Anne Kinsey-Henderson

River loads of suspended solids, nitrogen, phosphorus and herbicides delivered to the Great Barrier Reef lagoon

Degradation of coastal ecosystems in the Great Barrier Reef (GBR) lagoon, Australia, has been linked with increased land-based runoff of suspended solids, nutrients and pesticides since European settlement. This study estimated the increase in river loads for all 35 GBR basins, using the best available estimates of pre-European and current loads derived from catchment modelling and monitoring. The mean-annual load to the GBR lagoon for (i) total suspended solids has increased by 5.5 times to 17,000ktonnes/year, (ii) total nitrogen by 5.7 times to 80,000tonnes/year, (iii) total phosphorus by 8.9 times to 16,000tonnes/year, and (iv) PSII herbicides is 30,000kg/year. The increases in river loads differ across the 10 pollutants and 35 basins examined, reflecting differences in surface runoff, urbanisation, deforestation, agricultural practices, mining and retention by reservoirs. These estimates will facilitate target setting for water quality and desired ecosystem states, and enable prioritisation of critical sources for management.

Interpreting aerial gamma-ray surveys utilising geomorphological and weathering models

Abstract Applying weathering and geomorphological data can improve the interpretation of aerial gamma-ray surveys. Data from two potentially mineralised areas in eastern Australia are used to demonstrate the new approach. At Develin Creek, NW of Rockhampton in Queensland, Permian andesitic basalts host VMS-style Cu-Zn deposits. Topographic and SPOT data were used to generate a digital elevation model (DEM) over an area of approx. 125 km 2 . The Universal Soil Loss Equation (USLE) was then applied to model soil erosion and deposition. Surface estimates of the distribution of clay, iron-oxides and vegetation were derived from Landsat TM data. Multi-variate linear regression and analysis of residuals (i.e. the difference between predicted and observed maps for each radioelement) were then used to look for anomalies that may indicate near-surface mineralisation. This analysis indicated no surface indicators of undiscovered mineralisation, a finding in accord with extensive ground geophysics and mapping. At Barry, SW of Bathurst, NSW, Ordovician basalts potentially host porphyry-related gold deposits. These deposits are most probably seen in aerial gamma-ray survey data through elevated K in alteration haloes. A DEM was created over a study area of approx. 180 km 2 and the soil erosion/deposition characteristics modelled using the USLE. Laboratory analyses of rock and soil samples showed that weathering of the basalt (median radioelement contents of 1.4% K, 0.7 ppm eU, 1.6 ppm eTh) produced a soil with decreased K (0.8%) but increased eU (1.5 ppm) and eTh (7.0 ppm). These values were used along with the erosion model to predict the surface K distribution, which was then compared to the observed distribution using a linear regression model. Areas with high K residuals are considered prospective for Au mineralisation.

Modelling sources of sediment at sub-catchment scale: An example from the Burdekin catchment, North Queensland, Australia

A detailed sediment budget has been derived for the Weany Creek sub-catchment (13.5km^2) in North Queensland, Australia, using a sediment source, transport and depositional model known as SedNet (I. Prosser, P. Rustomji, W. Young, C. Moran, A. Hughes, CSIRO Land and Water Technical Report 15/01, Constructing River Basin Sediment Budgets for the National Land and Water Resources Audit, 2001, http://www.clw.csiro.au/publications/technical2001/tr15-01.pdf.). In this study, we have applied SedNet at a much finer scale than it has been applied previously in order to determine sources and sinks of sediment at a scale suitable for grazing land management. We have been able to show that a model such as SedNet can be applied to finer scales (with scale-appropriate modifications to inputs) and in so doing provide insight into sources and sinks of sediment at sub-catchment or paddock scale. The Weany Creek model has been able to show in detail which stream sections (or their associated watersheds) contribute most to suspended sediment loads, as well as where bedload deposits are most likely to accumulate. Additional detail regarding whether erosion is derived from hillslope, or combined gully/bank erosion, or both, has provided valuable insight into the sub-catchment-scale dynamics of erosion. The Weany Creek model has been also used to determine the impact of changes in management practice on sediment erosion, transport and accumulation. A scenario with reduced stocking rates was shown to reduce the delivery of sediment to stream, and consequently led to a lower delivery of fine sediment to downstream reaches.

Persistence of in-stream waterholes in ephemeral rivers of tropical northern Australia and potential impacts of climate change

Many northern Australian rivers have limited or non-existent dry season flow and rivers tend to dry to a series of pools, or waterholes, which become particularly important refugial habitat for aquatic biota during the periods between streamflow events. The present study developed techniques to identify in-stream waterholes across large and inaccessible areas of the Flinders and Gilbert catchments using Landsat imagery. Application of this technique to 400 scenes between 2003 and 2010 facilitated the identification of key waterhole refugia that are likely to persist during all years. Relationships for predicting total waterhole area from streamflow characteristics were produced for four river reaches. Using these relationships and streamflow predictions based upon climate data scaled using 15 global climate models, the potential impacts of future climate on waterhole persistence was assessed. Reductions in waterhole area of more than 60% were modelled in some years under drier scenarios and this represents a large reduction in available habitat for areas that already have limited in-stream refugia. Conversely, under wetter future climates the total area of waterholes increased. The approach developed here has applicability in other catchments, both in Australia and globally, and for assessing the impacts of changed flow resulting from water resource development.

Grazing impacts on gully dynamics indicate approaches for gully erosion control in northeast Australia: Grazing impacts on gully erosion in northeast Australia

Drainage network extension in semi-arid rangelands has contributed to a large increase in the amount of fine sediment delivered to the coastal lagoon of the Great Barrier Reef, but gully erosion rates and dynamics are poorly understood. This study monitored annual erosion, deposition and vegetation cover in six gullies for 13 years, in granite-derived soils of the tropical Burdekin River basin. We also monitored a further 11 gullies in three nearby catchments for 4 years to investigate the effects of grazing intensity. Under livestock grazing, the long-term fine sediment yield from the planform area of gullies was 6.1 t ha yr. This was 7.3 times the catchment sediment yield, indicating that gullies were erosion hotspots within the catchment. It was estimated that gully erosion supplied between 29 and 44% of catchment sediment yield from 4.5% of catchment area, of which 85%was derived from gully wall erosion. Under long-term livestock exclusion gully sediment yields were 77% lower than those of grazed gullies due to smaller gully extent, and lower erosion rates especially on gully walls. Gully wall erosion will continue to be a major landscape sediment source that is sensitive to grazing pressure, long after gully length and depth have stabilised. Wall erosion was generally lower at higher levels of wall vegetation cover, suggesting that yield could be reduced by increasing cover. Annual variations in gully head erosion and net sediment yield were strongly dependent on annual rainfall and runoff, suggesting that sediment yield would also be reduced if surface runoff could be reduced. Deposition occurred in the downstream valley segments of most gullies. This study concludes that reducing livestock grazing pressure within and around gullies in hillslope drainage lines is a primary method of gully erosion control, which could deliver substantial reductions in sediment yield. Copyright

Insights into the history and timing of post-European land use disturbance on sedimentation rates in catchments draining to the Great Barrier Reef

Sediment runoff has been cited as a major contributor to the declining health of the Great Barrier Reef (GBR), however, climate and land use drivers have not been jointly evaluated. This study used alluvial archives from fluvial benches in two tributaries of the Upper Burdekin catchment together with the best available land use history and climate proxy records to provide insights into the timing of depositional events in this region over the past 500 years. This study suggests that mining and the increased runoff variability in the latter half of the nineteenth century are the likely sources of the original excess sediment that was used to build the bench features in these catchments. Grazing also contributed to increased bench sedimentation prior to 1900, however, the contribution of grazing was likely more significant in the second half of the 20th century, and continues to be a dominant land use contributor today.

Optimising drainage from sugar cane fields using a one-dimensional flow routing model: a case study from Ripple Creek, North Queensland

Abstract Inundation of sugar cane fields in the Ripple Creek catchment is perceived by some cane farmers to be a major contributor to loss of production. To alleviate this inundation, drainage systems have been designed which are capable of removing surface water very rapidly. However, when the Herbert River is in flood this water may have nowhere to go. This combination of an efficient drainage system, the lack of an outlet for the water, and the growing of cane on land which may be unsuitable, has led to some farms facing extended periods of inundation. For this reason, proposals have been made to the Ripple Creek Drainage Board to modify the drainage system to produce a more equitable distribution of inundation. In this paper, we examined the application of the Mike-11 modelling system to the Ripple Creek catchment. We found that the modelled predictions of depth, discharge and velocity matched observational data collected as part of an intensive CSIRO monitoring program. The model developed was then used to examine the hydrologic impact of opening a new channel to divert flow into the Seymour River. We found that opening this channel had some impact on indundation in parts of the catchment, but that the impact depended on whether or not the Herbert River was in flood at the time.