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pyPrecipitation origin and evaporation of lakes insemi-arid Patagonia (Argentina) inferred fromstable isotopes (d18O, d2H)

Christoph Mayr a,*, Andreas Lucke a, Willibald Stichler b, Peter Trimborn b,Bettina Ercolano c, Gabriel Oliva d, Christian Ohlendorf e, Julio Soto c,Michael Fey e, Torsten Haberzettl e, Stephanie Janssen f, Frank Schabitz f,Gerhard H. Schleser a, Michael Wille f, Bernd Zolitschka e

a Institut fur Chemie und Dynamik der Geosphare: Sedimentare Systeme (ICG V), Forschungszentrum Julich, D-52425Julich, Germanyb Institut fur Grundwasserokologie, Ingolstadter Landstr. 1, GSF – Forschungszentrum fur Umwelt und Gesundheit, D-85764Neuherberg, Germanyc Universidad Nacional de Patagonia Austral (UNPA), Lisandro de la Torre 1070, 9400 Rıo Gallegos, Argentinad Estacıon Experimental Agropecuaria Santa Cruz (INTA), Chacra 45, CC 332, 9400 Rıo Gallegos, Argentinae Geomorphologie und Polarforschung (GEOPOLAR), Institut fur Geographie, Universitat Bremen, Celsiusstrasse FVG-M,D-28359 Bremen, Germanyf Seminar fur Geographie und ihre Didaktik, Universitat zu Koln, Gronewaldstrasse 2, D-50931 Koln, Germany

Received 2 November 2005; received in revised form 12 September 2006; accepted 28 September 2006

KEYWORDSOxygen-18;Deuterium;Precipitation;Lakes;Evaporation;Local meteoricwater line;Aridity;Patagonia

Summary Stable isotope approaches are often used for estimating water balances oflakes. Such studies require regional background information about hydrogen and oxygenisotope variability of lakes and their potential inflows. Here, a stable isotope database(d2H and d18O) is presented for estimating evaporation to inflow ratios (E/I) of lakes insemi-arid southern Patagonia. Water samples of 23 lakes and ponds located in the Patago-nian steppe at about 52�S were sampled during three subsequent austral summers. Twodeep crater lakes, Laguna Azul and Laguna Potrok Aike, were studied in more detail duringa two-years monitoring. Furthermore, precipitation, groundwater and atmospheric watervapor were sampled for isotope analyses. Presented data imply that the isotopic compo-sition of rainfall in southeastern Patagonia is predominantly determined by precipitationamount and moisture source area. For the investigated area, the first meteoric water and

0022-1694/$ - see front matter ª 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.jhydrol.2006.09.025

* Corresponding author. Present address: GeoBio-CenterLMU, Richard-Wagner-Strasse 10, D-80333 Munchen, Germany. Tel.: +49 8921806599; fax: +49 89 21806601.

E-mail address: c.mayr@lrz.uni-muenchen.de (C. Mayr).

Journal of Hydrology (2007) 334, 53–63

ava i lab le a t www.sc iencedi rec t . com

journal homepage: www.elsevier .com/ locate / jhydrol

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evaporation lines in d2H vs. d18O space are presented. The database was further used toestimate the water balances of the two crater lakes, Laguna Azul and Laguna Potrok Aike,which are in the focus of recent paleoclimatic investigations. According to that approachabout 50% and 60%, respectively, of the water entering Laguna Azul and Laguna PotrokAike via surface and subsurface inflow evaporates. These results testify a considerableflow of lake waters into the groundwater.ª 2006 Elsevier B.V. All rights reserved.

Introduction

We present results of an isotopic survey of groundwater,surface waters and precipitation from an area between51–53�S and 73–69�W in the steppe and steppe–forestecotone of extra-Andean southern Patagonia. Whereas re-gional-scale stable isotope data are largely available forthe northern hemisphere (e.g., Gibson and Edwards,2002; Kaiser et al., 2001), comparatively fewer studieshave been carried out in the southern hemisphere and inparticular in the south of South America. The only pub-lished d2H and d18O records of precipitation for southern-most South America are from Ushuaia (54.78�S, 68.28�W;period 1981–1999; IAEA, 2004) on Tierra del Fuego furthersouth of the investigated area. Ushuaia, however, has amore oceanic climate and receives considerably more pre-cipitation (�570 mm; Weischet, 1996) and thus the datacannot be considered as representative for the large areaof semi-arid Patagonia.

One aim of this study is to provide an isotope databasefor hydrologic investigations and climatological researchfor this region (Zolitschka et al., 2006). The original modelon which all water balance calculations with stable isotopesbase was established by Craig and Gordon (1965). It was fur-ther modified and applied to various lake systems by subse-quent authors (e.g., Dincer, 1968; Zuber, 1983; Gonfiantini,1986; Gat and Bowser, 1991; Gibson et al., 1993; Krabbenh-oft et al., 1990; Gat, 1995). Isotopic studies investigatingsurface waters and precipitation on a broader regionalscale, such as the study presented here, were recently car-ried out particularly in Canada (Gibson, 2002; Gibson andEdwards, 2002; Gibson et al., 2002). The stable isotope ap-proach not only characterizes present-day lake systems, butmay also give valuable input data for reconstructions of pasthydrologic conditions, if well-dated sediment componentsare found that have directly recorded lake water isotopecomposition. Ongoing studies at two of the lakes presentedhere (Laguna Azul and Laguna Potrok Aike) aim to recon-struct long-term hydrologic changes with stable isotope re-cords of lake sediment components such as lacustrinecellulose (e.g., Wolfe et al., 2001; Sauer et al., 2001) or bio-genic (e.g., von Grafenstein et al., 1999) and endogenic car-bonates (e.g., Teranes and McKenzie, 2001).

Climate

The area investigated is the southernmost part of semi-aridArgentina (Fig. 1), east of the Andes and west of the city ofRıo Gallegos. Most of the lakes studied are below 300 ma.s.l., with the exception of Laguna Travesıa (approxi-mately 550 m a.s.l.) and Laguna Esperanza (424 m a.s.l.)The climate is cool-temperate with an annual mean temper-

ature of 6–7 �C (Gonzalez and Rial, 2004). Monthly means ofdaily maximum temperatures approach 20 �C in the summermonths (December–February, DJF) and monthly means ofminima are as low as �6 �C during June–August (JJA; datarecorded between January 1999 and April 2005 at INTAmeteorological station, Laguna Potrok Aike). Relativehumidity is negatively correlated with temperature exhibit-ing average values around 57% in austral summer (DJF) and79% in austral winter (JJA). The main climatic characteristicof the region, however, is the predominant strong westerlywind. Wind records of the meteorological station at LagunaPotrok Aike recorded mean wind velocities of 4.6 m s�1 andonly 0.5% calms between January 1999 and April 2005. Pre-dominant wind directions are SW (24% of all wind directionsincluding calms) and WSW (19%). Monthly averages of windspeeds exhibit lower values for austral winters comparedto summers.

Annual precipitation amounts decrease from the westernslopes of the Andes to the interior steppe region where La-guna Potrok Aike is located and then slightly increase againtowards the Atlantic coast (Gonzalez and Rial, 2004). Long-term annual rainfall averages of Rıo Gallegos at the Atlanticcoast (51.62�S, 69.28�W, 19 m a.s.l.; mean of 1931–2005;Peterson and Vose, 1997) are 250 mm, whereas El Turbio(51.68�S, 72.15�W, 184 m a.s.l.) close to the foothills ofthe Andes receives about 410 mm (mean of 1941–1950; Ser-vicio Meteorologico Nacional, 1958).

Geographical setting

Some of the lakes investigated are of volcanic origin; oth-ers are located in depressions formed by erosion processessuch as eolian deflation. All volcanic lakes are located inthe Pali Aike Volcanic Field (PAVF), a 4500 km2 large volca-nic area of Upper Pliocene to Holocene age (Corbella,2002; D’Orazio et al., 2000). To our knowledge, no de-tailed information about regional hydrology so far existsfor that region. With a few exceptions, the lakes have nopermanent tributaries and no surface outflow. They arefed by precipitation and groundwater to a large extent.Rain-fed ponds and lakes show lake levels sometimes rap-idly fluctuating within a single year. A few crater lakes aresupposed to have mainly groundwater inflow. Neverthe-less, these groundwater-fed lakes also exhibit evidencesof severe hydrological changes on decadal to centennialtime scales, indicated by lake level terraces (Laguna Po-trok Aike: Haberzettl et al., 2005) or photographic docu-ments (Laguna Azul: Mayr et al., 2005). The large craterlake Laguna Potrok Aike has only marginal ice-cover duringwinter season, whereas the smaller crater lake Laguna Azulis regularly ice-covered in July. The smaller, shallowerlakes generally freeze in winter times.

54 C. Mayr et al.

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Stable isotope values are usually expressed in d notation,where d = (Rsample/Rstandard � 1) Æ 1000 with R as the ratioof the heavy (e.g., 18O, 2H) to the light (e.g., 16O, 1H) iso-tope. All d values of water samples are conventionally re-ported relative to the international standard VSMOW(Vienna standard mean ocean water).

The isotopic composition of precipitation changes rela-tive to the initial isotope composition of the atmosphericvapor from which it originates. This isotopic fractionationoccurs under isotopic equilibrium conditions and is depen-dent on temperature (T). The equilibrium isotope fraction-ation (e*) defined as

e� ¼ Rwater

R�1vapour

!� 103 ðClark and Fritz; 1997Þ ð1Þ

is well known for various T from experimental data (Horitaand Wesolowski, 1994; Majoube, 1971). Since e* of hydrogendiffers from e* of oxygen by a factor of 8 in the T range inwhich rain is usually formed, global precipitation data scat-

ter along a global meteoric water line (GMWL) with a slopeof approximately 8 and an ordinate intersection of 10 whenplotted in d2H vs. d18O space (Craig, 1961; Rozanski et al.,1993).

Surface waters are affected by evaporation leading toenrichment of heavy isotope species (2H, 18O) due to kineticisotope fractionation. The gradual heavy isotope enrich-ment involves both equilibrium and kinetic effects, as de-scribed by the linear resistance model of Craig and Gordon(1965). The kinetic isotope fractionation (eK) is calculated as

eK ¼ ð1� hÞ � CK ðGonfiantini; 1986Þ ð2Þ

with h as humidity and CK being empirically determined as12.5 for hydrogen and 14.2 for oxygen (Gonfiantini, 1986).The total isotopic fractionation during evaporation (e) sim-ply comprises equilibrium (e*) and kinetic (eK) isotope ef-fects by

e ¼ e� þ eK ðCraig and Gordon; 1965Þ: ð3Þ

Due to the similarity of CK values, the kinetic fractionationfactors of hydrogen and oxygen isotopes are rather similarin value. Therefore the slope of evaporated waters in d2H

68˚W70˚W72˚W51˚S

52˚S

54˚S

1718

19

2021

2223

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ARGENTINA

PuntaArenas

A

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Figure 1 Map of southernmost South America giving locations of lakes (Arabic numbers) and groundwater sampling sites (Romannumbers). Names of sampling locations are given in Appendix 1. (A) Sampling locations outside the Pali Aike Volcanic Field (PAVF)and (B) sampling locations within the PAVF. Numbers 14 and 16 in B refer to Laguna Potrok Aike and Laguna Azul, respectively.

Precipitation origin and evaporation of lakes in semi-arid Patagonia 55

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vs. d18O space is generally lower than the slope of the GMWL,i.e. less than 8. For the northern high latitudes, slopes ofthese so-called evaporation lines in the range of 5–7 are typ-ical (Gibson, 2002) and vary with changing humidity. Surfacewaters that have undergone evaporation under the sameambient conditions (primarily h, T and isotopic compositionof humidity) plot on the same local evaporation line (LEL),provided that the isotopic composition of their initial sourcewater was similar. The slope of this LEL (SLEL) is given as

SLEL ¼½hðdA � dIÞ þ e�H½hðdA � dIÞ þ e�O

ðGat; 1995Þ ð4Þ

with dI as isotope values of the inflows and dA as the isotoperatio of air humidity and subscripts H and O standing forhydrogen and oxygen, respectively. Among these parame-ters, dA is often calculated on the assumption of isotopicequilibrium between precipitation and atmospheric vapor.While this approximation is appropriate for humid areas,admixture of evaporated vapor to atmospheric moisture al-ters dA in dry continental environments making an estima-tion of dA from isotopic composition of precipitationdoubtful. Several other methods for the determination ofvalues for dA, such as evaporation pan experiments (Allisonand Leaney, 1982; Gibson et al., 1999) and trapping ofatmospheric vapor (Gibson et al., 1999; Jacob and Sonntag,1991) are not easily applicable in remote areas. dA may alsobe derived from the isotopic composition of a lake that rep-resents the end-member of evaporative isotope enrich-ment. In the course of evaporation, the remaining waterof a desiccating water body with no further inflow will be-come gradually enriched in the heavy isotope species untilisotopic steady state is reached (Gat, 1995). At this finalstate no further isotope enrichment will occur. The limitingisotopic composition d* for such a desiccating water bodywithout inflow is given by

d� ffi dA þehðCraig and Gordon; 1965Þ ð5Þ

Thus, Eq. (5) provides a value for dA if an appropriate lakeexists that reached the limiting isotopic composition (d*)or d* may be determined if the isotopic composition ofatmospheric vapor is known. The knowledge of h, lakewater isotopic composition (dL), dI, and d* in an evaporativesystem allows then to estimate evaporation to inflow ratios(E/I) as given by

E

I¼ 1� h

h� dL � dI

d� � dLðGat and Levy; 1978Þ ð6Þ

Material and methods

Meteorological and thermistor data

Meteorological data from two automatic weather stationsare used in this study, one located at the INTA station closeto Laguna Potrok Aike and another in the city of Rıo Gall-egos. Data from both stations are based on at least hourlyrecords starting in March 1999 at Laguna Potrok Aike, andJanuary 2001 at Rıo Gallegos. During a few periods datawere not recorded due to technical problems at Laguna Po-trok Aike weather station. Monthly data were only used ifmore than 70% of the data were available. Lake water tem-

peratures were recorded 4–12 times a day with thermistorsattached to moorings located in the centers of Laguna Azuland Laguna Potrok Aike in 0–0.5 m water depth. Humidityand temperature dependent isotope fractionation wereweighted with monthly evapotranspiration data (providedby FAO, in Oliva et al., 2001) to account for changing sea-sonal evaporation controls on isotopic fractionation as sug-gested by Gibson (2002). In the further these flux-weightedparameters are denoted with the subscript flw.

Sampling

Most surface water samples were collected during field sur-veys in February/March 2002, 2003 and 2004. Sampling loca-tions were recorded with a commercial global positioningsystem (GPS). Additional water samples from Laguna PotrokAike and Laguna Azul were taken in approximately monthlyintervals between February 2002 and March 2004. Daily rain-fall events were sampled for the same period in the city ofRıo Gallegos. Only a few precipitation samples could be col-lected directly at Laguna Potrok Aike, because the area isnot permanently inhabited. Thus, isotope data of precipita-tion at Laguna Potrok Aike are shown here for comparison,but data from Rıo Gallegos were used for all calculations.Water was immediately filled in PE bottles of appropriatesize (10–100 ml depending on water volume) and closedtightly to prevent evaporation. Whenever possible, sampleswere stored in a refrigerator before analysis.

Atmospheric vapor samples were collected at INTA sta-tion near Laguna Potrok Aike in March 2005 by freezingatmospheric moisture in a cold trap filled with glass globulesto enlarge the inner surface for condensation. The station islocated approximately 1.5 km W of Laguna Potrok Aike atthe windward side of the lake. Atmospheric vapor was quan-titatively frozen in a cold trap cooled to �90 �C with a Jula-boTM FT901 cryo-cooler in a Dewar vessel filled with ethanol.

Stable isotope analyses of water samples

25ll of water were injected with an automatic samplingsupply into an uranium oven for hydrogen isotope analyses.The water reacted quantitatively at 800 �C with uraniumand the resulting H2 gas was analyzed with an isotope ratiomass spectrometer (IR-MS, Delta S, Finnigan MATTM). Oxygenisotope analyses were performed with a method based onisotopic equilibrium exchange between oxygen of CO2 witha known isotopic composition and the water sample (Moserand Rauert, 1980) and analyzed with an IR-MS (Delta C, Finn-igan MATTM). Analytical uncertainty is 1& for d2H and 0.15&for d18O.

Results and discussion

Isotope composition of precipitation andgroundwater

The collected precipitation samples of Rıo Gallegos have awide range from �22.5& to �3.0& for d18O and from�177& to �22& for d2H (Fig. 2). Mean isotope values ofRıo Gallegos precipitation are �11.7& and �91& for d18Oand d2H, respectively. Isotope means are significantly lower

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(�12.6& and �97&) when weighted with the precipitationamount.

Unexpectedly, no correlation between isotope values ofprecipitation and temperature was found, as it is knownfrom many other continental stations (Rozanski et al.,1993). The values were not significantly higher in the australsummer (DJF) than in the austral winter (JJA). Fig. 2 dem-onstrates that the isotope variations between different pre-cipitation events clearly exceeded seasonal variations. Onepossible explanation is an ‘‘amount effect’’ as defined byDansgaard (1964), which is clearly visible in the deuteriumexcess (d) of samples from rainfalls with little amount.The deuterium excess defined as

d ¼ d2H� 8 � d18O ðClark and Fritz; 1997Þ ð7Þ

varies with humidity, wind speed and sea surface tempera-ture during primary evaporation and averages 10& on a glo-bal scale. The lowest d values (<�7) of our record occurredduring rainfalls with less than about 2 mm of water (Fig. 3).Such low excess values were reported also for evaporativeisotope enrichment during rainfall through dry air fromother arid regions, like e.g., Oman (Clark and Fritz, 1997).

Isotope composition of precipitation, however, is notonly influenced by temperature during condensation of rain-drops in a cloud and evaporation processes thereafter, butalso by the origin and trajectory of the moisture-bringingair mass (e.g., Aravena et al., 1999). Different sources ofmoisture should play an important role as cause for isotopevariations of rain in southeastern Patagonia, as air massescrossing the Andes are increasingly depleted in 18O due toRayleigh distillation during rainout of the ascending clouds(Stern and Blisniuk, 2002). Thus, we compared the isotopecomposition of precipitation events with wind directions.Since evaporation effects may have altered d18O and d2Hof rainfall events with less than 2 mm, data from such lowamounts, as well as those with no clear dominance of onewind direction (i.e. <50% winds from one direction duringrainfall) were excluded from further comparisons with

1/02 7/02 1/03 7/03 1/04-202468

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Tai

r (˚C

)δ18

O (

‰)

δ2 H (

‰)

d

Month /Year

Figure 2 d18O, d2H and deuterium excess (d) of precipitationevents collected at Rıo Gallegos (filled circles) and LagunaPotrok Aike (open symbols) between February 2002 and March2004 compared to monthly temperature averages of RıoGallegos.

0 2 4 6 8 10 12 14 16 18 20

-25

-20

-15

-10

-5

0

5

10

15

d

Precipitation Amount (mm)

Figure 3 Deuterium excess (d) values of precipitation events at Rıo Gallegos (filled circles) and Laguna Potrok Aike (open symbols)vs. precipitation amount. Rainfalls <2 mm often have extremely low d values.

Precipitation origin and evaporation of lakes in semi-arid Patagonia 57

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meteorological data. Although the choice of this thresholdis to a certain degree arbitrary, the main outliers of d whichare the result of evaporation are thus discarded. The major-ity of precipitation events in extra-Andean Patagonia comesfrom westerly directions and consequently should be de-pleted in 18O and 2H. In contrast, air masses from easterlydirections, i.e. from the Atlantic region, should bring rainwith higher isotope values, as no major relief causes en-hanced rainout of heavy isotopes. Fig. 4 shows that rainfallsfrom air masses coming from westerly directions were con-sistently more depleted in heavy isotopes than those fromeasterly directions, with only one exception. On averaged18O and d2H of rainfall events from NNE to SE were 7&and 65& more enriched, respectively, than those fromSSW to NNW directions. Our data suggest that these chang-ing sources of moisture indeed play an important role forthe isotopic composition of precipitation in semi-arid Pata-gonia. Thus, short-term variability of the isotopic composi-tion of precipitation may originate to a large extent fromvarying sources of moisture in the investigated area.

All precipitation samples scatter along a regression linein d2H vs. d18O space (Fig. 5), the Local Meteoric Water Lineof Rıo Gallegos (LMWLRG), given by

d2H ¼ ð7:92� 0:24Þ � d18Oþ ð1:22� 3:03ÞR2 ¼ 0:95 ðn ¼ 56; p < 0:0001; t-testÞ ð8Þ

The slope of this line approaches the global averagewhereas its intercept is lower than the intercept of theGMWL. Regional isotopic survey of lake waters and isotopic

composition of vapor

The isotopic compositions of collected lake waters rangefrom 5.7& (d18O) and 15& (d2H) for Laguna Tres in 2002to �8.8& (d18O) and �77& (d2H) for Laguna Arenas Negrasin 2003 (Fig. 5 and Appendix). The wide range of isotope val-ues for water bodies only a few kilometers apart is best ex-plained by different E/I ratios due to diverse basinmorphologies, inflow rates and wind exposure. Large in-ter-site as well as inter-annual isotopic variations are ob-served for the shallow lakes (Fig. 7 and Appendix). Incontrast, the two deep crater lakes Laguna Azul and LagunaPotrok Aike exhibit similar isotope values and little inter-an-nual variations (Fig. 7). In spite of these differences be-tween lake types and years, the isotope values of allwater bodies plot along a regression line given by

d2H ¼ ð6:19� 0:18Þ � d18O� ð22:00� 0:67ÞR2 ¼ 0:97 ðn ¼ 34; p < 0:0001; t-testÞ ð9Þ

in d2H vs. d18O space (Fig. 5). The regression line representsthe local evaporation line (LEL) and the high correlationcoefficient confirms the similarity of the isotopic composi-tion of the source waters of the investigated lakes. Thus,the isotopic composition of inflow (dI) for all lakes can beapproximated from Eqs. (8) and (9) providing dI values of�105& for d2H and �13.4& for d18O. The LEL–LMWL inter-section is close to the main cluster (i.e. sites IV–IX in Fig. 1)of ground waters centered around �99& and �13.2& ford2H and d18O. Only a few other groundwaters exhibit highervalues possibly due to re-infiltrated lake water or due tosubfossil groundwater formed during times with differentisotope composition of precipitation. In good agreement

N NE E SE S SW W NW-25

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N NE E SE S SW W NW-200

-150

-100

-50

0

δ18O

(‰

)

Wind Direction

δ2 H (

‰)

Wind Direction

Figure 4 d18O and d2H values of precipitation events at RıoGallegos (filled circles) and Laguna Potrok Aike (open circles)vs. wind directions during the respective days. Only days withpredominance (>50%) of winds from one direction are given.Wind data are from hourly measurements at Laguna Potrok Aikeand at Rıo Gallegos. Bars give standard deviations for isotopemeans of rainfalls from westerly and easterly air masses,respectively.

Figure 5 Stable isotope values of precipitation (Rıo Gall-egos), surface water, groundwaters and atmospheric vapor ind2H vs. d18O space. Precipitation data plot on the given localmeteoric water line (LMWL), surface waters on a local evap-oration line (LEL).

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with the calculated value of dI is the mean isotopic compo-sition of Rıo Gallegos precipitation weighted with the pre-cipitation amount (dP wgt.mean) with �97& for d2H and�12.6& for d18O (Fig. 5). Thus, the main sources for lakewaters, i.e. ground water and precipitation, are isotopicallyindistinguishable. This is the reason why deep crater lakeslike Laguna Azul and Laguna Potrok Aike which are assumedto be fed to a large part by groundwater plot on the sameline as ephemeral lakes (e.g. Laguna Maar Tito) which havewater only after intense heavy rainfall events.

The dA values plot on a line left of the LMWL with a sim-ilar slope as the LEL in d2H–d18O space (Fig. 5) indicatingthat water vapor in semi-arid Patagonia is to a large extentderived from recycled moisture evaporated over the conti-nent. Hence, these results confirm that the calculations ofdA from dP assuming isotopic equilibrium often applied inhumid regions are invalid for semi-arid regions such as ex-tra-Andean Patagonia.

Depth profiles and seasonal isotope variations ofselected lakes

Depth profiles of Laguna Azul and Laguna Potrok Aike reflectthe different circulation patterns of the two lakes (Fig. 6).Laguna Azul is dimictic and has a stratified water body dur-ing austral summer and an inverse temperature gradient inwinter, whereas Laguna Potrok Aike is polymictic and exhib-its weak stratification only during short periods with calmsin extraordinary warm summers. Accordingly, stable isotopeprofiles of the lakes show distinct differences. The 56 mdeep Laguna Azul had d18O values enriched by 0.5& in theepilimnion (uppermost 18 m of the water column) relativeto the hypolimnion (below 20 m) in March 2002 (Fig. 6A,B). In the subsequent years epilimnic heavy isotope enrich-ment was lower and reached only 0.3&. The small but dis-tinct differences in epilimnic isotope enrichment betweenthe summer 2002 and the two subsequent summers are pos-

Figure 6 Depth profiles of d18O for Laguna Azul (a) and Laguna Potrok Aike (c). Temperature profiles for Laguna Azul (b) andLaguna Potrok Aike (d) are shown to demonstrate the different mixing types of the two lakes.

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sibly due to higher isotopic enrichment during a dry periodthat prevailed previous to sampling in February 2002. Thedepth profiles of the 100 m deep Laguna Potrok Aike exhibitno clear variations with depth in all three summers (Fig. 6C,D), except for the summer of 2004, when a weak stratifica-tion was established in the water body for a short windlessperiod.

The respective lake surface areas and crater morpholo-gies and thus wind exposures are most likely responsiblefor the different mixing behaviors of the two lakes. Thesmaller (maximum lake diameter of 560 m) and relativelyyoung (several thousand years) crater lake Laguna Azul issurrounded by up to 60 m high steep crater walls shelteringthe lake surface from extreme wind exposure. The consider-ably older and larger (maximum lake diameter of 3470 m)crater lake Laguna Potrok Aike has gentle slopes and is fullyexposed to the wind.

Surface water isotope variations are in a similar range ofless than 1& for both crater lakes during different years inthe monitoring interval (Fig. 7). The course of seasonal vari-ations, however, is distinct for the two lakes. This is proba-bly also the effect of different mixing patterns. Less

evaporated water from the hypolimnion admixes with theepilimnic water during spring and fall turnovers. d18O valuesof surface waters from Laguna Azul exhibit clear seasonalvariations following water temperature variations (Fig. 7).Higher isotope values were observed during late australsummer and autumn (December–May), lower values duringwinter and early spring (July–October) roughly followingwater temperature. The largest heavy isotope enrichmentduring the monitoring period was recorded at the beginningof March 2002, right at the end of the extremely dry period.

In contrast, the record of Laguna Potrok Aike exhibitsonly one shift in lake water isotope composition and no sea-sonal variations. The meteorological data suggest that ex-treme rainfalls might be responsible for the isotope shiftbetween February 27th and April 9th, 2002. The extremelydry year 2001 (199 mm of precipitation at Rıo Gallegos)was followed by a comparatively wet year 2002 (397 mmof precipitation at Rıo Gallegos) with heavy rainfalls startingjust after the sampling period in March 2002. However, in-ter-annual and intra-annual isotope variations for both, La-guna Azul and Laguna Potrok Aike, are relatively low due totheir large water volumes (0.01 km3 and 0.41 km3, respec-tively) compared to lake surface areas (0.15 km2 and7.58 km2) and an assumed permanent inflow of groundwa-ter. In contrast, the isotope record of the shallow (presum-ably less than 2 m deep) Laguna Tres de Enero exhibitsvariations that exceed those of the two crater lakes by morethan a factor of 10 (Fig. 7). Thus, the irregular water supplymainly by precipitation and surface run-off to Laguna Tresde Enero is directly reflected in its isotope composition.

Therefore, isotopic enrichment of lake waters in thesouthern Patagonian steppe is largely dependent on the laketype. Differences between lakes can be explained by vari-able ratios of surface area to volume and by different mix-ing patterns of the lakes affecting the isotopic compositionof the lake waters. Furthermore, the balance between con-tinuous groundwater inflow and episodic surface run-off andprecipitation in this respect is important. In the following, afirst approach is made to estimate evaporation to inflow ra-tios (E/I) for the deep crater lakes Laguna Azul and LagunaPotrok Aike based on the available isotope and meteorolog-ical data.

Estimations of E/I

For E/I estimations the data given in Table 1 were insertedinto Eq. (6). Atmospheric vapor was trapped at differentdays and daytimes at the INTA station near Laguna PotrokAike (see Appendix). Lake water evaporation could not con-tribute to the vapor trapped, as the station is west of Lagu-na Potrok Aike and west winds prevailed during the trappingperiod. The respective means of measured dA values are�19.3& for d18O and �142& for d2H. Based on these data,d* can be calculated using the known values of water tem-perature and humidity as the input data for Eq. (5). Theresulting d* values of 6.5& (d18O) and 24& (d2H) are closeto the isotope composition of Laguna Tres in the extremelydry February 2002, 5.7& and 15&, respectively (see Appen-dix). We thus assume that Laguna Tres during that time waslike a desiccating water body without further inflow and wasclose to the limiting isotopic enrichment. The intersectionpoint between LEL and LMWL was taken as mean isotope

-4.0

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Figure 7 Seasonal and inter-annual variations of d18O forthree lakes (d18OL of Laguna Ea. Tres de Enero, Laguna PotrokAike and Laguna Azul) and parameters influencing isotopevariations. Precipitation (P) data given are monthly means ofRıo Gallegos. Water temperatures (Twater) are monthly meansof direct measurements with thermistors (filled circles refer toLaguna Azul, open squares to Laguna Potrok Aike).

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pycomposition of the inflow waters (dI). This is reasonable, assurface waters plot on or next to the LEL.

E/I estimations based on hydrogen and oxygen isotopes,respectively, agree for Laguna Azul and Laguna Potrok Aikewithin 3–5% (Table 1). Small differences between hydrogenand oxygen estimations might be evoked by slight deviationsof lake water isotope composition from the LEL. No majordifferences were found between E/I calculated for 2002and 2003 (calendar years) which is consistent with the smalllake level variations between these years at Laguna Azul. AtLaguna Potrok Aike the lake level increased between Febru-ary 2002 and February 2003 by about 1 m. Nevertheless E/Ivalues of 2002 and 2003 are almost identical, because an-nual means of dL were used for the calculation and the lakelevel rise already took place in early 2002 contemporane-ously with the drop of dL (Fig. 7). The model implies that58% (2002) and 57% (2003) of the water flowing into LagunaPotrok Aike undergoes evaporation (Table 1). The values ofLaguna Azul are lower, namely 51% in 2002 and 50% in 2003.In a first approximation these lakes can be considered closeto hydrologic steady state for the two years. In this case, aconsiderable amount (ca. 42% and 49%, for Laguna PotrokAike and Laguna Azul, respectively) of the inflowing waterre-infiltrates into the groundwater. Thus, both lakes arenot terminal lakes in the sense defined by Gat (1995), butrather groundwater-fed lakes with considerable groundwa-ter through-flow.

Sensitivity tests

The reliability of the presented E/I calculations is stronglydependent on the validity of the model input data. Someof the variables necessary for the model calculations are

well known due to extensive monitoring (T, h, dL), othersbase on less data (e.g., dA). Moreover, uncertainty in somevariables might have a greater influence on E/I estimationsthan in others. Thus, sensitivity analyses were carried out toelaborate the limitations and uncertainties of the approach.Changes of the main input parameters by ±10% demonstratethe varying sensitivity of E/I depending on the variable(Table 2). The least critical parameter according to thisapproach is d*. An increase of d* of 10% results in a decreaseof E/I of 4%. In contrast, a 10% increase of hflw decreasesE/I by about 25% (Table 2). The actual errors for hflw, how-ever, are far less than the assumed 10% as the data base iscomparatively good. In contrast, the actual error for d*

might be critical as the value is calculated from a varietyof parameters each with its own error. The main source oferror of the calculated d* may be dA, which bases on few,but consistent values. Additionally, a main source of erroris dI, which is defined by the crossing point between LMWLand LEL. The combined errors of the single parametersmay either increase or reduce the overall error, as thevarious parameters have in part opposing effects on E/I(Table 2). An exact determination of the total error is thusnot possible.

However, the principle assumptions can also be tested bycalculating the slope of the LEL using Eq. (4) and comparingit with the slope given by the regression through the lakeisotope data (Eq. (9)). As input the values of dA, e, h anddI used for E/I estimations were used, too (Table 1). Theresulting slope of the LEL is 6.49 and thus close to the ob-served value of 6.19 ± 0.18. Calculations of SLEL with dA de-rived from dI (or likewise from dP) assuming isotopicequilibrium with precipitation give lower slopes around4.5 and do not approach the observed slope of the LEL. This

Table 1 Input parameters and calculated E/I for Laguna Azul and Laguna Potrok Aike

Input parameters Hydrogen Oxygen Data source

hflw 0.63 0.63 INTA meteorological station (mean 1999–2005),weighted with evapotranspiration data of FAO (inOliva et al., 2001)

eK 4.63 5.25 Calculated according to Eq. (2)e*flw (PTA) 100.01 10.95 Horita and Wesolowski, 1994; water temperature

from thermistors, weighted with evapotranspirationdata of FAO (in Oliva et al., 2001)

e*flw (AZU) 97.91 10.80 Horita and Wesolowski, 1994* water temperaturefrom thermistors, weighted with evapotranspirationdata of FAO (in Oliva et al., 2001)

T (PTA; �C) 7.30 �C 7.30 �C Annual average from monthly meansT (AZU; �C) 7.58 �C 7.58 �C Annual average from monthly meansdI �105.2 �13.4 Calculated from Eqs. (7) and (8)dA �142.0 �19.3 Mean of analysesd* 24.1 6.5 Calculated from Eq. (5)dL Annual means from Table 1

Site E/I calculated from hydrogen isotopes E/I calculated from oxygen isotopes Mean E/I

Potrok Aike (2002) 0.59 0.56 0.58Potrok Aike (2003) 0.59 0.55 0.57Laguna Azul (2002) 0.49 0.53 0.51Laguna Azul (2003) 0.49 0.52 0.50

Precipitation origin and evaporation of lakes in semi-arid Patagonia 61

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result evidences the validity of the given input data, espe-cially of dA.

Conclusions and outlook

Isotopic composition of precipitation in semi-arid southernPatagonia is highly variable and barely seasonally con-trolled. This variability is at least in part explained by dif-ferent pathways of the rain-bringing air masses. We wereable to show that air masses coming from southern and wes-tern directions in general have significantly lower d valuesthan rains from northern and eastern directions due to rain-out effects of air-masses passing the southern Andes. Evap-oration effects dominate the isotopic composition of manyrainfall events with less than 2 mm, as indicated by theirlow d values.

Isotope values of water bodies in this region plot on awell-defined evaporation line, independently of theirrespective depth, mixing, surface area or lake type. Thisis possible only in the case that isotopic compositions ofthe inflow waters are similar for all lakes, regardless if cur-rent precipitation or groundwater is the main source of in-flow. The majority of groundwater samples cluster aroundthe intersection of the Local Evaporation Line with theLMWL proving that groundwater is one of the main sourcesof water for many of the lakes. This is especially true forthe deep crater lakes, Laguna Azul and Laguna Potrok Aike,which were investigated in greater detail. The usually ap-plied assumption of isotopic equilibrium between vaporand precipitation is invalid for the semi-arid region of south-ern Patagonia. The E/I estimations given from model calcu-lations have shown that both lakes are ground water-through-flow lakes and have considerable leaking of lakewater into the groundwater.

Ongoing studies will focus on isotope records of the sed-iments from both lakes to provide reconstructions of waterbalances for the past. These studies should allow decipher-ing hydrological changes that took place in semi-arid Pata-gonia during the last millennia.

Acknowledgements

We are much indebted to Jose Larosa, Guillermo Clifton andPedro Tiberi for collecting water samples, as well as to

Beatriz Milicic for providing meteorological data from RıoGallegos. We thank Hugo Corbella, Jorge Moreteau and Cris-tobal Kennard for logistic help. Two anonymous reviewersand especially T.W.D. Edwards are thanked for constructivecomments on an earlier version of the manuscript. Financialsupport (Grants 01 LD 0034/35) of the German Federal Min-istry of Education and Research (BMBF) in the framework ofthe German Climate Research Programme and by the DFGPriority Programme ‘‘ICDP’’ (Zo 102/5–1,2) is gratefullyacknowledged.

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at doi:10.1016/j.jhydrol.2006.09.025.

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