Aaron Hawdon

Calibration and correction procedures for cosmic-ray neutron soil moisture probes located across Australia

The cosmic-ray probe (CRP) provides continuous estimates of soil moisture over an area of ~30 ha by counting fast neutrons produced from cosmic rays which are predominantly moderated by water molecules in the soil. This paper describes the setup, measurement correction procedures, and field calibration of CRPs at nine locations across Australia with contrasting soil type, climate, and land cover. These probes form the inaugural Australian CRP network, which is known as CosmOz. CRP measurements require neutron count rates to be corrected for effects of atmospheric pressure, water vapor pressure changes, and variations in incoming neutron intensity. We assess the magnitude and importance of these corrections and present standardized approaches for network-wide analysis. In particular, we present a new approach to correct for incoming neutron intensity variations and test its performance against existing procedures used in other studies. Our field calibration results indicate that a generalized calibration function for relating neutron counts to soil moisture is suitable for all soil types, with the possible exception of very sandy soils with low water content. Using multiple calibration data sets, we demonstrate that the generalized calibration function only applies after accounting for persistent sources of hydrogen in the soil profile. Finally, we demonstrate that by following standardized correction procedures and scaling neutron counting rates of all CRPs to a single reference location, differences in calibrations between sites are related to site biomass. This observation provides a means for estimating biomass at a given location or for deriving coefficients for the calibration function in the absence of field calibration data.

Flood water quality and marine sediment and nutrient loads from the Tully and Murray catchments in north Queensland, Australia

Current estimates of sediment and nutrient loads from the Tully–Murray floodplain to the Great Barrier Reef lagoon are updated by taking explicit account of flood events. New estimates of flood discharge that include over-bank flows are combined with direct measurements of sediment and nutrient concentrations in flood waters to calculate the loads of sediment and nutrient delivered to the ocean during 13 floods that occurred between 2006 and 2008. Although absolute concentrations of sediment and nutrient were quite low, the large volume of water discharged during floods means that they make a large contribution (30–50%) to the marine load. By not accounting for flood flows correctly, previous estimates of the annual average discharge are 15% too low and annual loads of nitrogen and phosphorus are 47% and 32% too low respectively. However, because sediments may be source-limited, accounting for flood flows simply dilutes their concentration and the resulting annual average load is similar to that previously estimated. Flood waters also carry more dissolved organic nitrogen than dissolved inorganic nitrogen and this is the opposite of their concentrations in river water. Consequently, dissolved organic nitrogen loads to the ocean may be around twice those previously estimated from riverine data.

Field testing of the universal calibration function for determination of soil moisture with cosmic‐ray neutrons

The semitheoretical universal calibration function (UCF) for estimating soil moisture using cosmic-ray neutron sensors was tested by comparing to field measurements made with the same neutron detector across a range of climates, soil, latitude, altitude, and biomass. There was a strong correlation between neutron intensity and the total amount of hydrogen at each site; however, the relationship differed from that predicted by the UCF. A linear fit to field measurements explained 99% of the observed variation and provides a robust empirical means to estimate soil moisture at other sites. It was concluded that measurement errors, neutron count corrections, and scaling to remove altitudinal and geomagnetic differences were unlikely to explain differences between observations and the UCF. The differences may be attributable to the representation of organic carbon, biomass or detector geometry in the neutron particle code, or to differences in the neutron energy levels being measured by the cosmic-ray sensor and modeled using the particle code. The UCF was derived using simulations of epithermal neutrons; however, lower energy thermal neutrons may also be important. Using neutron transport code, we show the differences in response of thermal and epithermal neutrons to the relative size of the hydrogen pool. Including a thermal neutron component in addition to epithermal neutrons in a modified UCF provided a better match to field measurements; however, thermal neutron measurements are needed to confirm these results. A simpler generalized relationship for estimating soil moisture from neutron counts was also tested with encouraging results for low biomass sites.

Bulk soil moisture estimation using CosmOz cosmic ray sensor and ANFIS

This paper investigates a novel technique based on Fuzzy C means (FCM) algorithm and Adaptive Neuro-Fuzzy Inference System (ANFIS) to estimate soil moisture using cosmic ray soil moisture probes deployed across Australia. These probes are a brand new sensing technology still being evaluated. Preliminary results indicate ANFIS is able to estimate soil moisture with 90% accuracy.

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

The Lucinda Jetty Coastal Observatory's role in satellite ocean colour calibration and validation for Great Barrier Reef coastal waters

The Lucinda Jetty Coastal Observatory (LJCO) was established as part of Australias Integrated Marine Observing System, to provide reliable data streams in coastal waters to unravel the inaccuracies in remotely-sensed satellite ocean colour products, LJCO is located at the end of a 5.76 km long Lucinda Jetty (18.52S, 146.39E) in the coastal waters of the Great Barrier Reef World Heritage Area close to the Herbert River Estuary and the Hinchinbrook Channel. The Lucinda Jetty Coastal Observatory generates two complementary data streams: above water radiometry and in water measurement of the optical properties. An autonomous above-water radiometer performs radiometric measurements for determining water-leaving radiance and atmospheric measurements for retrieving aerosol optical properties. An in situ underwater instrument package is deployed to characterize the inherent optical properties of these complex coastal waters. The instruments were commissioned in October 2009. Details on the instrument setup, data acquisition and some preliminary data acquired for LJCO are presented.