Amazonian climatic change: water isotope detection of deforestation and greenhouse impacts
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Date
2004-10-25
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International Atomic Energy Agency
Abstract
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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Keywords
Climatic change, Isotopes, Water, Amazon River, Hydrology, Rivers, Human populations, Watersheds, Recycling
Citation
Henderson-Sellers, A., & McGuffie, K. (2004). Amazonian climatic change: water isotope detection of deforestation and greenhouse impacts. Paper presented to International Conference on Isotopes in Environmental Studies – Aquatic Forum 2004 Monte-Carlo, Monaco 25–29 October 2004. In Book of extended synopses. Retrieved from https://inis.iaea.org/collection/NCLCollectionStore/_Public/36/003/36003223.pdf?r=1#page=5&zoom=auto,-15,800