Browsing by Author "McGuffie, K"

Now showing 1 - 7 of 7
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    Amazonian climatic change: water isotope detection of deforestation and greenhouse impacts
    (International Atomic Energy Agency, 2004-10-25) Henderson-Sellers, A; McGuffie, K
    Land use change in the Amazon basin, the largest and most biologically diverse river system in the world, has the potential to cause significant disruption to hydrological, biogeochemical and human systems. The naturally occurring isotopologues of water, commonly, but incorrectly, termed ‘isotopes’, of interest as possible tracing and validation tools in hydrological simulations are 1H2 18O and 1H2H16O. Large catchment simulations of water resources where isotopes could be applicable include water re-cycling as a function of precipitation type and variability [1, 2]; evaporation sourcing (i.e. whether water vapour comes from transpiration or from evaporation from rivers, lakes, soil water or the vegetation canopy) [3]; ice and snow temperature deposition determination; and aquifer and soil processes including those dependent upon precipitation intensity and melt-water contributions [4]. coupled with measurement of isotopes in water sources, SWI characteristics in river discharge now provide insight into basin- integrated hydro-climates [3, 5]. New data from the Global Network for Isotopes in Precipitation (GNIP) database, and previously published data now fully analysed, reveal significant changes in seasonal isotopic characteristics in the upper reaches of the Amazon basin underlining the use of stable water isotopes as a means of validating and improving numerical models. Despite observational limitations, which make determination of correctness difficult, some global models are shown here to be too poor to be of value in the Amazon. For example, isotopic depletions, a strong function of rainfall amount, are incorrect when precipitation is inadequately predicted seasonally or following ENSO circulation shifts. Isotopic enrichments of d18O and dD exhibit systematic variations in the Amazonian water cycle as a result of forest and flooding changes. We find signatures of both circulation and land-use change impacts in the isotopic record: ENSO events cause decreased depletion in the dry season, due to a decreased emphasis on convective precipitation, while increases in upper basin isotope depletions in the wet season result from relatively less non-fractionating recycling (i.e. less transpiration and full canopy evaporation) because there are fewer trees. Prediction of d18O and dD depletions by an isotope AGCM, while being adequate when averaged over the whole 17-year AMIP II period, are found to be less plausible for shorter periods. An isotope LSS is shown to be very sensitive to the prescription of boundary layer atmospheric water vapour isotopic depletion. We conclude that efforts to evaluate model simulations of the Amazon against isotopic data are currently seriously hampered by: (i) poor simulation of the gross water budget (e.g. lack of surface water conservation in models); (ii) considerable model differences in surface water distribution (i.e. between evaporation and runoff); (iii) wide ranging characterization of other possible causes of water isotopic fluctuations, such as El Niño and La Niña events; and (iv) significantly different characterization by current land-surface schemes of the partition of evaporation between fractionating and non- fractionating processes. While our results show great promise for isotopic evaluation of near-surface continental water cycle impacts in the Amazon, they also underline the need to address existing shortcomings in both atmospheric and land-surface models before isotopic finger-printing can be fully achieved.
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    Atmospheric isotopes: evolution of stable water isotopologues as an applicable data source
    (Australian Institute of Physics, 2005-01-31) Henderson-Sellers, A; McGuffie, K
    Stable water isotopes have been employed as a means of challenging, validating and improving numerical models of basin-scale water processes since the 1980s. Two rare but naturally occurring isotopologues of water, 1H218O and 1H2H16O, are coming to be of practical use in diagnosis of water cycle system changes. Recent developments have served to illustrate how detection and attribution of both human impacts and natural variations in surface-atmosphere water exchanges can beneficially exploit stable water isotope observations and simulations. The promise for isotopic finger-printing of near-surface water cycle changes is illustrated here for three important basins. © 2005 AIP
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    Nuclear geophysiology: stable water isotopes as evaluators of hydroclimate predictions in the Murray-Darling basin
    (Australian Institute of Physics, 2005-01-31) Henderson-Sellers, A; Airey, PL; McGuffie, K
    Isotopic data from two end-range and one central aquifer in the Murray Darling Basin are used to determine precipitation intensity thresholds for evaluation of GCM predictions. Applying these to ‘good’ and ‘poor’ Atmospheric Model Intercomparison Project (AMIP) simulations of the Murray Darling gives rise to large differences in rainfall amount (30% to 62%). Selecting only ‘good’ models shows a >150mm annual groundwater recharge loss in El Niño cf. La Niña climates. These isotopic techniques are applicable to future model scenarios of basin-scale hydrology, especially in difficult to simulate semi-arid basins.
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    Shift in stable water isotopes in precipitation in the Andean Amazon: Implications of deforestation or greenhouse impacts?
    (Elsevier Ltd, 2006) Henderson-Sellers, A; McGuffie, K
    Changes in the O and H isotopes in precipitation have been linked to greenhouse warming, but no signal attributable to Amazonian deforestation has been reported. Recent data from the Andes exhibit a seasonally contrasting signal which is consistent with large-area removal of forest. Specifically, at Izobamba, in the far west of the basin, the seasonality in isotopic depletions has become enhanced between 1972 and 2000. The observed more negative isotopic ratios in the wet season are consistent with increases in runoff fraction and/or reductions in recycling through non-fractionating processes. The dry season result (statistically significant less negative isotopic ratios) is harder to explain and could be due to a decrease in fractionating recycling (i.e. partial evaporation from water bodies). Application of a simple isotopic catchment model suggests that these isotopic changes in precipitation may be the result of large-scale deforestation in the Amazon Basin. Isotopically-enabled numerical models are needed to establish regional validity. © 2006 Elsevier B.V.
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    Stable water isotopes as tools for basin-scale water cycle: diagnosis of the Murray–Darling
    (Elsevier Ltd, 2006) Henderson-Sellers, A; Airey, PL; McGuffie, K; Stone, DJM
    We examine the hypothesis that isotopic techniques are applicable to hydrological predictions in difficult-to-simulate semi-arid basins, using the Murray–Darling Basin as an example. Isotopic data from three aquifers in the Murray–Darling characterize precipitation intensity for evaluation of GCMs. Applying these to ‘good’ (water conserving) and ‘poor’ (non-water-conserving) climate model simulations of the Murray–Darling gives rise to large differences in rainfall amount (30–62%). Selecting only ‘good’ models shows a greater than 150 mm annual groundwater recharge loss in El Niño cf. La Niña climates. 2002–2003 El Niño drought data are used to refine isotopic calculation of water lost in evaporation from rivers and irrigation, giving a cumulative loss of 64% of river water during 2002 (cf. 80% using a previous method). This substantiates recent identification of this El Niño drought as evaporatively most extreme and we conclude that stable water isotopes, used synergistically with hydro-climate models, have great potential in future water resource predictions. © 2006 Elsevier B.V.
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    Stable water isotopes: revolutionary tools for global water cycle disturbance diagnosis
    (International Atomic Energy Agency, 2004-10-25) Henderson-Sellers, A; McGuffie, K
    Here we assess the simulation of isotopic fluxes in basin-scale hydrology, focusing on the ‘big leaf’ representation of land surfaces in numerical models as the current mechanism for incorporating water isotopes. Applications of the simulation of stable isotopic behaviour simulated by global climate or earth system models, including river isotopic characterization of basin changes and plant-respired oxygen isotope ‘tagging’, to resolving uncertainty are limited until more basic criteria such as conservation, current mean climate and capture of observed variability are demonstrated. We find that surface water budgets are still rather poorly simulated and inadequately constrained at the scale of large basins; yet surface energy partition can be apparently well captured by models with inadequate land-surface parameterization.
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    Temporal analysis of stable water isotopic characteristics in the Murray Darling Basin
    (International Atomic Energy Agency, 2004-10-24) Henderson-Sellers, A; Airey, PL; Stone, DJM; Bradd, J; McGuffie, K
    “Water shortages, especially in tropical countries, are the climate challenge for this century” [1]. The isotopic composition of water and carbon in e.g. ice cores, ground water and biomass has been recognized as relevant to hydro-climates on timescales from glacial [2] to extreme weather [3]. We present Australian stable water isotope (SWI) research capability and exploit it in novel ways in order to establish objective validation of and improvement in existing water resource models ultimately reducing uncertainty in predictions. The use of stable water isotopes in hydro-climate modelling is refined on three timescales for the Murray Darling Basin. Isotopes demonstrate that in semi-arid regions, groundwater recharge occurs when the rainfall intensities exceed a threshold suggesting improvement of aquifer predictions over tens to thousands of years using isotopic threshold estimates. A range of atmospheric global circulation models ’ simulations of key hydrological parameters over years to decades reveals poor results for the majority (13 in 20) and underlines the value of isotopic constraints on basin hydrology. Modelling minute to monthly isotope fluxes using land surface schemes and a steady state (phenomenological) model of river hydrology allows comparison of the partitions of precipitation between transpiration, run-off and ‘lake’ evaporation with isotope observations from June 2002 to January 2003. These results will have the greatest importance if combined to improve the dynamics of simulations of regional water cycles [4]. Three timescales have been used here to explore the role of stable water isotopes in refining climate and hydrological models of the Murray Darling Basin. Firstly, over tens to thousands of years, we have examined the processes leading to the effective recharge of groundwater. The isotope data clearly indicate that in the warm arid/semi-arid regions, in contrast to the behaviour in cool temperate zones, effective recharge only occurs when the rainfall intensities exceed a threshold value. Isotopic estimates of this recharge threshold rainfall intensity could be applied to predictions of future groundwater resources. Secondly, over years to decades, we have assessed the success of a range of atmospheric global circulation models in simulating key hydrological parameters over the AMIP II period including El Niño and La Niña forcing. The results are rather poor for the majority (13 out of 20) GCMs suggesting that further constraints on the basin’s hydrology, such as from isotopes, may be valuable. In our third approach, we have modelled minute to monthly isotope fluxes using (a) land surface schemes (LSSs) at particular grid points within the Murray Darling Basin and (b) a steady state (phenomenological) model of river hydrology. Model conservation, climatic variations and ‘plausibility’, all pre-requisites for good simulations, have been investigated here for the Murray Darling. Models’ partitions of precipitation between transpiration, run-off and ‘lake’ evaporation are compared with isotope observations from the Darling River between June 2002 and January 2003. We find that: (i) more work is needed on gross water fluxes first; (ii) simple isotopic models generate plausible values but more complex ones, as yet, do not; and (iii) isotopes have potential for evaluation of whether LSSs are (in)correctly recharging and accessing groundwater reservoirs and for evaluation of the partitioning of water into runoff cf. re-evaporation. Tests based around these concepts offer a novel addition to the traditional methods of validating climate models and their sub-components.