Browsing by Author "Azcurra, CS"
Now showing 1 - 6 of 6
Results Per Page
Sort Options
- ItemA comparison between direct and pan-derived measurements of the isotopic composition of atmospheric waters(The Modelling and Simulation Society of Australia and New Zealand Inc., 2011-12-12) Azcurra, CS; Hughes, CE; Parkes, SD; Hollins, SE; Gibson, JJ; McCabe, MF; Evans, JPThe stable isotopes of water can be used to examine and quantify the contribution to atmospheric moisture from evaporation, transpiration and surface-waters. However, obtaining extensive and ongoing time series data of the isotopic composition of atmospheric moisture has been difficult. Presented here is an alternate method using an isotope mass balance approach to estimate the isotopic composition of atmospheric moisture using water samples collected from class A evaporation pans. While this evaporation pan method does not provide the high-resolution time series data that can be obtained from an isotope analyser taking in-situ measurements of atmospheric moisture, the method is relatively simple and inexpensive to set-up and maintain. In this preliminary investigation, a comparison between the isotopic composition of atmospheric moisture estimated from the evaporation pan method and in-situ measurements of the isotopic composition of water vapour using a Fourier Transform Infrared (FTIR) spectrometer deployed at the Lucas Heights weather station in New South Wales is undertaken. Through comparison of the two series of hydrogen isotope data, an assessment of the evaporation pan method can be made. Although there was some agreement between the isotopic composition of vapour measured by the FTIR spectrometer and the estimation for the atmospheric moisture (R2 = 0.49), the comparison is sensitive to climatic parameters that vary significantly within a 24-hour period such as the relative humidity of air and the air and pan temperatures. Inverting the model to use the FTIR spectrometer measurements at an hourly resolution improved the performance of the model (R2 =0.57). However, this also revealed that the model produced more depleted values of the evaporation pan water isotopes than those observed. In contrast, there was a variable relationship between the modelled and observed isotope values of atmospheric moisture. These conflicting results will need to be resolved before the evaporation pan method is broadly applied in isotope hydrology. © 2011 The Modelling and Simulation Society of Australia and New Zealand Inc.
- ItemComparison of atmospheric water vapour δ18O and δ2H estimated using evaporation pan, rainfall equilibrium and continuous measurements(Elsevier B. V., 2019-09) Crawford, J; Azcurra, CS; Hughes, CE; Gibson, JJ; Parkes, SDFor a period of 16 months in Sydney, Australia, the variations of 2H/1H and 18O/16O in atmospheric vapour (δ2HA and δ18OA) were estimated using an evaporation pan method as well as using the isotopic precipitation-equilibrium approach. These calculations were then compared with δ2HA values measured at 10 m above ground surface using a Fourier Transform Infrared Spectrometer (FTIR). As pan isotopic composition was available on a weekly time scale, the evaporation rates were measured daily, and the atmospheric variables were available hourly, the weekly time scale was used to calculate the arithmetic averages of the atmospheric variables that were used in the estimation of the pan-derived δ2HA. Good agreement (r = 0.7, P-value = 0.00) was found between the pan-derived and the FTIR measured δ2HA for weekly intervals, although individual differences ranged from −25.0 to 20.4‰, with the absolute difference averaging 8.0‰. A sensitivity analysis showed that the determination of δ2HA is most sensitive to air temperature, relative humidity and the isotopic composition of the pan water. While the precipitation-equilibrium approach only appears to be representative of atmospheric conditions close to times of precipitation events, the pan-derived isotopic composition of atmospheric vapour was found to be closer to the FTIR averages over longer periods including intervals with no precipitation. Overall, this means that the pan method is far more effective for uninterrupted estimation of δ2HA and δ18OA of atmospheric water vapour, as required for water budget studies, than the precipitation-equilibrium method, and it is more cost effective and robust than continuous measurement. Crown Copyright © 2019 Published by Elsevier B.V.
- ItemDaily observations of rainfall, vapour and pan water δ2H for improved quantification of atmospheric and terrestrial water interactions(American Geophysical Union, 2012-12-03) Hughes, CE; Azcurra, CS; Parkes, SD; Hollins, SE; McCabe, MF; Evans, JP; Pickering, D; Gibson, JJ; Edwards, KThe stable isotopes of water (δ2H, δ18O) have been used extensively to track the movement of water through the hydrological cycle. As water moves through the hydrological cycle, its isotopic composition changes (fractionation) as a result of phase changes and interactions with other waters. When combined with other measurements of water fluxes and volumes, the stable isotopes can provide information about sources, processing, and transport of water suitable for determining water and energy balances. While the isotopic composition of atmospheric moisture influences the isotopic composition of terrestrial waters, obtaining ongoing time series data of this has been difficult. One method for estimating the isotopic composition of atmospheric moisture combines the Craig-Gordon isotopic evaporation model with a mass balance approach, using water samples collected from Class A evaporation pans at weekly intervals. The method assumes steady-state conditions and thus does not represent the highly variable meteorological conditions that can strongly influence fractionation. Although water-sampling regimes cannot match the frequency of the observed variability in meteorological conditions, increasing the sampling resolution to a sub-weekly resolution will more closely align isotopic measurements with meteorological conditions and allow the assessment of established water isotope relationships at a greater temporal frequency. An alternate method for estimating the isotopic composition of atmospheric moisture is based on local precipitation being in equilibrium with atmospheric moisture. As per the evaporation pan method, it is assumed that steady-state conditions apply. At the Lucas Heights weather station in south-eastern Australia, atmospheric moisture water isotopes (δA) have been directly measured at sub-hourly intervals using a Fourier Transform Infrared (FTIR) spectrometer. This hydrogen isotope time series was evaluated against daily observations of isotopes in water from a Class A evaporation pan (δL) and in precipitation (δP) collected from the same site. This study assesses the validity of established (steady-state) relationships between liquid and vapor water isotopes for interactions that have been measured at the medium-resolution temporal scale, and provides a basis for improving flux estimates based on an isotope mass balance approach. ©Author(s) 2012
- ItemInterpreting past climate using southwest Australian speleothems(Australian Meterological & Oceanographic Society, 2012-01-31) Treble, PC; Azcurra, CS; Baker, AA; Bradley, C; Wood, A; Fischer, MJ; Fairchild, IJ; Hellstrom, JC; Gagan, MKThere is an identified need to extend our baseline climate information beyond the relatively short duration of instrumental records in Australia. An improved knowledge of natural rainfall variability would assist in our understanding of climate change. SW Australia (SWWA) is one region that has been identified as having a changing climate since the 1970s. Speleothems (cave stalagmites) are an effective archive of past climate variability and caves from the coastal region of SWWA are being studied for paleoclimate records. The modern speleothem record from this region has been assessed and shown to record the post-1970s rainfall decrease (Treble et al., 2003; 2005; Fischer and Treble, 2008). The extension of the speleothem record is currently underway, however, a long-term cave monitoring program was also deemed necessary to separate the climatic from non-climatic signals i.e. to reduce uncertainty when interpreting these records. This presentation outlines what we have learnt about the possible hydrological modification of the climate signal in speleothems. In particular, we present results from a five-year long monitoring study of rainfall and cave drip water O isotopes (!18O) from Golgotha Cave, SWWA. From this study, we have been able to characterize the probable flow paths feeding stalagmites in our monitored cave. These flow paths range from slow diffuse flow of isotopically-averaged rainfall to preferential routing of high-magnitude, 18O-depleted, events along fast flow routes into the cave. Hence, we offer a possible explanation for why paleoclimate records from coeval speleothems in our cave may differ. Our study suggests that this disagreement may simply be due to different flow paths resulting in a bias towards the preservation of high or low magnitude rainfall events.
- ItemAn isotopic and modelling study of flow paths and storage in quaternary calcarenite, SW Australia: implications for speleothem paleoclimate records(Elsevier, 2013-03-15) Treble, PC; Bradley, C; Wood, A; Baker, AA; Jex, CN; Fairchild, IJ; Gagan, MK; Cowley, J; Azcurra, CSWe investigated the distinctive shallow sub-surface hydrology of the southwest Western Australia (SWWA) dune calcarenite using observed rainfall and rainfall δ18O; soil moisture, cave drip rate and dripwater δ18O over a six-year period: August 2005–March 2012. A lumped parameter hydrological model is developed to describe water fluxes and drip δ18O. Comparison of observed data and model output allow us to assess the critical non-climatic karst hydrological processes that modify the precipitation δ18O signal and discuss the implications for speleothem paleoclimate records from this cave and those with a similar karst setting. Our findings include evidence of multiple reservoirs, characterised by distinct δ18O values and recharge responses (‘low’ and ‘high’ flow sites). Dripwaters exhibit δ18O variations in wet versus dry years at low-flow sites receiving diffuse seepage from the epikarst with an attenuated isotopic composition that approximates mean rainfall. Recharge from high-magnitude rain events is stored in a secondary reservoir which is associated with high-flow dripwater that is 1‰ lower than our monitored low-flow sites (δ18O). One drip site is characterised by mixed-flow behaviour and exhibits a non-linear threshold response after the cessation of drainage from a secondary reservoir following a record dry year (2006). Additionally, our results yield a better understanding of the vadose zone hydrology and dripwater characteristics in Quaternary age dune limestones. We show that flow to our monitored sites is dominated by diffuse flow with inferred transit times of less than one year. Diffuse flow appears to follow vertical preferential paths through the limestone reflecting differences in permeability and deep recharge into the host rock. © 2013, Elsevier Ltd.
- ItemAn isotopic and modelling study of recharge to the Tamala Limestone, SW Australia: implications for speleothem paleoclimate records(International Association of Hydrogeologists, 2013-09-20) Treble, PC; Bradley, C; Fairchild, IJ; Baker, AA; Jex, CN; Azcurra, CS; Wood, AWe present a modern calibration of recharge pathways in the vadose zone of Quaternary dune calcarenite (Tamala Limestone in the Margaret River region, SW Western Australia. This study aimed to i. better understand groundwater recharge in these calcarenites, which has wider relevance to SW WA calcarenites and their use as a groundwater source; and to ii. interpret the O isotopic composition of these waters in order to assess the suitability of Golgotha Cave for reconstructing records of paleoclimate using speleothems. We present six years of data from our monitored site at Golgotha Cave (Treble et al. 2013). A lumped parameter hydrological model is developed to describe water fluxes and dripwater O isotopic composition. Comparison of observed data and model output allow us to assess the critical non-climatic karst hydrological processes that modify the precipitation isotopic signal and discuss the implications for speleothem records from this cave and those with a similar karst setting. Our findings include evidence of multiple reservoirs, characterised by distinct isotopic values and recharge responses ('low’ and ‘high’ flow sites). Dripwaters exhibit isotopic variations in wet versus dry years at low-flow sites receiving diffuse seepage from the epikarst with an attenuated isotopic composition that approximates mean rainfall. Recharge from high-magnitude rain events is stored <1%o lower in a secondary reservoir which is associated with high-flow dripwater that is 1%o lower than our monitored low-ow sites. One drip site is characterised by mixed-flow behaviour and exhibits a non-linear threshold response after the cessation of drainage from a secondary reservoir following a record dry year (2006). We show that ow to our monitored sites is dominated by diffuse ow with inferred transit times of less than one year. Diffuse flow appears to follow vertical preferential paths through the limestone reflecting differences in permeability and deep recharge into the host rock. Reference Treble et al. (Z013). An isotopic and modelling study of flow paths and storage in Quaternary calcarenite, SW Australia: implications for speleothem paleoclimate records, Quaternary Science Reviews 64; 90-103.