Normanby Basin is the fourth largest on Cape York (~24,200 km2), while the Endeavour River catchment is much smaller (1315 km2). Cattle grazing is the most extensive land use across both catchments, with areas of horticulture occurring on the rich red basalt soils around Lakeland (Normanby Basin) and along the Endeavour River. Previous water quality studies have shown elevated levels of sediments and nutrients associated with these land use developments, particularly in the Normanby Basin. Further details on the basin characteristics and water quality issues can be found in reports from other studies.
A rainfall simulation study was undertaken in April 2015 to provide supporting data for extension activities in grazing and horticulture across the Cape York region. Simulated rainfall was used to assess and compare the runoff and water quality from various soil types and management practices (including grazing, horticulture and internal property roads) within the Normanby and Endeavour River catchments. Simulations were undertaken on a combination of 19 differing soil type/land use/management practice combinations.
Simulated rainfall was applied to each plot at a rate of approximately 80 mm/hr for one hour. Runoff rates were manually measured from the outlet of each plot by recording the time taken to fill a measured volume. Water quality samples were collected at 5 or 10 minute intervals (6 samples collected from each simulation run), depending on when runoff commenced. Water samples were chilled on collection, and sub-sampled and filtered at the end of each day. Nutrient samples were then frozen, and the sediment samples kept cool, and submitted to the laboratory for analyses (electrical conductivity, sediment and nutrient (total and filtered) concentrations).
Water quality loads (kg/ha) and event mean concentrations (EMC; mg/L) were calculated for each parameter. In summary, these results highlight:
- Runoff – the combination of soil types and management showed that uncompacted highly permeable soils did not run off (banana row and ungrazed pastures), ranging up to almost 100% runoff from a scalded gully head on a sodic duplex soil.
- Sediment loss (TSS) – sediment loss from those plots that ran off ranged from 24 kg/ha (brown earth pasture; low runoff) to 9260 kg/ha (gully head on sodic duplex soil; high runoff and sediment concentration). Roads produced at least 16 times more sediment loss than pasture sites on the same soil type due to more runoff and higher sediment concentrations. The gully head on the sodic duplex soil produced ~42 times more sediment loss than the pasture site on the same soil. These results highlight the need for erosion management of roads, and maintaining pasture cover on fragile sodic soils to prevent gully initiation.
- Nutrients – total N and P loads in runoff from horticultural sites were within the range of the loads measured from the pasture sites. Roads produced more TN and TP than pasture, due to the higher runoff and sediment concentrations. The elevated TKN (and therefore TN) loads from the brown earth road may be due to the influence of cattle excrement. Runoff of nitrogen and phosphorus was dominated by organic forms at all sites, except the dragon fruit site which may have been due to applied fertiliser.
The results of this rainfall simulation study are supported by those observed from other studies. The results highlight the potential for elevated sediment and nutrient runoff losses if the soil type is not managed appropriately.
Client – Cape York NRM for Queensland Government
Read the Final Report on Rainfall simulations of land management practices in the Normanby and Endeavour River catchments.