Isotopic estimation of the evapo-transpiration flux in a plain agricultural region (Po plain, Northern Italy)

Applied Geochemistry 34 (2013) 53–64

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Isotopic estimation of the evapo-transpiration flux in a plain agricultural region (Po plain, Northern Italy) Giovanni Elmi a,⇑, Elisa Sacchi b, Gian Maria Zuppi c,1, Marcello Cerasuolo a, Enrico Allais d a

Dipartimento di Scienze Ambientali, Università Ca’ Foscari di Venezia, Italy Dipartimento di Scienze della Terra e dell’Ambiente, Università di Pavia e CNR-IGG, U.O. di Pavia, Italy c CNR-IGAG, Roma, Italy d ISO4 s.n.c., Torino, Italy b

a r t i c l e

i n f o

Article history: Available online 7 January 2013

a b s t r a c t Samples of water vapour and monthly precipitation were collected in Pavia, located 50 km south of Milan (Western Po plain, Northern Italy), over a period of 19 months, from March 2006 to September 2007. Results are interpreted in relation to the local climatic factors (temperature and precipitation rates), and to air mass circulation patterns, derived from sea level pressure maps, geopotential maps and satellite images. Since most water vapour samples represent a mixture of continental air masses and local evapo-transpiration fluxes, a computational method based on the stable isotope content (EMMA) has been used to evaluate the percentage of the different components and to quantify the local vapour fraction. The regression line equation for rainwater samples is:

d2 Hvs:VSMOW ¼ 8:8 ð0:5Þ  d18 Ovs:SMOW þ 14:5 ð3:5Þ‰ ðR2 ¼ 0:96; n ¼ 17Þ The slope of the line is extremely high and probably related to the dataset used, which includes two summer seasons and one winter season. In addition, the latter was somewhat anomalous, with recorded average temperatures higher than the average calculated for the years 1970–2002. The mean annual weighted d18O in rainwater samples is equal to 6.90 ± 2.2‰. The regression line equation for water vapour samples is:

d2 Hvs:VSMOW ¼ 6:8 ð0:3Þ  d18 Ovs:SMOW  7:4 ð4:9Þ‰ ðR2 ¼ 0:92; n ¼ 37Þ: The two regression lines meet at d18O = 10.82 ± 13.97‰. This value appears more depleted than the mean annual weighted precipitation value, but is close to the isotope composition of the phreatic aquifer (d18O = 9.0 ± 0.5 to 10.4 ± 0.3‰). In addition, the slope of the water vapour regression line (6.8) indicates evaporation under high relative humidity (Rh = 95%). The isotope composition of the water vapour in the Pavia area results from three main components: moisture carried by continental cold circulation, by maritime (Atlantic and Mediterranean) circulations and by the local evapo-transpiration flux. The latter is more intense in late spring and summer, due the maximum vegetation activity but also to irrigation and rice field flooding, and to the consequent maximum rise of the water table level. The turbulence due to the dynamic nature of the troposphere mixes local atmospheric moisture with water vapour carried by advection from other regions. Even in the most obvious situations of no circulation, phenomena of exclusive local evapo-transpiration are seldom observed. Circulation of air masses from the Mediterranean and the Atlantic carry the major part of atmospheric moisture towards the Po plain, while only cold and dry air masses from the NE, which are not found in the summer, are able to completely lower the local isotopic signal of the vapour, because of the substitution effect caused by their higher density. The expected correlation between the evapo-transpiration flux intensity and the precipitation amount is significant, and little difference in the local evapotranspiration flux is observed between 2006 and 2007. On the other hand, the isotope composition of water vapour testifies to the importance of irrigation as a source of local vapour, evidencing, even at mid-latitudes, a regional scale feedback between land use and climate. Ó 2013 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +39 0412348666; fax: +39 0412348584. 1

E-mail address: [email protected] (G. Elmi). Deceased.

0883-2927/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apgeochem.2012.12.010

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G. Elmi et al. / Applied Geochemistry 34 (2013) 53–64

1. Introduction Water vapour is an important components of the atmosphere, indeed, it is at the fourth most abundant, after N2 (78% of the atmosphere), O2 (21%) and Ar (0.9%). The atmospheric abundance of water vapour varies from 0.4 to 400 102 ppm (IAEA, 2001). This variability is due to physical transformations and chemical reactions which involve atmospheric moisture. On the other hand, water vapour is important also because of its environmental effects. Together with CO2, N oxides and CH4, water vapour is classified among the so-called greenhouse gases, i. e. the gases (and other compounds) responsible for the greenhouse effect and the subsequent global warming. Although atmospheric moisture is less powerful than other greenhouse gases in reflecting terrestrial thermal radiation, its contribution is no less important for its abundance. According to some researchers, one effect of global warming could be a massif intensification of the evapo-traspiration processes on Earth’s surface. The larger presence of vapour in the atmosphere could constitute a positive feedback mechanism, although the presence of clouds may counterbalance this effect by raising the albedo, i. e. the percentage of solar radiation reflected by bright surfaces (Loáiciga, 2003). Atmospheric water vapour (or, more precisely, that which is in the troposphere) has dubious origins because of the extremely dynamic nature of air masses. The vapour mostly originates from the tropical seas and can be transported by global circulation through a process known as advection; nevertheless, a certain quantity may originate through processes of local recirculation. The latter involves mainly continental areas, where atmospheric moisture can originate either from surface waters, from shallow aquifers and even from raindrops (evaporation), or from living organisms, and particularly from plants (transpiration). Evapo-transpiration from continental waters is a relevant climatic feature in terrestrial areas (Ter Maat et al., 2006; Froehlich et al., 2008). The relative importance of the advection process and the local evapo-transpiration flux thus contribute to the hydrological budget. Stable isotope analysis of atmospheric waters has proven to be a valid instrument of scientific investigation. Deuterium and 18O ratios allow the investigator to distinguish between vapour carried by atmospheric circulation and vapour of local origin; furthermore, they are good tracers of air mass circulation. Many experiences have demonstrated that, after the evaporation process, water vapour is significantly depleted in 2H and 18O, if compared with the original liquid water, because the lighter isotopes are kinetically favoured in this process. The latter, being a thermodynamic process, is controlled by temperature and relative air humidity, so that approaching conditions of thermodynamic equilibrium, vapour and liquid water tend to be more similar in composition (Clark and Fritz, 1997 and references therein). Isotopic fractionation of atmospheric waters, according to the origin and pathway of air masses by which they are transported, is, nowadays, well known. Changes in the isotopic composition of water vapour moving in the troposphere are less studied than changes affecting rainwater, for evident practical reasons. Heavier isotopes are more involved in rainout processes and atmospheric moisture can be considered a residual of water vapour condensed and possibly deposited to the soil from the same air mass, so that it is possible to estimate that water vapour isotopic composition varies in troposphere according to the same latitude, altitude and continental effects which have been reported in literature for rainwater. The factors which determine the stable isotope content of rainwater have been investigated in a more detailed way and many data have been collected since 1960 by the GNIP, i.e. the Global Network for Isotopes in Precipitation, operated by the International Atomic Energy Agency (IAEA) and the World Meteorological

Organization (WMO). On the other hand, far fewer studies found in the literature have dealt with atmospheric moisture. Recently, the IAEA initiated efforts to improve the availability of isotope data on other water cycle components, by constituting the group for Moisture Isotopes in the Biosphere and Atmosphere (IAEA–MIBA). The collected data are expected to provide an alternative to present dependency on model output for some key variables, further advancing the understanding of regional scale hydrological budgets and enhancing the development of new global change indicators: indeed, 18O and 2H in atmospheric vapour can be used as indicators for regional to global-scale reductions in evaporation perhaps in response to changes in global dimming and brightening (IAEA, 2005b). From the global perspective, Central Europe, the Mediterranean Basin, North Africa and the Sahel, reveal a relatively simple pattern of spatial and temporal composition of precipitation. Indeed, the displacement of the Intertropical Convergence Zone (ITCZ) and the Azores anticyclone control the climatological regime and, consequently, the isotopic composition of rainfall (Zuppi and Sacchi, 2004). A precise knowledge of the distribution of the isotopic composition of local precipitation and its relationship with local environmental conditions is crucial for hydrological studies on a regional scale. In the case of the Po plain, one of the largest sedimentary basins in Europe, hosting a multilayer aquifer of strategic importance, the situation is complicated by the presence of the Alps and Apennines ridges, bordering the plain on three sides, and the Adriatic sea, bordering the plain on the eastern side (Fig. 1). A first comprehensive map of the isotopic composition of precipitation in Italy was produced by Longinelli and Selmo (2003), and data from 36 stations allowed defining the equation for the Meteoric Water Line in Northern Italy. Results also evidence an addition of water vapour from the Adriatic sea, causing an enrichment in the isotopic composition of precipitation in the eastern and central section of the Po plain, and a marked ‘‘shadow effect’’ related to westerly air masses moving over the Apennines, causing a depletion in the isotopic composition of precipitation in the southern part of the Po plain. In a following paper, Longinelli et al. (2006) compared the long-term weighted yearly mean values with precipitation data from the years 2002–2004, to find that the latter were considerably more depleted than those calculated for the previous years. This is attributed to the climatic anomaly of 2003, coupling the hottest summer of the last century to a marked decrease in the amount of spring and summer precipitation, and, therefore, increasing the relative weight of the isotopically depleted winter and autumn precipitations. The shift was particularly marked at the collection sites located along the northern slope of the Apennines affected by the previously mentioned shadow effect. The primary goal of the present study is to produce some preliminary information about the isotopic composition of water vapour and precipitation and to interpret the isotopic variations as a function of air mass circulation as well as meteo-climatic parameters. Isotopic data, collected over a period of 19 months (from March 2006 to September 2007) have been compared with variability in local climatic factors (temperature and precipitation rates), and the relationship with circulation patterns has been taken into account. Since most water vapour samples represent a mixture of continental air masses and local evapo-transpiration fluxes, a computational method based on the stable isotope content has been used to evaluate the percentage of the different components and to quantify the local vapour fraction. The identification of the sources of water vapour in this area is a clue to understanding the hydrological cycle over the Mediterranean region, characterised by varying wind directions during the year. In addition, the study of atmospheric circulation is very useful, as it allows tracing the origin of atmospheric pollutants and

G. Elmi et al. / Applied Geochemistry 34 (2013) 53–64

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Fig. 1. Location (A) and land use map (B) of the study area.

forecasting the distribution of contaminants released into the atmosphere from a certain source. Therefore, this investigation may provide important information for studies on atmospheric pollution transport and on the role of atmospheric moisture in the field of climatic change, particularly referring to the greenhouse effect.

2. Study area The low-lying region around Pavia, in the western Po plain just south of Milan, is mainly exploited for agriculture and is rich in water resources (Fig. 1). The plain sector of the Pavia province, covering an area of about 1600 km2, is cultivated with mainly rice (51%) and corn and wheat (30%) (data for 2007, source Provincia di Pavia, Settore agricoltura), all requiring great quantities of water for their growth: rice fields are in fact flooded with water for several months per year (March–August); often corn is cultivated through flooding as well. The entire area is crossed by several rivers and streams (the most important being the Po and Ticino rivers), as well as by canals which carry water from rivers and springs to cultivated fields. The region is characterised by an elevated potential infiltration, and soils are arenosols and luvisols, with acidic pH, mainly constituted by quartz and feldspars (ERSAF, 2004). The water table is not very deep (generally about 1–2 m), and in the surroundings of Pavia, the existence of a perched aquifer has been demonstrated (Pilla, 1998). The phreatic aquifer is recharged locally, but strong recharge takes place from seepages from the channel network and from rice ponds. This is evidenced by the seasonal fluctuation of the water table, showing a steep rise, locally up to 1.6 m, related to field waterlogging. Data collected from local irrigation agencies (Associazione Irrigazione Est-Sesia,

1984) allowed calculating the entire season recharge which is about 38,000 m3/ha, greatly exceeding natural recharge from precipitation (Sacchi et al., 2007). The climate, in the western sector of the Po plain, is classified as temperate continental. Normally, winters are cold and summers are very hot and wet, because the mitigating effect of the sea is absent. Spring and autumn are characterised by the highest precipitation. The temperature and precipitation regime characterising this region are illustrated in Fig. 2. Since the latitude of this region is between the sub-tropical high pressures zone and the sub-polar low pressures zone, westerly winds blow constantly (the so-called westerly zonal winds). Their direction from west to east is determined by dynamic high pressure centres and terrestrial rotation. Thermal cyclonic and anticyclonic systems, which originate from differential warming on the terrestrial surface by the Sun, are important for air mass circulation, too. The Po plain is surrounded on three sides by the Alps and the Apennines: for this reason, it is less affected by the pressure disequilibria common in northerncentral Europe. An important factor which determines the climate of all the Mediterranean area is the Azores high, which originates over the Atlantic Ocean and moves from SW to NE and vice versa, according to the seasons. The anticyclone influences southern Europe primarily in summer, and determines the conditions of fine weather typical of that season. A typical winter situation involves the movement of air masses from the NE due to the Russian-Siberian high, which originates over the Siberian plains due to their extreme winter cooling. Anticyclonic systems above northern Africa often cause the movement of air masses from the south. Winds blowing from the south or SE may also be the consequence of a cyclogenesis over the gulf of Genoa, which is relatively frequent, mainly during fall: a low pressure centre may originate on the Ligurian sea from the evolution of Atlantic perturbations or leewards

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Fig. 2. Climate diagram for Pavia, based on data collected between 1970 and 2002 (Source: ARPA Lombardia. U.O. Idrografia), compared to average temperature and monthly precipitation data from 2006 and 2007.

from the Alps because of the advection and whirling of cold air through the Rhone valley (the latter called lee cyclogenesis; Trigo et al., 2002). The Po Valley, due to its geographical settings, is commonly affected by the formation of persistent fog. Several studies have been conducted to understand the mechanisms of formation (e.g. Wobrock et al., 1992), the fog chemical characteristics (Fuzzi et al., 1992, 1996) and its role in the formation of building corrosion crusts (DelMonte and Rossi, 1997).

Oxygen isotope composition was measured by water-CO2 equilibration at 25 °C (Epstein and Mayeda, 1953) using 3 mL of sample. The analytical errors are ±1‰ for H isotope analyses and ±0.1‰ for O isotope analyses. All gases were analysed on a Finnigan MAT 250 Mass Spectrometer at ISO4 s.n.c., Torino, Italy. The results are expressed in ‰ vs. the international standard V-SMOW (Gonfiantini, 1978; Gonfiantini et al., 1995). In order to help determine the provenance of air masses during water vapour sampling operations, different types of images have been collected:

3. Methods

1. Sea-level pressure maps, useful in understanding low elevation circulation conditions. 2. Geopotential maps at 850 and 500 hPa, which help to reconstruct air mass movements in the medium and low troposphere, respectively; these maps and those mentioned in point 1 have been provided by the Wetterzentrale database. 3. Satellite images, taken in the high resolution visible (HRV) band and provided by the satellite Eumetsat, which are useful to reconstruct the movements of air masses at different altitudes in the troposphere at the precise moment of each sampling operation, as the pressure maps were all referred to 0:00 GMT, while the sampling operations took place in the morning of the same day.

Water vapour samples were collected every 15–20 days, in Pavia, within the University campus. The sampling period was designed in order to cover at regular intervals two crop growing seasons (spring and summer) and one winter. Indeed, it is expected that, due to agronomical practices, the evapo-transpiration flux related to field flooding would be much greater during summer. Samples were collected through a vacuum line composed of two Pyrex traps immersed in liquid N2, as recommended by the IAEA (2005a). At the beginning and end of each sampling operation, lasting typically 2–3 h, data of air temperature and air relative humidity were recorded, as well as other meteorological conditions. Water vapour was frozen upon collection, then melted at room temperature to let the vapour and the liquid phase equilibrate thermodynamically and isotopically, providing at least 4 mL of water. Monthly precipitation samples, on the other hand, have been collected by a pluviometer installed on the same university campus and preserved in tanks covered with vaseline oil to prevent evaporation. The condensed water vapour, as well as rainwater samples, were analysed by IRMS. Hydrogen isotope composition was determined by water reduction over metallic Zn (Coleman et al., 1982).

The origin of air masses has been estimated approximately by analysing the sea-level pressure maps, under the hypothesis that air flux is parallel to isobars, with a movement in a clockwise direction from the high pressure centres and in an anticlockwise direction approaching the low pressure ones. The categorisation made upon the first analysis has been improved with the observation of satellite images and geopotential maps. Here the movement of air masses, roughly, is parallel to isolines so that isolines with higher values lay on the right of the air flux.

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G. Elmi et al. / Applied Geochemistry 34 (2013) 53–64 Table 1 Amount and isotope composition of monthly precipitation samples. Sample

Month

d18O (‰) vs. VSMOW

d2H (‰) vs. VSMOW

d Excess

Amount of precipitation (mm)

PREC1 PREC2 – PREC3 PREC4 PREC5 PREC6 PREC7 PREC8 PREC9 PREC10 PREC11 PREC12 PREC13 PREC16 PREC17 PREC18 PREC19

March 2006 April 2006 May 2006 June 2006 July 2006 August 2006 September 2006 October 2006 November 2006 December 2006 January 2007 February 2007 March 2007 April 2007 May 2007 June 2007 July 2007 August 2007

7.86 8.89 n.d. 4.69 5.07 5.06 5.69 6.18 4.04 6.16 6.07 8.33 13.37 7.26 8.84 7.63 5.74 6.53

58.2 64.4 n.d. 30.6 32.7 32.3 35.3 33.2 12.9 37.3 41.4 57.7 98.8 55.0 68.1 56.9 35.6 39.4

4.7 6.7 n.d. 6.9 7.8 8.2 10.2 16.2 19.4 12.0 7.1 9.0 8.2 3.1 2.6 4.1 10.3 12.8

15.7 55.9 13.7 22.6 35.3 110.2 122.2 32.5 25.7 40.1 47.5 18.9 69.7 10.9 85.4 81.0 7.4 122.1

n.d.: not determined.

Table 2 Results of the atmospheric water vapour samples and climatic parameters measured during sampling. Sample

Date

d18O (‰) vs. VSMOW

d2H (‰) vs. VSMOW

Temp. (°C)

Relative humidity (%)

Weather

Wind direction

Wind speed (km h1)

VAP1 VAP2 VAP3 VAP4 VAP5 VAP6 VAP7 VAP8 VAP9 VAP10 VAP11 VAP12 VAP13 VAP14 VAP15 VAP16 VAP17 VAP18 VAP19 VAP20 VAP21 VAP22 VAP23 VAP24 VAP25 VAP26 VAP27 VAP28 VAP29 VAP30 VAP31 VAP32 VAP33 VAP34 VAP35 VAP36 VAP37

15/3/2006 31/3/2006 18/4/2006 2/5/1006 15/5/2006 30/5/2006 14/6/2006 3/7/2006 17/7/2006 2/8/2006 21/8/2006 4/9/2006 20/9/2006 4/10/2006 18/10/2006 6/11/2006 20/11/2006 5/12/2006 20/12/2006 4/1/2007 22/1/2007 5/2/2007 20/2/2007 7/3/2007 27/3/2007 10/4/2007 19/4/2007 8/5/2007 17/5/2007 12/6/2007 21/6/2007 16/7/2007 26/7/2007 3/8/2007 15/8/2007 7/9/2007 24/9/2007

22.58 12.10 14.92 18.38 14.10 22.13 16.13 16.60 15.36 13.49 11.34 11.27 14.75 13.36 13.95 14.79 15.34 15.05 20.65 18.02 14.15 17.72 19.33 12.05 20.17 13.40 11.64 14.70 12.76 11.63 12.04 13.25 14.90 13.72 11.73 12.39 12.52

163.3 88.9 104.7 129.0 102.5 152.9 107.6 108.0 96.7 94.3 86.9 79.6 109.6 98.8 103.2 94.9 110.6 109.2 151.1 133.4 112.7 134.4 147.9 95.3 152.7 103.3 91.3 112.9 98.9 89.7 90.7 98.9 106.4 98.4 87.1 89.8 96.5

12.5 19.0 20.0 16.5 25.0 24.5 36.0 37.0 33.0 32.5 27.0 28.5 24.5 23.0 14.0 14.0 12.5 9.5 6.5 3.0 8.5 4.0 6.0 13.0 16.5 20.0 19.5 28.5 22.5 29.0 32.5 33.0 29.0 28.0 29.0 24.0 19.0

92 76 65 67 63 51 40 56 70 55 72 51 64 60 70 72 95 93 85 72 84 76 77 90 80 50 38 89 96 68 62 55 60 53 49 77 95

Sunny Slightly cloudy Rainy Rainy Partly cloudy Slightly cloudy Sunny Sunny Sunny Slightly cloudy Sunny Sunny Sunny Partly cloudy Partly cloudy Cloudy Foggy Foggy Sunny Partly cloudy Cloudy Foggy Foggy Rainy Slightly cloudy Sunny Cloudy Cloudy Cloudy Cloudy Sunny Sunny Sunny Slightly cloudy Sunny Sunny Foggy

WSW NW S SW ENE N SW NNE NE NE SSW SW NE SW E WSW WSW ENE NE NE NE SSW SW E NE SW E WSW E WSW S NNW NE NE WSW WSW ENE

0.9 5.2 2.8 9.2 1.5 6.8 9.3 2.5 2.5 1.6 3.3 1.8 1.4 7.0 6.4 6.0 8.5 7.1 0.2 4.1 3.1 4.2 11.2 7.0 5.4 3.2 5.0 3.2 1.1 3.1 2.6 1.2 3.1 5.3 3.7 4.8 4.8

4. Results Data obtained from the isotopic analyses of rainwater samples are presented in Table 1, together with the monthly precipitation data. In Table 2, on the other hand, the isotope values determined on water vapour samples are presented with the observed data of temperature, relative humidity and weather conditions.

Climatic parameters and precipitation amounts measured onsite were validated by comparison with those recorded by an automatic monitoring station (Davis Vantage Pro 2 Wireless) located approximately 500 m SW of the University campus at 12 m altitude above ground, belonging to the Regional meteorological network (Rete Meteorologica Lombarda). Data sets are in general good agreement. Nevertheless, temperature data measured during

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sampling operations (Ts) appear to be higher, in comparison with those of the reference monitoring station (Tr), but directly proportional, according to the equation

T s ¼ 1:28 ð0:06Þ  T r þ 1:38 ð1:07Þ ðR2 ¼ 0:92; n ¼ 37Þ The discrepancy is attributed to the presence of concrete walls and pavements and to the altitude of the water vapour sampling point (1 m above ground level) with respect to the weather monitoring station (12 m). For all calculations, the more precise data from the automatic monitoring station were used, with the exception of temperature, measured during the water vapour sampling (Table 1). On the basis of the observation of the meteorologic maps and satellite images, water vapour samplings were classified according to the estimated air mass origin. The 37 sampling circulations have been divided as follows:  Absence of circulation (11 cases).  Winds from Siberia (NE/East, 6 cases) or from northern Europe (North, 1 case). These situations have been classified in the same category because they are both continentally-derived air masses, and are cold and dry.  Winds from the northern Atlantic (NW 6 cases), cold and wet.  Winds from northern Africa and the Mediterranean (SW/South/ SE, 5 cases), which are warm and have varying levels of humidity.  Winds from the middle Atlantic (West, 5 cases), wet with varying temperature.  Cyclogenesis on the gulf of Genoa (3 cases). Some examples of maps representing these categories are shown in Fig. 3. 5. Discussion Results of the isotope composition of rainwater and water vapour (Table 1 and 2) belong to two distinct groups of values, with the exception of the rainwater value of March 2007, which lies in the isotope range of the water vapour group. This separation into two groups is in agreement with the principle that light isotopes are favoured in the evaporation process, while heavy isotopes are favoured in the condensation process. On a d2H vs. d18O plot, results shown in Tables 1 and 2 are displaced around two straight lines (Figs. 4 and 5).

recorded during September 2006 to May 2007 was considerably higher than the average calculated for the years 1970–2002 (Fig. 2). The most depleted isotope composition, recorded in March 2007 (Table 1), corresponds to a major rain event (35.6 mm precipitation on March 25 over a total in March of 69.7 mm) related to the transit of a cold front over the area. These climatic conditions persisted during the water vapour sampling performed on March 27, 2007, also showing one of the most depleted isotopic compositions. The mean annual weighted d18O in rainwater samples is equal to 6.90 ± 2.2. This value is slightly enriched but comparable to the values indicated by Longinelli and Selmo (2003) for precipitation in the area (Fig. 4), ranging between 7.73‰ (Parma station) and 7.13‰ (Milano station). This again is an effect of the anomalous winter season included in the observation period, since the isotopic composition of monthly precipitation is higher than expected. Longinelli et al. (2006) have demonstrated that in Northern Italy anomalous climatic events may affect the mean annual weighted d18O values, causing a significant shift from values calculated for previous years. The authors compared data obtained for the years 2002–2004, which includes the hottest summer of the last century, with those obtained in the previous years. This climatic event should have determined a marked enrichment in heavy isotopes in the annual weighted means for that year. On the contrary, yearly mean isotopic values were considerably more negative than the previously calculated values, due to the marked decrease in the yearly amount of precipitation and particularly that of spring and summer. Isotope data variation over time shows no correlation with seasonal temperature fluctuations, as shown in Fig. 6. The variation range in isotope values is lower in summer and early autumn, probably due to a stronger local evapo-transpiration flux, which could have a buffer-effect, stabilizing the oscillation of these values. Deuterium excess, calculated using the Dansgaard (1964) formula

d ¼ d2 H  8d18 O

5.1. Precipitation data

is also reported in Table 1. This parameter reflects the prevailing conditions during evolution and interaction of air masses throughout the storm trajectories (Froehlich et al., 2002). Its seasonal variation is in agreement with what is expected, i.e. high in summer and low in winter. Indeed, the d value is inversely correlated with the humidity, which is lower in winter and higher in summer (Rozanski et al., 1993).

The regression line of rainwater samples is shown by the equation:

5.2. Water vapour data

d2 Hvs:VSMOW ¼ 8:8 ð0:5Þ  d18 Ovs:SMOW þ 14:5 ð3:5Þ‰ ðR2 ¼ 0:96; n ¼ 17Þ The slope of the rainwater regression line is considerably higher than that of the Global Meteoric Water Line (which is equal to 8.17 ± 0.07, Rozanski et al., 1993), and much higher than that measured by Longinelli and Selmo (2003) in northern Italy, whose value is 7.71. Even the slope measured by Pilla et al. (2006) in groundwater of the Lomellina region is lower, being equal to 7.78. Nevertheless, Longinelli and Selmo (2003) report for their stations slope values ranging from 5.7 to 8.9, confirming that the equation of the Global Meteoric Water Line is generally inadequate to describe precipitation over the Mediterranean area. In the present case, this high slope is probably related to the dataset used, which includes two summer seasons and one winter season. In addition, the latter was anomalous, since the average temperature

The water vapour regression line is represented by the equation

d2 Hvs:VSMOW ¼ 6:8 ð0:5Þ  d18 Ovs:SMOW  7:4 ð4:9Þ‰ ðR2 ¼ 0:92; n ¼ 37Þ The slope is higher than that calculated at the Lisbon station (Carreira et al., 2005), and similar to that evaluated for the air moisture at the Rabat station (Ouda et al., 2005). This is due to the different origin and trajectories of the air masses during the sampling periods. According to the synoptic weather maps presented by the above mentioned authors, the air moisture sampled for isotope analyses was in most cases related to three distinct meteorological situations over the western Mediterranean basin: a strong movement of air masses from SE to NW, an orthogonal movement from the North Atlantic and a third movement from NE to SW.

G. Elmi et al. / Applied Geochemistry 34 (2013) 53–64

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Fig. 3. Pressure maps illustrating the most frequent meteorological conditions.

5.3. Impact of the local hydrological parameters According to Clark and Fritz (1997), during evaporative enrichment of liquid water, the vapour will have a reciprocal depletion,

and plot on the same evaporative line, but opposite to the initial composition of the water, on the left side of the meteoric water line. Therefore, the ‘‘initial water’’ isotope composition is found where the water vapour and the precipitation regression lines

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Fig. 4. Isotopic composition of rainwater samples, compared to the GMWL. Squares indicate the mean annual weighted d18O for precipitation collected at the Milano, Casale Monferrato and Parma stations (Longinelli and Selmo, 2003).

Fig. 5. Isotopic composition of water vapour samples, compared to the GMWL.

meet. In the present case, the value obtained is d18O = 10.82 ± 13.97‰. Despite the high uncertainty, due to the restricted number of observations including the above mentioned anomalous winter period, this value appears more depleted than the mean annual weighted precipitation value (d18O = 6.90‰ ± 2.2, see above), but is close to the isotope composition of the phreatic aquifer (d18O = 9.0 ± 0.5 to 10.4 ± 0.3‰) (Pilla et al., 2006). Indeed, although recharged locally, the phreatic aquifer receives a great input from seeping irrigation channels and rice ponds, which derive water from Alpine rivers such as the Po, Sesia and Ticino ranging in d18O from 10.0‰ to 11.0‰ (Zuppi and Bortolami, 1982; Pilla et al., 2006). In addition, the slope of the water vapour regression line (6.8) line would indicate evaporation under high relative humidity (Rh = 95%) (Gonfiantini, 1986). No correlation was found between atmospheric water isotopic composition and temperature, or weather conditions during sampling. The graph shown in Fig. 7 reports d18O composition of water vapour vs. the partial water vapour saturation pressure (ps in kPa), calculated using temperature (t in °C), according to the equation (Ward and Elliott, 1995):

ps ¼ exp

  16:78  t  116:9 t þ 237:3

All points show a d18O vs. VSMOW lower than the value of 10.4‰, corresponding to the isotope signature of a water vapour sample in thermodynamic equilibrium with shallow groundwater mainly fed by irrigation channels in the plains around Pavia (Pilla et al., 2006). Previous studies, indeed, demonstrate that water vapour isotope composition, in thermodynamic equilibrium, tends to be equal to that of evaporative basins (Bariac et al., 1990; AraguásAraguás et al., 1998). 5.4. Air masses origin and trajectory Experimental data indicate that samples which fall closer to the ‘‘local vapour endmember’’ (Fig. 7) do not correspond to situations classified as ‘‘no circulation’’ only. Instead, these belong to air masses of different origin. Only samples collected during conditions of atmospheric circulation bringing air masses from Siberia or northern Europe are systematically depleted in heavy isotopes, and are, therefore, far from the local vapour endmember. Such a depletion is also observed in Fig. 8, where the isotopic composition of the water vapour is reported as a function of the partial water vapour pressure Pv, which depends on both temperature and relative humidity (Rh) conditions, according to the equation

G. Elmi et al. / Applied Geochemistry 34 (2013) 53–64

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Fig. 6. d18O variability range in rainwater and water vapour, compared to the average monthly temperature.

Fig. 7. d18O of water vapour samples as a function of the partial water vapour saturation pressure, according to the origin of air masses.

pv ¼ ps  Rh Water vapour samples, excluding those collected in summer, with winds blowing from the NE, are further depleted in heavy isotopes, due to the presence of cold and dry air masses from these regions. These air masses, because of their higher density, displace warmer air masses previously present in the study area. This significantly lowers the local vapour isotopic signal, accordingly with the low temperatures and the continental nature of the regions from which these air masses are derived.

The water vapour isotopic signature does not allow clear distinction of situations when circulation is absent from the others, most likely due to the fact that turbulence phenomena (at medium-small scale) are always present in the atmosphere. These phenomena lead to mixing between local vapour and water of different origin. The graph presented in Fig. 8 also reveals that samples with the highest values of vapour pressure are those collected in late spring and summer. As already stated, in this period the evapo-transpiration flux is most intense and the local vapour

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Fig. 8. d18O of water vapour as a function of the vapour pressure, according to the origin of air masses.

contribution is highest. This period corresponds to the maximum vegetation activity but also to rice field flooding and to the consequent maximum rise of the water table level, which in several areas of the irrigated plain may be as shallow as 60 cm below the soil surface (Sacchi et al., 2007). 5.5. Evaluation of the local evapo-traspiration flux In order to verify this hypothesis, a method to apportion the contribution, for each water vapour sample, of moisture carried by continental cold circulation, by maritime (Atlantic and Mediterranean) circulation and by the local evapo-transpiration flux was devised. All the data points in Fig. 8 are approximate to a triangle,

whose vertices correspond to three extreme conditions. Vapour carried by cold circulation is represented by d18O values and vapour pressure reported in the literature for the arctic region (Schriber et al., 1977) and for the IAEA survey stations in Greenland (IAEA/WMO, 2004). It is hypothesised that the arctic air masses formed on site under near equilibrium conditions, and their isotopic composition were modified by picking up additional moisture while migrating SE towards the Mediterranean basin. Vapour carried by maritime circulation is represented by data collected by CNESTEN in Rabat (Morocco) in the years 2002–2003 (Ouda, B., pers. comm.). These data also allowed calculating, for this station, the error bars, shown in Fig. 8. Local vapour is represented by data estimated under conditions of the intense evapo-transpiration flux

Fig. 9. Estimated percentage of vapour of different origins in atmospheric moisture.

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in Pavia: its d18O vs. VSMOW value is assumed to be equivalent to that reported in the literature for freshwaters and shallow groundwater in the Pavia plain (Pilla et al., 2006). After defining the three end-members (represented by star symbols in Fig. 8), the contribution of each fraction in generating atmospheric water vapour was estimated through a mass balance approach using the End-Member Mixing Analysis (EMMA) equations (Christophersen et al., 1990; Hooper et al., 1990; Weltje, 1997). The variation of the three components in time is shown in Fig. 9. The maximum local contribution in 2006 was recorded in July, and generally the highest values were observed in summer and early autumn. A similar situation was recorded in 2007. Relative maxima were observed after the heaviest precipitation events, when a certain amount of water is retained by the top soil, and can successively evaporate. The difference in the estimated contribution of the locally produced vapour in 2006 and 2007 is due to the different climatic condition: the 2006 summer was somewhat warm, compared to that of 2007. On the other hand, the late spring of 2007 was significantly warmer than the monthly average temperature (Fig. 2), resulting in the maximum contribution of local water vapour being recorded on May 8th (Fig. 9). It should be noted also that irrigation of rice paddies generally starts at the end of April, at the beginning of Spring, and terminates around mid-August, causing a steep rise of the water table (Sacchi et al., 2007). In order to test if evaporation from the shallow water table could also be responsible for this increase in the local vapour

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contribution, data from the continuous monitoring of several piezometers in the Pavia province were obtained from the local authorities (Provincia di Pavia). Data are shown in Fig. 10. No appreciable difference in the depth of the water table between 2006 and 2007 can be observed for the Porta Chiossa piezometer, located about 10 km NE of the University campus, while another piezometer, located about 5 km N of the sampling site, shows a greater water table depth in 2006 than in 2007. From these observations, it is possible that the depth of the water table plays a role in the evapo-transpiration process, as it may be important for vegetation growth. 6. Conclusions Collected data have demonstrated that the isotope composition of the water vapour may help in understanding the origin of the air masses. In the Pavia area, three main components have been identified: moisture carried by continental cold circulation, by maritime (Atlantic and Mediterranean) circulation and by the local evapo-transpiration flux. The latter is more intense in summer and early autumn. This could be explained by two types of factors, namely: 1. Climatic-geographical factors: i.e. the ‘‘belt’’ formed by the Alps and the Apennines around the Po plain keeps the latter sheltered from perturbations which take place in the rest of Europe. This happens most frequently in summer, when warming by the Sun is more intense, and tropospheric circulation is weaker. Also, the frequent influence of the Azores high, which often develops over southern Europe as a field of high and rather uniform pressure, can modify any disturbances. 2. Agricultural practices: between August and September, a long period of irrigation with field flooding ends and plant transpiration reaches a maximum (as plants are at the climax of their vegetative phase). Moreover, turbulence due to the dynamic nature of the troposphere makes local atmospheric moisture mix with water vapour carried by advection from other regions. Even in the most obvious situations of no circulation, phenomena of exclusive local evapotranspiration are seldom observed. Circulation of air masses from the Mediterranean and the Atlantic carry the major part of atmospheric moisture towards the Po plain, while only cold and dry air masses from the NE, which are not found in the summer, are able to completely lower the local isotopic signal of the vapour, because of the substitution effect caused by their higher density. The expected correlation between the evapo-transpiration flux intensity and the amount of precipitation is quite significant, and may in part explain the differences in the local evapo-transpiration flux between 2006 and 2007. On the other hand, the isotope composition of water vapour testifies to the importance of irrigation as a source of local vapour, evidencing, even at mid-latitudes, a regional scale feedback between land use and climate (Ter Maat et al., 2006). Acknowledgements

Fig. 10. Water table fluctuations observed in two continuously monitored piezometers in the Pavia area (source: Provincia di Pavia).

We wish to acknowledge ARPA Lombardia, Provincia di Pavia, and Tommaso Grieco, for providing data about land use, piezometric monitoring stations and climatic parameters. Cristiano Corte is acknowledged for help in the interpretation of pressure maps. We also wish to thank Bouchera Ouda (CNESTEN, Morocco) for providing water vapour data from Rabat. The manuscript was greatly improved by the suggestions provided by Prof. Roberto Gonfiantini and Dr. Luis Araguás-Araguás.

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