The paper evaluates long-term seasonal variations of the deuterium excess (d-excess = delta(2)H - 8. delta(18)O) in precipitation of stations located north and south of the main ridge of the Austrian Alps. It demonstrates that sub-cloud evaporation during precipitation and continental moisture recycling are local, respectively, regional processes controlling these variations. In general, sub-cloud evaporation decreases and moisture recycling increases the d-excess. Therefore, evaluation of d-excess variations in terms of moisture recycling, the main aim of this paper, includes determination of the effect of sub-cloud evaporation. Since sub-cloud evaporation is governed by saturation deficit and distance between cloud base and the ground, its effect on the d-excess is expected to be lower at mountain than at lowland/valley stations. To determine quantitatively this difference, we examined long-term seasonal d-excess variations measured at three selected mountain and adjoining valley stations. The altitude differences between mountain and valley stations ranged from 470 to 1665 m. Adapting the 'falling water drop' model by Stewart [J. Geophys. Res., 80(9), 1133-1146 (1975).], we estimated that the long-term average of sub-cloud evaporation at the selected mountain stations (altitudes between about 1600 and 2250 m.a.s.l.) is less than 1 % of the precipitation and causes a decrease of the d-excess of less than 2 per thousand. For the selected valley stations, the corresponding evaporated fraction is at maximum 7 % and the difference in d-excess ranges up to 8 per thousand. The estimated d-excess differences have been used to correct the measured long-term d-excess values at the selected stations. Finally, the corresponding fraction of water vapour has been estimated that recycled by evaporation of surface water including soil water from the ground. For the two mountain stations Patscherkofel and Feuerkogel, which are located north of the main ridge of the Alps, the maximum seasonal change of the corrected d-excess (July/August) has been estimated to be between 5 and 6 per thousand, and the corresponding recycled fraction between 2.5-3 % of the local precipitation. It has been found that the estimated recycled fractions are in good agreement with values derived from other approaches.

As a follow-up to the meeting of experts convened at the International Atomic Energy Agency (IAEA) in February 1989, and the International 14C Workshop held in Glasgow in September 1989, the 14C Quality Assurance Program was formulated. In a joint effort of several radiocarbon teams and IAEA staff, we have prepared a set of five new intercomparison materials. These are natural materials frequently used by radiocarbon laboratories. The materials were distributed to 137 laboratories in May 1990. In February 1991, a meeting of experts was convened in Vienna to evaluate the results, to determine the radiocarbon activity of the five samples expressed in % Modern (pMC) terms and to define the 13C/12C ratio, and to make recommendations on further use of these materials. We present here the results of the exercise and the agreed consensus values for each of the five materials and discuss the different analyses that were undertaken.

Abstract. Sublimation dominates the ablation process on cold, high-altitude glaciers in the tropical Andes. Transport of water vapor through the firn and exchange with ambient moisture alter the stable isotope composition of the surface layers. A sublimation experiment carried out during an ice core drilling campaign on Cerro Tapado (5536 m above sea level, 30ø08'S, 69ø55'W) revealed a strong enrichment in the 2H and •80 content in the surface layer. Concerning the deuterium excess, a decrease occurred at daytime, while during the night, the values remained comparatively constant. At daytime the sublimation is enhanced due to the higher moisture deficit of the ambient air accompanied by relatively high firn surface temperatures. Low surface temperatures at night cause condensation of water vapor in the firn pores near the surface and thus inhibit penetration of the isotopically enriched surface front into deeper firn layers. Measuring an isotope profile obtained through detailed sampling between the surface and 38 cm depth proved this mechanism. The observed modification of the isotopic composition at the surface was quantitatively described by a model, which also reproduced the mass loss measured with sublimation pans and calculated from relevant meteorological data. The results of this study suggest that the influence of sublimation on the preserved isotope record of ice cores under comparable environmental conditions is rather limited. In any case, simultaneous measurements of 82H and 8•80 help to identify layers in an ice core which might be effected by sublimation. However, since the mass loss due to sublimation was of the order of 2-4 mm per day during the experiment, important palaeoinformation from an isotope record could be eliminated during extended dry periods.

Abstract:The first results of a regional circulation model REMO iso fitted with water isotope diagnostics are compared with various isotope series from central Europe. A 2 year case study is conducted from March 1997 to February 1999 centred over Europe, analysing daily and monthly measurements. Isotope signals over Europe are dominated by the typical isotopic effects such as temperature, continental and altitude effects, both on annual and seasonal scales. These well-known isotopic effects are successfully reproduced by REMO iso , using two different boundary data sets. In a first simulation, the European Centre for Medium-range Weather Forecasts (ECMWF) analyses serve as boundary conditions, where water isotopes were parameterized by a simple temperature dependence. In a second simulation, boundary conditions both for climatic and isotopic variables are taken from the ECHAM iso general circulation model output. The comparison of both simulations shows a very high sensitivity of the simulated υ 18 O signal to boundary conditions. The ECMWF-nested simulation shows an average offset of 4Ð5‰ in mean υ18 O values and exaggerated seasonal amplitude. The ECHAM-nested simulation represents correctly the observed mean υ18 O values, although with a dampened seasonality. REMO iso 's isotope module is further validated against daily υ 18 O measurements at selected stations (Nordeney, Arkona and Hohenpeissenberg) situated in Germany.

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Land cover and agricultural management practices can significantly influence soil structure. However, little is known about how fertilizer applications and land cover affect soil hydrology and groundwater recharge over long time periods. The objective of this study was to use stable water isotopes as environmental tracers to provide additional information required for better understanding of water flow and solute transport processes in the unsaturated zone influenced by land cover and type of fertilizer applications. Five lysimeters containing undisturbed soil monoliths from the same agricultural field site were investigated over a period of 5 yr. Liquid cattle slurry and solid animal manure were applied to the maize (Zea mays L.) and winter rye (Secale cereale L.) lysimeters. The grass–clover lysimeter was treated with mineral fertilizer. Quantitative influence of land cover and type of fertilizer application on water flow and solute transport was evaluated for all lysimeters using a modified version of HYDRUS‐1D. The highest drainage was observed in the maize lysimeter treated with cattle slurry, and the lowest in the grass lysimeter treated with mineral fertilizer. Pronounced differences in water contents and estimated saturated hydraulic conductivities between the lysimeters were restricted to the upper 25 to 30 cm of the soil. In particular, the lysimeters treated with animal manure had higher porosities, indicating a higher content of organic matter. Main differences in discharge between the lysimeters were observed in spring and during the plant growth periods, indicating the importance of nonuniform, patchy infiltration patterns during snow melt and of root water uptake, respectively. Mean water flow velocities, transit times, and effective water contents were estimated from the stable water isotope data, providing evidence on the impact of land cover and type of fertilizer application. We found smaller mean transit times in the maize lysimeters and for soils with liquid cattle slurry applications. Simulations indicate that numerical modeling can reproduce the general trend of water flow and isotope transport. Despite differences in mean transit times, fitted dispersivities were all in the same range, suggesting similar soil structures in the five lysimeters. However, more data for calibration and more information about heterogeneous infiltration would be required to improve the model accuracy. In general, stable water isotopes clearly contributed an added value, elucidating differences in mean flow parameters between the lysimeters. Thus, they provided evidence of the impacts of land cover and fertilizer applications, which are not obvious from water balance and mean discharge rates alone.

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