A comparative study of GRACE with continental evapotranspiration estimates in Australian semi-arid and arid basins: sensitivity to climate variability and extremes

This study examines the dynamics and robustness of large-scale evapotranspiration products in water-limited environments. Four types of ET products are tested against rainfall in two large semi-arid to arid Australian basins from 2003 to 2010: two energy balance ET methods which are forced by optical satellite retrievals from MODIS; a newly developed land surface model (AWRA); and one approach based on observations from the Gravity Recovery and Climate Experiment (GRACE) and rainfall data. The two basins are quasi (Murray-Darling Basin: 1.06 million km(2)) and completely (Lake Eyre Basin: 1.14 million km(2)) endorheic. During the study period, two extreme climatic events-the Millennium drought and the strongest La Nina event-were recorded in the basins and are used in our assessment. The two remotely-sensed ET products constrained by the energy balance tended to overestimate ET flux over water-stressed regions. They had low sensitivity to climatic extremes and poor capability to close the water balance. However, these two remotely-sensed and energy balance products demonstrated their superiority in capturing spatial features including over small-scale and complicated landscapes. AWRA and GRACE formulated in the water balance framework were more sensitive to rainfall variability and yielded more realistic ET estimates during climate extremes. GRACE demonstrated its ability to account for seasonal and inter-annual change in water storage for ET evaluation

Early human settlement of Sahul was not an accident

The first peopling of Sahul (Australia, New Guinea and the Aru Islands joined at lower sea levels) by anatomically modern humans required multiple maritime crossings through Wallacea, with at least one approaching 100 km. Whether these crossings were accidental or intentional is unknown. Using coastal-viewshed analysis and ocean drift modelling combined with population projections, we show that the probability of randomly reaching Sahul by any route is <5% until ≥40 adults are ‘washed off’ an island at least once every 20 years. We then demonstrate that choosing a time of departure and making minimal headway (0.5 knots) toward a destination greatly increases the likelihood of arrival. While drift modelling demonstrates the existence of ‘bottleneck’ crossings on all routes, arrival via New Guinea is more likely than via northwestern Australia. We conclude that anatomically modern humans had the capacity to plan and make open-sea voyages lasting several days by at least 50,000 years ago

Australian marine radiocarbon reservoir effects: ΔR atlas and ΔR calculator for Australian mainland coasts and near-shore islands

Studies of pre-bomb mollusks live-collected around the Australian coastline have concluded that near-shore marine radiocarbon reservoir effects are small and relatively uniform. These studies are based on limited samples of sometimes dubious quality representing only selective parts of Australia’s lengthy coastline. We systematically examine spatial variability in the marine radiocarbon reservoir effect (ΔR) through analysis of 292 live-collected mollusk samples across the Australian mainland coasts and near-shore islands subject to strict selection criteria. This study presents 233 new ΔR values combined with an evaluation of 59 previously published values. Results demonstrate significant spatial variability in marine radiocarbon reservoir effects across the study region. ΔR values range from 68 ± 24 14C years off the Pilbara region of Western Australia to –337 ± 46 14C years in the southern Gulf of Carpentaria in Queensland. Most sets of local values exhibit internal consistency, reflecting the dominant influence of regional oceanography, including depletion in ΔR values southwards along the eastern Australian coastline coincident with the East Australian Current. Anomalous values are attributed to inaccurate documentation, species-specific relationships with the carbon cycle and/or short-term fluctuations in marine radiocarbon activities. To account for the heterogeneous distribution of marine 14C, we recommend using a location specific ΔR value calculated using the Australian ΔR Calculator, available at: https://delta-r-calc.jcu.io/

Targeting Global Protected Area Expansion for Imperiled Biodiversity

Governments have agreed to expand the global protected area network from 13% to 17% of the world's land surface by 2020 (Aichi target 11) and to prevent the further loss of known threatened species (Aichi target 12). These targets are interdependent, as protected areas can stem biodiversity loss when strategically located and effectively managed. However, the global protected area estate is currently biased toward locations that are cheap to protect and away from important areas for biodiversity. Here we use data on the distribution of protected areas and threatened terrestrial birds, mammals, and amphibians to assess current and possible future coverage of these species under the convention. We discover that 17% of the 4,118 threatened vertebrates are not found in a single protected area and that fully 85% are not adequately covered (i.e., to a level consistent with their likely persistence). Using systematic conservation planning, we show that expanding protected areas to reach 17% coverage by protecting the cheapest land, even if ecoregionally representative, would increase the number of threatened vertebrates covered by only 6%. However, the nonlinear relationship between the cost of acquiring land and species coverage means that fivefold more threatened vertebrates could be adequately covered for only 1.5 times the cost of the cheapest solution, if cost efficiency and threatened vertebrates are both incorporated into protected area decision making. These results are robust to known errors in the vertebrate range maps. The Convention on Biological Diversity targets may stimulate major expansion of the global protected area estate. If this expansion is to secure a future for imperiled species, new protected areas must be sited more strategically than is presently the case

The use of Envisat ASAR Global Monitoring Mode data to map rapid broad-scale flood events

This thesis seeks to enhance our ability to map the extent of large floods in near real time using coarse resolution C-band radar remote sensing. The microwave part of the electromagnetic spectrum has a great advantage over visible and infrared light in its ability to penetrate cloud cover, and as radar is an active system, it does not rely on daylight hours for reflected solar radiation. The European Space Agency's Advanced Synthetic Aperture Radar aboard the Envisat satellite, operating in Global Monitoring Mode (GM), is targeted for particular consideration due to its high temporal frequency, comprehensive coverage and ease of acquisition. Challenges are identified which relate both to the use of radar generally, and also in particular to GM data, in the demarcation of water and land, as well as to the practical business of data processing.\ud \ud These challenges relate to the way that water is identified, which can be by a low signal where specular surface reflection away from the sensor occurs, or by a high signal where multiple interactions occur between the water surface and emergent structures such as vegetation. Thresholds must make the distinction between the two cases, and as such, some prior knowledge of land cover is needed in the segmentation process. With such coarse data as GM, mixed pixels comprising both high and low water signals are often encountered, which result in a mid-range pixel value that masks the presence of water. The thresholding process is further complicated by the relationship of the signal returned to the sensor with incidence angle, which varies between about 14-44° with GM data. Under some wind conditions, waves of a particular pitch and orientation on the surface of open water cause resonance effects, returning a very high signal - sometimes even a gain - to the sensor. In particular circumstances, where Flood waters flow through arid land, the low signal returned from open water due to specular reflection cannot be distinguished from the low signal returned from desert due to attenuation and absorption. In literature surrounding research in this field, results from observations of radar response in wetlands and flooded grasslands are mixed, pointing to the importance for further work in this area. In Australia, the need for a better understanding of the expected backscatter response from inundated areas in tropical savanna, which covers one third of its landmass, is clear.\ud \ud The computational framework was set up for the efficient download, registration and orthorectification of GM data using scripting and open source software. Full advantage was made of the parallel processing capabilities of James Cook University's High Performance Computing network, scripts were tailored to GM data's characteristics and test results proved the method appropriate for the high volume processing required by the large GM dataset. This capability was used to carry out regression on a pixel-wise basis across a year's worth of GM data, categorised by seasonal rainfall periods, in order to normalise backscatter values with respect to incidence angle. Correlation of the resulting characteristics with surface parameters, such as regolith, vegetation and soil type were observed. The potential confusion between absorption in dry, homogeneous soils, and specular reflection on surface water was predicted. It was observed that the degree of change of backscatter with incidence angle on open water appeared independent of the presence of Bragg Resonance, despite absolute values being at opposite ends of the scale, depending on whether resonance did, or did not, occur.\ud \ud A major flood event in Pakistan was successfully mapped and made available in near-real time for the disaster relief effort. An image differencing technique allowed the successful separation of low backscatter response from open water with that from the immediately surrounding desert. GM data were found to fill a gap in the period where the flood was obscured to visible and infrared sensors, during the crucial first week of the event. Definition of the extremities of the flood were tackled with a spatial threshold using a region growing algorithm, and the radiometric backscatter threshold was established using an incremental convergence technique, employing multiple κ -statistic calculations with contemporaneous MODIS SWIR data. Both the stability of the radar threshold, and the instability of the MODIS SWIR reflectance threshold, were highlighted.\ud \ud The backscatter responses to two large flood events in the tropical savanna of northern Australia were investigated, showing markedly different results. One flood, in the floodplain of Queensland's Flinders River, involved total inundation of tussock grasslands over an area of 9000 km², allowing accurate classification using GM data (κ = 0.7), with predictable dihedral scattering returns as the flood receded and the emergent tussock grasses caused multiple interactions between the radar signal and the surface water. Inundated areas covered by emergent vegetation in the other flood, in Cape York's Staaten River floodplain, were almost completely indistinguishable from the surrounding wet vegetation. Data from water height loggers established in the neighbouring Mitchell floodplain over a dry/wet season period provided an insight into the interaction of these particular vegetation conditions under flood. Results concurred with the work of others, that backscatter response is a complex combination of effects depending on relative water height, vegetation spacial density, biomass, and verticality, or enmeshment, of super-surface grasses.\ud \ud The need for further work is discussed, together with spin-off opportunities, in the context of current and planned alternative C-band satellite data sources. The planned contribution of C-band data, along with contemporary visible/infrared products in the upscaling of current and ongoing JCU research into greenhouse gas emissions in the Mary River in the Northern Territory is outlined, together with the possible use of C-band radar to gauge fuel moisture content and fire potential, in the light of our findings in the tropical savanna. The potential use of GM data to explore correlation between Gravity Recovery and Climate Experminent (GRACE) data and surface water and soil moisture over time is discussed

Radar mapping of broad-scale inundation: challenges and opportunities in Australia

This paper explores our ability to map the extent of large floods in near real time using coarse resolution C-band radar remote sensing. The European Space Agency’s advanced synthetic aperture radar aboard the Envisat satellite, operating in global monitoring mode (GM), is considered for Australia due to its high temporal frequency, comprehensive coverage and ease of acquisition. Challenges are identified which relate both to the use of radar generally, and also in particular to GM data, in the demarcation of water and land. In Australia, the need for a better understanding of the expected backscatter response from inundated areas in tropical savanna, which covers one third of its landmass, is targeted. The backscatter responses to two large flood events in the tropical savanna of northern Australia are investigated, showing markedly different results. One flood allows the accurate classification of inundated extents, while the other is almost completely indistinguishable from the surrounding wet vegetation. Data from water height loggers established in the neighbouring Mitchell floodplain over a dry/wet season period provide an insight into the interaction of these particular vegetation conditions under flood. Results concur with the work of others, that backscatter response is a complex combination of effects depending on relative water height, vegetation spatial density, biomass, and verticality, or enmeshment, of super-surface grasses. Opportunities are also identified that relate to future space missions, the synoptic use with optical data, and better knowledge of the processes that govern the applicability of radar data for mapping large flood events

The use of radar satellite data from multiple incidence angles improves surface water mapping

Satellite radar data has been employed extensively to monitor flood extents, where cloud cover often prohibits the use of satellite sensors operating at other wavelengths. Where total inundation occurs, a low backscatter return is expected due to the specular reflection of the radar signal on the water surface. However, wind-induced waves can cause a roughening of the water surface which results in a high return signal. Additionally, in arid regions, very dry sand absorbs microwave energy, resulting in low backscatter returns. Where such conditions occur adjacent to open water, this can make the separation of water and land problematic using radar. In the past, we have shown how this latter problem can be mitigated, by making use of the difference in the relationship between the incidence angle of the radar signal, and backscatter, over land and water. The mitigation of wind-induced effects, however, remains elusive. In this paper, we examine how the variability in radar backscatter with incidence angle may be used to differentiate water from land overcoming, to a large extent, both of the above problems.\ud \ud We carry out regression over multiple sets of time series data, determined by a moving window encompassing consecutively-acquired Envisat ASAR Global Monitoring Mode data, to derive three surfaces for each data set: the slope β of a linear model fitting backscatter against local incidence angle; the backscatter normalised to 30° using the linear model coefficients (σ₃₀⁰), and the ratio of the standard deviations of backscatter and local incidence angle over the window sample (SDR). The results are new time series data sets which are characterised by the moving window sample size.\ud \ud A comparison of the three metrics shows SDR to provide the most robust means to segregate land from water by thresholding. From this resultant data set, using a single step water–land classification employing a simple (and consistent) threshold applied to SDR values, we produced monthly maps of total inundation of the variable south-western basin of the Aral Sea through 2011, with an average pixel accuracy of 94% (kappa = 0.75) when checked against MODIS-derived reference maps

Humans, water, and the colonization of Australia

The Pleistocene global dispersal of modern humans required the transit of arid and semiarid regions where the distribution of potable water provided a primary constraint on dispersal pathways. Here, we provide a spatially explicit continental-scale assessment of the opportunities for Pleistocene human occupation of Australia, the driest inhabited continent on Earth. We establish the location and connectedness of persistent water in the landscape using the Australian Water Observations from Space dataset combined with the distribution of small permanent water bodies (springs, gnammas, native wells, waterholes, and rockholes). Results demonstrate a high degree of directed landscape connectivity during wet periods and a high density of permanent water points widely but unevenly distributed across the continental interior. A connected network representing the least-cost distance between water bodies and graded according to terrain cost shows that 84% of archaeological sites >30,000 y old are within 20 km of modern permanent water. We further show that multiple, well-watered routes into the semiarid and arid continental interior were available throughout the period of early human occupation. Depletion of high-ranked resources over time in these paleohydrological corridors potentially drove a wave of dispersal farther along well-watered routes to patches with higher foraging returns

Soil sampling in oil palm plantations: a practical design that accounts for lateral variability at the tree scale

Aims: The aim was to devise a practical soil sampling design for oil palm plantations that takes into account tree-scale variability, thus facilitating detection of trends in soil properties over time.\ud \ud Methods: We geometrically evaluated the ability of linear sampling transects to represent the distribution of typical management zones and radial patterns known to influence soil properties. The effect of sampling point density was tested using interpolated surfaces of soil biological, chemical and physical properties derived from values measured on a 35-point sampling grid covering the repeating tree unit in plantations with 15–25-year old palms.\ud \ud Results: The ability of sampling transects to represent the proportion of the plantation in various zones improved with increasing transect length and sampling density. Increasing the number of sampling points from 10 to 50 (using an acceptably long transect with length 5.57 × palm spacing) decreased the maximum deviation between the overall mean and the transect-derived mean from 15.9 to 5.6 % for the most variable parameter, respiration, and 3.2 to 0.6 % for the least variable parameter, bulk density.\ud \ud Conclusions: Transect sampling provides an efficient means of obtaining a composite soil sample that accounts for tree-scale variability in oil palm plantations. The method is readily adaptable for other tree crops

Continuous shipboard measurements of oceanic δ^18O, δD and δ^13C(DIC) along a transect from New Zealand to Antarctica using cavity ring-down isotope spectrometry

Cavity ring-down spectrometers, with automated sampling interfaces, were deployed to allow measurements of water isotopes (δ^18O, δD) and dissolved inorganic carbon (δ^13(CDIC)) stable isotope ratios at high temporal resolution along a transect from New Zealand to the Antarctic continental shelf. Measurements every 10 min for δ^18O and δD, 15 min for DIC yielded 2499 and 2289 discrete measurements respectively. High resolution data enabled the delineation of water mass boundaries as well as revealing insights into surface hydrological and biological processes. δ^18O, δD, and δ^13(CDIC) decreased southwards, dropping by approximately 1.0‰, 7.0‰, and 0.5‰, respectively. Though the decline in δ^13(CDIC) with latitude was generally linear, the drop in δ^18O and δD was punctuated by areas of rapid, significant change corresponding to the Sub-Tropical, Sub-Antarctic and Polar Fronts. North of the Sub-Antarctic Front (approx. 54.5°S) the dominant control on water and DIC isotopes was the precipitation–evaporation balance and the contribution of upwelling waters, respectively. Further south, in close proximity to the sea ice and on the Antarctic shelf, water isotope values were more variable and predominantly influenced by the melting/freezing of sea-ice coupled to inputs from glacial/snow melt water. Local increases in δ^13(CDIC) were likely due to photosynthetic enrichment of the DIC pool. Using this new instrumentation has provided one of the most comprehensive oceanic transect data sets yet achieved and illustrates the potential of these methods to delineate discrete water masses and advance our knowledge of both water and inorganic carbon cycling processes in the ocean. This methodology, combining high-resolution isotopic measurements with hydrographic data, has significant benefits in modelling water mixing in locations with multiple sources and controlling processes