a conceptual and numerical model for groundwater management

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A conceptual and numerical model for groundwater management: a case study on a coastal aquifer in southern Tuscany, Italy Piero Barazzuoli & Monica Nocchi & Roberto Rigati & Massimo Salleolini Abstract Ongoing hydrogeological research aims to develop a correct management model for the Plio- Pleistocene multi-aquifer system of the Albegna River coastal plain (southern Tuscany, Italy); overexploitation of this aquifer for irrigation and tourism has caused seawater intrusion. The conceptual model is based on eld and laboratory data collected during the 19952003 period. Meteoric inltration and ows from the adjoining carbon- ate aquifer recharge the aquifer. Natural outow occurs through a diffuse ow into the sea and river; articial outow occurs through intensive extraction of groundwa- ter from wells. Water exchanges in the aquifer occur naturally (leakage, closing of aquitard) and articially (multiscreened wells). The aquifer was represented by a three-dimensional nite element model using the FEFLOW numerical code. The model was calibrated for steady-state and transient conditions by matching com- puted and measured piezometric levels (February 1995February 1996). The model helped establish that seawater intrusion is essentially due to withdrawals near the coast during the irrigation season and that it occurs above all in the Osa-Albegna sector, as well as along the river that at times feeds the aquifer. The effects of hypothetical aquifer exploitation were assessed in terms of water budget and hydraulic head evolution. Keywords Groundwater modeling . Coastal aquifer . Salinization . Water budget . Italy Introduction In many coastal areas the growth of human settlements, together with the development of agricultural, industrial and tourist activities, has led to the overexploitation of aquifers. Such overexploitation induces a rise in the freshwatersaltwater interface (seawater intrusion) and thus the degradation of the chemical quality of ground- water; the problem will be aggravated by the expected rise in sea level associated with global warming (IPCC 2007). This situation occurs in several areas of the Mediterranean and will worsen due to the increase in the resident population and in coastal tourism (López-Geta et al. 2003). The quality and quantity of groundwater resources along the Italian coasts has been degrading for some time; the impact of the growing population is alarming, especially in the southern regions, where 45% of the total resident population lives in coastal zones (Barrocu 2003). The southern coast of Tuscany (central Italy) is largely affected by seawater intrusion (Bencini and Pranzini 1992, 1996; Barazzuoli et al. 1999; Angelini et al. 2000; Bencini et al. 2001) and by the consequent deterioration of the quality of groundwater and the local anomalous accumu- lation of heavy metals (Grassi and Netti 2000; Protano et al. 2000; Agati et al. 2001); this is due to intense pumping for different purposes (above all irrigation and domestic use), especially during summer, when the water demand for agriculture and tourism is highest and the natural availability of water is lowest. The problem has been aggravated in the last few decades by the progressive decrease in the potential renewable water resources of southern Tuscany (Barazzuoli et al. 2002) due to a reduction in total annual precipitation. The coastal plain of the Albegna River is currently experiencing seawater intrusion owing to an irrational exploitation of the aquifer through hundreds of wells of different types and depths and with different pumping rates. The deterioration of groundwater quality is currently a limiting factor for local economic growth; agriculture has either been completely abandoned or has been directed towards crops which can tolerate saltwater but are of inferior quality. The intrusion of seawater in coastal aquifers was rst conceptualized independently by Badon-Ghijben (1889) and Herzberg (1901) assuming hydrostatic equilibrium, immiscible uids and the existence of a sharp interface Received: 2 March 2007 / Accepted: 16 May 2008 Published online: 1 July 2008 * Springer-Verlag 2008 P. Barazzuoli : M. Nocchi : R. Rigati : M. Salleolini ()) Department of Earth Sciences, University of Siena, Via Laterina 8, 53100 Siena, Italy e-mail: [email protected] Tel.: +39-577-233811 Fax: +39-577-233938 Hydrogeology Journal (2008) 16: 15571576 DOI 10.1007/s10040-008-0324-z

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  • A conceptual and numerical model for groundwater management:a case study on a coastal aquifer in southern Tuscany, Italy

    Piero Barazzuoli & Monica Nocchi & Roberto Rigati &Massimo Salleolini

    Abstract Ongoing hydrogeological research aims todevelop a correct management model for the Plio-Pleistocene multi-aquifer system of the Albegna Rivercoastal plain (southern Tuscany, Italy); overexploitation ofthis aquifer for irrigation and tourism has caused seawaterintrusion. The conceptual model is based on eld andlaboratory data collected during the 19952003 period.Meteoric inltration and ows from the adjoining carbon-ate aquifer recharge the aquifer. Natural outow occursthrough a diffuse ow into the sea and river; articialoutow occurs through intensive extraction of groundwa-ter from wells. Water exchanges in the aquifer occurnaturally (leakage, closing of aquitard) and articially(multiscreened wells). The aquifer was represented by athree-dimensional nite element model using theFEFLOW numerical code. The model was calibrated forsteady-state and transient conditions by matching com-puted and measured piezometric levels (February 1995February 1996). The model helped establish that seawaterintrusion is essentially due to withdrawals near the coastduring the irrigation season and that it occurs above all inthe Osa-Albegna sector, as well as along the river that attimes feeds the aquifer. The effects of hypothetical aquiferexploitation were assessed in terms of water budget andhydraulic head evolution.

    Keywords Groundwater modeling . Coastal aquifer .Salinization . Water budget . Italy

    Introduction

    In many coastal areas the growth of human settlements,together with the development of agricultural, industrialand tourist activities, has led to the overexploitation ofaquifers. Such overexploitation induces a rise in thefreshwatersaltwater interface (seawater intrusion) andthus the degradation of the chemical quality of ground-water; the problem will be aggravated by the expected risein sea level associated with global warming (IPCC 2007).This situation occurs in several areas of the Mediterraneanand will worsen due to the increase in the residentpopulation and in coastal tourism (Lpez-Geta et al.2003). The quality and quantity of groundwater resourcesalong the Italian coasts has been degrading for some time;the impact of the growing population is alarming,especially in the southern regions, where 45% of the totalresident population lives in coastal zones (Barrocu 2003).

    The southern coast of Tuscany (central Italy) is largelyaffected by seawater intrusion (Bencini and Pranzini 1992,1996; Barazzuoli et al. 1999; Angelini et al. 2000; Benciniet al. 2001) and by the consequent deterioration of thequality of groundwater and the local anomalous accumu-lation of heavy metals (Grassi and Netti 2000; Protano etal. 2000; Agati et al. 2001); this is due to intense pumpingfor different purposes (above all irrigation and domesticuse), especially during summer, when the water demandfor agriculture and tourism is highest and the naturalavailability of water is lowest. The problem has beenaggravated in the last few decades by the progressivedecrease in the potential renewable water resources ofsouthern Tuscany (Barazzuoli et al. 2002) due to areduction in total annual precipitation. The coastal plainof the Albegna River is currently experiencing seawaterintrusion owing to an irrational exploitation of the aquiferthrough hundreds of wells of different types and depthsand with different pumping rates. The deterioration ofgroundwater quality is currently a limiting factor for localeconomic growth; agriculture has either been completelyabandoned or has been directed towards crops which cantolerate saltwater but are of inferior quality.

    The intrusion of seawater in coastal aquifers was rstconceptualized independently by Badon-Ghijben (1889)and Herzberg (1901) assuming hydrostatic equilibrium,immiscible uids and the existence of a sharp interface

    Received: 2 March 2007 /Accepted: 16 May 2008Published online: 1 July 2008

    * Springer-Verlag 2008

    P. Barazzuoli :M. Nocchi :R. Rigati :M. Salleolini ())Department of Earth Sciences,University of Siena,Via Laterina 8, 53100 Siena, Italye-mail: [email protected].: +39-577-233811Fax: +39-577-233938

    Hydrogeology Journal (2008) 16: 15571576 DOI 10.1007/s10040-008-0324-z

  • between fresh- and saltwater in a homogeneous uncon-ned aquifer. They found that the depth of the freshwatersaltwater interface below sea level (zs) is given by:

    zs hf fs f

    where f is the density of freshwater, s is the density ofsaltwater, and hf is the elevation of the water table abovesea level. When the equation is applied correctly, theestimated depth closely approximates the real one (Chengand Ouazar 1999); it is still widely used to simulatesaltwater intrusion (Essaid 1990; Cheng and Chen 2001)and, especially for educational purposes, to gain clearinsight into the behaviour of fresh and saline groundwaterin coastal aquifer systems (Oude Essink 2003). Due tomolecular diffusion and hydrodynamic dispersion, freshand salt water are actually miscible liquids: the contactbetween the two uids is therefore a transition zone ratherthan a sharp interface (Gambolati et al. 1999; Cheng andChen 2001). The situation is further complicated by thefact that the saltwater intrusion itself changes the uiddensity, so that this parameter varies in space and time as afunction of changes in concentration, temperature andpressure in the uid. Furthermore, the porous mediumitself is usually stochastically heterogeneous. In order toproperly reproduce the mechanism of saltwater encroach-ment, a variable density ow and transport modellingapproach is therefore currently adopted (Voss and Souza1987; Koch and Zhang 1992; Diersch 1998b; Holzbecher1998; Bear et al. 1999; Diersch and Kolditz 2002).

    The medium- and long-term effects of land manage-ment policies are difcult to foresee due to interactionamong numerous elements and variables of differentnature, especially as far as seawater intrusion is concernedbecause many aspects of this problem are not completelyunderstood (Custodio and Bruggeman 1987; Custodio andGalofr 1993; FAO 1997; Bear et al 1999; Cheng andOuazar 2004). Groundwater management thus requires theuse of numerical models to test present and alternativeexploitation scenarios taking into account not onlytechnical aspects but also economic, legal, social andpolitical ones (Wang and Anderson 1982; Bear andVerruijt 1987; Emch and Yeh 1998; Custodio and Galofr1993; van Dam 1999; Maimone et al. 2004; Bear 2004).

    In this context, the authors developed a long-term hydro-geological research program, the preliminary results of whichwere published by Angelini et al. (2000) and Barazzuoli etal. (2003, 2004). This work presents a conceptual andnumerical model for simulating the hydrodynamics of themulti-aquifer system of the lower Albegna River valley. Themodel will be used for hydrochemical simulations andthe correct management of local water resources. Thisaquifer system provides a good example of the situation inrecent coastal plains (Custodio 2002; Morell 2003).

    The study area

    The Albegna River coastal plain, located in southernTuscany, consists of aeolian and alluvial sediments covering

    an area of about 100 km2 (Fig. 1). The river crosses the plainfrom ENEWSW with an average annual discharge ofabout 5 m3/s at the mouth. The area north of the Osa Riveris characterized by gentle hills alternating with short atsectors where there are sometimes both natural and articialditches resulting from land reclamation works. The highestpeaks, no more than 354 metres above sea level (m asl), arein the south-eastern sector (Poggio del Leccio hill).

    The average annual precipitation in the plain is about630 mm, and the average annual temperature is about 16C.The effective precipitation is rather low, varying from150 mm/year in the low plain to 250 mm/year in thesurrounding hilly areas. As occurs along the entire coastlineof southern Tuscany, most precipitation is returnedto the atmosphere through actual evapotranspiration, withan average of more than 70% (Barazzuoli et al. 1993).According to the climatic classication proposed byThornthwaite (1948), the investigated area can be consid-ered subarid C1 (moisture index from 33.30).

    This area is a tectonic depression made up of continental,transition andmarine sediments (MioceneQuaternary). Thesecover a pre-Neogene substratum composed of Liguride units(an argillaceous-calcareous-ysch complex), cropping out onthe northern and eastern borders of the plain, as well asTuscan units on the southeastern and northern borders(Mancini 1960; Tozzi and Zanchi 1987; Bonazzi et al.1992; Bossio et al. 2004). Miocene (essentially Messinian)sedimentary sequences consist of strongly eroded con-glomerates that can be found only in the eastern sector.Pliocene sediments prevalently consist of clay, togetherwith regressive sands, gravel and conglomerates. In theeastern sector, the limestones, lacustrine clays andtravertine deposits formed in the Pleistocene are inter-ngered with the terraced Albegna River deposits consist-ing of gravels, sands and conglomerates. The aeoliansands and nest uvial deposits (clays and silts) date backto the Holocene period.

    The outcropping rocks can be divided into two maingroups with different permeability through a qualitativeclassication according to formation:

    Quaternary and Neogene complexes. These depositshave weak or non-existent cementation, and showpredominantly primary permeability due to interstitialporosity. The degree of permeability varies: it ismoderate-high in the Neogene conglomerates, sand-stone and sand, travertine, terraced alluvial deposits,shores, and in the horizons of coarse aeolian sediments,but zero to low in the Pliocene clays, transitiondeposits, actual and recent alluvial deposits, and inthe horizons of ne aeolian sediments.

    The pre-Neogene complex. Characterized by diageneticformations showing predominantly secondary perme-ability due to ssuring or ssuring and karst. Thedegree of permeability varies: it is medium-high in theTriassic dolomitic limestones and Cretaceous-Eocenecalcarenites, but zero to low in the Palaeozoic-Triassicmetamorphic rocks and Cretaceous-Eocene argilla-ceous calcareous and arenaceous ysch.

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  • Conceptualization

    Geometry and structureThe main geometric-structural and hydrogeological charac-teristics of the Plio-Pleistocene multi-aquifer system of theAlbegna River coastal plain were reconstructed on the basisof the general geologic reconstruction and the 61 well/borehole and 31 geo-electric data points (Fig. 2). The systemis made up of several gravely and sandy layers which canbe combined into three main aquifer layers; these aquifersare generally separated by aquitards composed of clayeydeposits with silt or sand in variable proportions, but theysometimes combine to form a single-layer aquifer. Another

    aquitard consisting of sandy-clayey silt is present at the top.The upper aquifer layer (SE), consisting mainly of well-sortedsand with a hydraulic conductivity (K) of 104105 m/s,extends only up to 6 km inland from the coast. It overlies theA2 aquitard consisting of clay and silty clay. The interme-diate aquifer layer (SG) consists of gravely sediments with asandy-clayey matrix that have a very heterogeneouslateral distribution due to shifts in the course of theAlbegna River;K varies from 103 to 105 m/s, accord-ing to grain size. In almost the entire area covered bythe model, the SG layer lies above the rather thick(about 2025 m), clayey A3 aquitard that separates itfrom the lower aquifer layer (GL). The latter is made up

    Fig. 1 Geological-hydrogeological sketch map of the Albegna River coastal plain in southern Tuscany, Italy

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  • of sandy-clayey gravel of constant thickness (810 m),with K ranging from 104105 m/s. The hydraulicconnection between SG and GL is articially ensured bynumerous multiscreened wells, especially along thecoast where they usually reach a depth of 4050 m.

    This hydrogeological system overlies an imperviousclayey basement, except at the borders of the plain, wherehydraulic connections with the outside are possible throughpermeable rocks. In the south-eastern sector, water exchangesbetween the cavernous limestone (K of 103 to 104 m/s;Nocchi 2002; Nocchi and Salleolini 2007) and the aquiferlayers (especially outow from the limestone towards thealluvial deposits) occur through faults, as conrmed byhydrodynamic and hydrochemical evidence (Angelini et al.2000; Nocchi 2002, 2004). A similar situation, but with areduced ow of water, also occurs in the northern sector,where the fault along the Osa River causes the yschformations and cavernous limestone to crop out; further-more, at the Melosella locality, the GL aquifer layer liesdirectly above arenaceous formations (K of about 105 m/s).

    The aquifer system is mostly recharged by the directinltration of precipitation falling in the modelled area; itis also recharged by inltration into the aquifer layersoutside the modelled area (lateral ows, in the northernand eastern sector) and by the ow of water through thecontact alluvial deposits and the cavernous limestone(lateral and vertical ows in the south-eastern sector). Thenatural outow is discharged into the sea and the AlbegnaRiver; the intensive withdrawal from the aquifer throughwells is the articial outow. Water exchange within the

    aquifer occurs due to both natural (leakage, close ofaquitards) and articial causes (multiscreened wells).

    Piezometric surface and hydraulic propertiesThe piezometric level in 62 wells was measured monthlyfrom February 1995 to February 1996 (Angelini et al. 2000).Note that, due to continuous groundwater extraction fromthe various wells in the area, it was only possible tomeasure the dynamic water level; hydraulic head data thusdo not reect the natural equilibrium conditions, especiallyin the summer months, when the demand for waterincreases due to irrigation and tourism. Since both freshand salt water occur in the area, a method similar to that ofPost et al. (2007) was used to convert water levelmeasurements to fresh water heads; a maximum correctionof about 0.15 m was obtained in the higher salinity zones.

    It was impossible to complete a detailed study of thehydrodynamics of the Plio-Pleistocene aquifer due to thelack of hydraulic head measurements at various depths.Nevertheless, the partial stratigraphic continuity of theaquifer layers, the widespread presence of semiperviousinterlayers, and the wide variety of connections between thedifferent layers due to the hundreds of multiscreened wellseffectively guarantee the hydraulic continuity within theaquifer complex. This continuity was conrmed by the goodlocal correlation among hydraulic head measurements. Eachpiezometric measurement therefore corresponds to the totalhydraulic head of all aquifer layers constituting a singlehydrological system in hydraulic equilibrium, and piezo-

    Fig. 2 Schematic cross-sections and conceptual model of the multi-aquifer system in aeolian and alluvial sediments of the Albegna Rivercoastal plain (redrawn from Nocchi 2004). The vertical scale is exaggerated about 20 times

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  • metric surfaces were reconstructed using hydraulic headmeasurements from all observation wells.

    Collected data show that the aquifer is unconned (orsemi-conned) in the north-eastern and southern sectors, butsemi-conned (or conned) downstream due to the presenceof semipervious (or impervious) covers and/or intercala-tions. The reconstructed piezometric surfaces (Fig. 3) showthat withdrawals strongly affect groundwaters, causingwide areas of negative hydraulic head in the main welleld. Major depressions southeast of Fonteblanda and ENE of Albinia are well below sea level from spring toautumn, with a maximum depth of 7 m above sea level(asl) and maximum extension in July. In June and July thewhole coastline between the Osa River and Albinia, as farinland as the conuence Albegna-Magione Radicata (about6 km from the coast), is marked by a negative hydraulichead; this depression does not spread southeastward owingto the important ow of water from the cavernouslimestone. Groundwater always has a centripetal movementtowards the valley bottom; the various ows originate inthe hills surrounding the plain and move towards theAlbegna River (the main groundwater drainage axis),through which the waters nally reach the sea. Hydraulichead measurements carried out in September 2002 andFebruary 2003 yielded piezometric surfaces very similar tothe previous ones (Nocchi 2002, 2004).

    The hydraulic properties of the aquifer are hardly known.Angelini et al. (2000) report on the only pumping tests carriedout in the area (Fig. 4): tests A and B refer to wells screened

    in all aquifer layers, which have an average hydraulictransmissivity (Ta) of 5.710

    3 m2/s, average hydraulicconductivity (Ka) of 5.610

    4 m/s and average storativity(Sa) of 310

    3; test C refers to a well only screened in the SEand SG Pleistocene layers (T=9104 m2/s,K=3.8105 m/s,S=3104). The authors estimate the aquifer transmissivitydistribution by multiplyingKa and the saturated thicknessmatrix; the relative map (see Fig. 4) shows transmissivityvalues ranging from zero, where the pre-Neogene basementcrops out, to 21103 m2/s in the Albinia area, with anaverage value of 8.5103 m2/s. This reconstruction wasconrmed indirectly and locally by means of the goodcorrelation between transmissivity and the normalizedtransverse resistance obtained through vertical resistivitysounding in the Osa-Albegna coastal sector following theprocedure proposed by Ahmed et al. (1988), and inagreement with the direct relationships derived by Urish(1981), Ponzini et al. (1983), Gorman and Kelly (1990) andBarazzuoli et al. (1999).

    Groundwater qualityThe main physicalchemical characteristics of groundwa-ter were dened through electrical conductivity surveyscompleted at the time of the piezometric surveys andanalysis of water samples from 38 wells to determine Cl

    and NO3 concentrations (Angelini et al. 2000); further-

    more, 12 samples of groundwater, seawater and thermo-mineral spring water were taken to estimate major ion

    Fig. 3 Groundwater level contour map for the Plio-Pleistocene aquifer (redrawn from Angelini et al. 2000): a February 1995; b July 1995

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  • concentrations. The distribution of chloride (Fig. 5)reveals how the quality of groundwater has deterioratedsubstantially throughout the coastline up to about 4 kminland from the shore, where local values of 2,000 mg/Lare reached even in spring; lower values of 100 mg/Lwere observed in the southeastern sector (where lowerelectrical conductivity values were also observed), con-rming the ow of freshwater from the cavernouslimestone. The waters coming from wells screened in thelower aquifer layer (Pliocene gravel and sand) are thosewith the highest chloride concentrations; this layertherefore seems to be the one most affected by seawaterintrusion. These wells are located in the sector facing thecoast, where the piezometric depression is greatest, andtheir structure (with more screens) facilitates seawaterintrusion within the aquifer system. Moreover, based onchemical and isotopic data, Bencini and Pranzini (1996)suggested that the cation exchange process and inltrationof sulphate river waters (originated by dissolution of thegypsum formation and mixing with thermal waters) may

    be another cause of groundwater salinization in theAlbegna plain.

    Due to the high correlation observed between electricalconductivity and chloride concentrations, in September2002 only electrical conductivity was monitored in surfacewater and groundwater (Fig. 6). Measurements show thatseawater intrusion mostly affects natural and articialwatercourses, even inland from the sea, whereas theelectrical conductivity (and thus the salinity) of ground-water is generally lower, even near the coast (Nocchi2002). At the Barca del Grazi locality, the followingvalues were recorded for the Albegna River: 12,565 S/cm(at 20C) at low tide and 29,135 S/cm at high tide.

    In March 2003, surface water was sampled in four riversections in order to determine major ion concentrations(see Fig. 6). Analytical data were processed according toPiper (1944) and compared with the water quality of theSaturnia thermomineral spring (Fig. 7), which dischargesinto the upper course of the Albegna River, largelycontributing to its baseow (0.30.6 m3/s; Mancini

    Fig. 4 Hydraulic transmissivity map of the Plio-Pleistocene aquifer (modied after Angelini et al. 2000). The gure shows the location ofpumping tests (star with upper-case letter)

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  • 1960; Fanelli et al. 1982). Albegna River water in thisstretch has the same earthy alkalinesulphate compositionof the above-mentioned thermomineral water, but lowerabsolute concentrations (Nocchi 2004); Patrignone water(sect. 3), with its earthy bicarbonatealkaline composition,has very different characteristics.

    Role of surface waterThe 20022003 eld surveys were carried out with theaim of better understanding the interaction betweensurface water and groundwater. The investigated parame-ters therefore also include (Nocchi 2002, 2004): the depthof the river bottom (comparing data provided by the Osa-Albegna Land Reclamation Consortium with that reportedon 1:5,000 scale regional technical cartography), therelative water level, and the discharge rate (by placing aspinner in the previously mentioned sections of theAlbegna River, see Fig. 6).

    The water level of the Albegna River in the investigatedarea was everywhere lower than the piezometric level inwinter, conrming the role of the river as the aquiferdrainage axis (the situation is locally reversed in the summermonths by withdrawal from wells near the river). Further-more, in its nal stretch (from the conuence with theMagione-Radicata Stream to the mouth), the river bottom ofthe Albegna River is below sea level; consequently, seawater ascends the river and determines the free-surface water

    level. Electrical conductivity measurements at the Barca delGrazi locality corroborate this theory (see Fig. 6).

    Discharge measurements in the river were completed inthe absence of precipitation, so that watercourses receivedno runoff and only the baseow was measured; results aresummarized in Fig. 7. Collected data reveal a substantialincrease in discharge downstream, with greater drainage inthe stretch between sections 4 and 1 (about 100 l/s/km),where the difference between the height of the piezometriclevel and that of the water surface is greatest.

    Numerical modelling

    SoftwareThe groundwater numerical ow model was developedusing FEFLOW (nite element subsurface ow system)working under both steady-state and transient conditions.For theoretical and practical information concerning soft-ware use and the solution of equations, the reader can refer tothe respective manuals (Diersch 1998a, b). The niteelement method was adopted for its exibility and capacityto simulate complex geometric forms and to rene thenodal grid around points and/or single lines (observationpoints, coastline, etc.). Geolithological, hydrogeologicaland hydrochemical data processing was carried out in aGIS environment (ArcView) that can be totally interfacedwith FEFLOW; this was very useful in the development of

    Fig. 5 Chloride concentration contour map for the Plio-Pleistocene aquifer (redrawn from Angelini et al. 2000): a MarchApril 1995;b OctoberNovember 1995

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  • the conceptual model, in the creation of the numericalmodel and in the analysis of simulation results.

    Discretization and boundary conditionsThe aquifer system was discretized using a grid of triangularelements made up of 6,794 nodes and 13,231 elementsranging from 80 to 170 m in size and covering an area ofabout 90 km2. A higher degree of renement was adoptedfor the control points (observation wells, which wereplaced in the grid as xed nodes representing their trueposition) and along the river and coast (important bound-aries for the correct simulation of ow). The three-dimensional grid consists of six layers corresponding tothe above-mentioned hydrogeological layers; it is therefore

    made up of nearly 80,000 linear prismatic elements with atriangular base, for a total volume of about 4.3 km3. Thereconstructed geometry matches that represented in geo-logical sections constructed by Angelini et al. (2000). Theboundary conditions assigned to the numerical modelderive directly from the conceptual reconstruction of theaquifer system (Fig. 8).

    A constant head value was assigned to nodes along thecoastline (or lagoon), where groundwater is in contact withthe free surface of the sea. A hydraulic head of 0 m asl wasassigned to grid nodes coinciding with the topographicsurface; starting from the underlying grids (along the verticalof the coast line), the overpressures deriving from the higherdensity of sea water were calculated considering a typicalfreshwatersaltwater interface geometry (Diersch 1998c),

    Fig. 6 Electrical conductivity measured in surface waters and groundwaters during the September 2002 survey (redrawn from Nocchi2004); surface water values at Barca del Grazi refer to low tide (12,565 S/cm) and high tide (29,135 S/cm). The gure shows the riversections in which discharge measurements and water sampling for chemical analysis were completed in March 2003; only sections 1, 2, 3and 5 were sampled

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  • and results were assigned to the nodes as head increments.This procedure was adopted because of the lack ofinformation on the density of uids at various depths(obtainable only through specic soundings and surveys)the greatest limitation of the model.

    In the modelled area, the Albegna River is in contact withthe aquifer; the specied head was therefore also assigned tothe nodes representing the course of the river. In this case,constant hydraulic heads were established on the basis of theheight of the river bottom and the average height of the freesurface of water measured in sections and interpolated alongthe river; head values were considered constant throughoutthe year due to the rivers regular and substantial baseowwith small hydrometric oscillations. The test application ofvariable functions to the river nodes always produced scant

    Fig. 7 Results of the March 2003 survey for river sections shownin Fig. 6 (redrawn from Nocchi 2004): a Piper diagramthe crossrepresents the Saturnia thermomineral spring (values by Fanelli etal. 1982), which discharges into the upper course of the AlbegnaRiver (about 30 km from the coast)]; b discharge values

    Fig. 8 Boundary conditions of the numerical model

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  • variations in the piezometric surface, in terms of both leveland spatial distribution.

    Based on head data, the following ux conditions wereassigned to the boundaries crossed by water from outsidethe modelled area: uxes of 4106 m/day from the northand 7106 m/day from the east, assigned to layers GLand SG respectively, simulate meteoric inltration in theirareas of recharge outside the model; a ux of 2106

    m/day from the southeast, assigned to layer SG, representsthe contribution of the local carbonate formation. East ofthe lagoon, where the permeable alluvial sediments liedirectly above the cavernous limestone, water exchangesbetween the two aquifers were simulated through avertical inow at the base of the model (see Fig. 2) thatvaries according to piezometric relationships. Where nocondition was specied, the model uses no-ow boundaryconditions (impervious basement, no evidence of ow).

    Flow conditions were also used to assign inows oroutows, which vary in time and space; they were thus used

    to simulate meteoric inltration in the model (arealrecharge). Effective precipitation (Pe) was initially estimatedaccording to Thornthwaite and Mather (1957) on an annualor monthly basis, depending on simulation conditions(steady-state or transient); inltration was then estimatedas the percentage of Pe according to soil type and on thebasis of hydrologic classications proposed by Favi andRossi (1991) and Civita et al. (1999). The percentagecalculations were rened through calibration as follows(Fig. 9): 96% for the sandy deposits cropping out in thesouthern and western sectors (hydrologic group A: soilswith high inltration rates even under conditions of fullimbibition and with high transmission rates); 576474%for sandy sediments cropping out in the northern sector(usually belonging to group B: moderate inltration andtransmission rates); 21% for the river terrace outcrops inthe eastern sector (group C: low inltration and transmis-sion rates); 2% for the ner deposits of the Albegna River(group D: very low inltration and transmission rates).

    Fig. 9 Percent meteoric inltration zoning with respect to the effective precipitation assigned to the model; letters (A, B1, etc.) representsthe soil hydrologic group according to Favi and Rossi (1991)

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  • The discharge condition was assigned to the wells whichexploit the aquifer; water extractions at individual nodessimulate withdrawal, which was estimated using statisticaldata and distributed according to the density of wells. Theability of FEFLOW to simulate hydraulic connectionbetween aquifer layers through multiscreened wells wasused to determine discharge rates; a very high hydraulicconductivity is assigned to the well node, and the calculateddischarge is automatically distributed according to thepermeability of the various layers. Water requirements inthis area are determined chiey by agriculture needs, whiletourism needs are met by the aqueduct, which exploits otheraquifers. Due to the lack of reliable data on surface water andgroundwater withdrawals for agriculture purposes, thesequantities were estimated on the basis of informationprovided by the last censuses completed by the Central

    Institute of Statistics and processing the following data(Bilardo et al. 1997): in the agricultural sector, the extensionof cultivated lands according to crop type, water require-ments and the irrigation system; in the zootechnical sector,the type and number of livestock and water requirements.Withdrawals for irrigation of back gardens and parks andfor industrial purposes were not taken into consideration,since they are not signicant in the investigated area.

    CalibrationThe main objective of the calibration stage is to obtainresults as much as possible in agreement with the elddata by acting on the variables which characterize thesystem. The parameters calibrated with the model were the

    Fig. 10 Hydraulic conductivity zoning (K) within aquifer layers: a upper SE (1) layer, b intermediate SG (2 and 3) layer, and c lower GL(4 and 5) layer. The blank zone within the model boundary represents a non-existent aquifer (aquifer layers are not continuous throughoutthe modelled area). The table shows the assigned value and the range of K values. For the aquitard, Kvert=510

    8 m/s

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  • hydraulic conductivity and specic storage of aquiferlayers and the vertical hydraulic conductivity (Kvert) ofaquitards; aquifers were further subdivided according totheir hydrogeological characteristics.

    The model was initially calibrated for steady-stateconditions, taking into consideration the piezometric levelmeasured in February 1995 by Angelini et al. (2000); asthe latter is the seasonal maximum, it was possible toexclude withdrawals from the initial uncertainties. Thefew known values of hydraulic properties were used asinput parameters; by varying the hydraulic conductivity,simulations were carried out so as to achieve the best tamong estimated and measured piezometric values for asufcient number of control points. The PEST software(Doherty 1998), which is able to optimize the parametersof any model, was used to achieve optimum calibrations

    for different starting conditions. The obtained results areshown in Fig. 10, together with the adopted subdivision ofthe aquifer layers.

    The results of this calibration are plotted in Fig. 11,which shows the good t between measurements andcalculations in the 60 observation wells inside themodelled area; the two sets of values are highly correlatedand plot very close to the perfect correspondence line. Theaverage absolute difference is about 0.50 m; this is ahighly satisfactory value, since the conceptual modelcannot take into account local changes occurring in thereal system, and hydraulic head values were determinedon the basis of altimetry reported in the 1:5,000 scaleregional technical cartography. The same considerationcan be made when comparing the simulated piezometricsurface with the one measured in February 1995 (see also

    Fig. 11 a Groundwater level contour map and b calibration curve obtained for steady-state conditions (comparison is made with February1995 values for 60 observation wells)

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  • Fig. 3a): the general framework is very similar, withpractically the same gradients and heads.

    The model was subsequently calibrated for transientconditions. In this case, the simulation calculates piezo-metric surface variations in time; it is therefore possible tocalculate the parameters coming into play in the temporalequation (specic yield or specic storage). The evolutionof inows (ux at boundary, meteoric recharge) andoutows (withdrawals) at aquifer boundaries was alsotaken into consideration. The aim was to simulatepiezometric oscillations registered at the control pointsduring the observation period as accurately as possible. Thesimulation reference period is the one in which groundwa-ter level measurements were carried out; the computedpiezometric surface in steady-state conditions was assignedas the initial condition, and comparison was made on thesame days in which measurements were completed. Thesimulation started on 19 February 1995 and ended on 28February 1996, lasting a total of 375 days.

    The hydraulic heads assigned to the sea were constant(since the small tidal oscillations of 0.150.20 m were not

    taken into consideration), as were the ones assigned to theriver, since no registration of its level is available. Lateralows were also assumed to be constant in time, whereas thevertical ow coming from the cavernous limestone increasedprogressively and peaked in July, when the local wells reachthe piezometric minimum (this simulates the recharge effectthat occurs here in August, i.e. before meteoric inltration).This simulation uses the wells which pump water mostly forirrigation and livestock farming; since irrigation starts inMarch and continues through to June or September, depend-ing on the crop type, withdrawals vary in time and wereassigned considering piezometric uctuations. Meteoricrecharge was assigned by applying the mentioned inltrationpercentages to the effective monthly precipitation; themonthly inltration value was distributed evenly throughoutthe respective month.

    The adopted specic yield of 0.12 was determined byestimating porosity through the empirical relationshipproposed by Archie (1942) and considering a specicretention of 0.10 (Angelini et al. 2000). Using this method,Gabbani and Gargini (1992) found a similar value for

    Fig. 12 Specic storage zoning. The gure indicates the location of observation wells shown in Fig. 13 (dot with number)

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  • another alluvial coastal plain of southern Tuscany. Thisvalue was assigned to all aquifer layers because of its minorinuence on simulation results. Specic storage is a highlysignicant parameter, because calculations were completedfor a conned aquifer. Figure 12 shows the calibrationresults for this parameter, which depend essentially on thesystems degree of connement and thus on the grain-sizedistribution of the cover sediments (see also Fig. 9).

    The results of the calibration under transient conditionsare shown in the plots of Fig. 13, where results arecompared with measurements in selected control points.Note the good match between the two sets of values (theaverage absolute difference is about 0.50 m in July 1995);in some cases, it is better than the match between controlpoint measurements and the results of the simulationunder steady-state conditions. Pumping causes a substan-tial drawdown of the groundwater level in MarchJune;the end of irrigation and autumn rains subsequentlydetermine a rise in the water level. Comparison betweencomputed and measured piezometric surfaces in the 19951996 period leads to the same considerations made forcalibration under steady-state conditions.

    Water budgetBy drawing up the aquifer water budget it is possible tocalculate the volumes of water exchanged at the modelboundaries (Fig. 14 and Table 1). In the period of 19February 199528 February 1996, the aquifer systemshows a negative balance of over 6105 m3 (about 1,700m3/day). Note that this period was characterized bybelow-average meteoric recharge ( 25%); consideringan average inltration, there would be equilibriumbetween inow and outow (also considering thatapproximations are unavoidable in such calculations).The most interesting issue is the considerable quantity ofwater crossing the coastline (over 142106 m3 inow and143106 m3 outow) through the typical ownet to thefreshwatersaltwater contact (inow from the bottom,ascent along the interface and outow from the top); thedifference (about 5.5105 m3) represents the net outowof groundwater towards the sea, which occurs mostlysouth of the Albegna River. The sea water that actuallyenters the aquifer system, causing the decay of thechemical quality of groundwater (seawater intrusion), ismostly the balance of ows crossing the coast in the Osa-

    Fig. 13 Comparison between computed (red line) and measured (blue line) hydraulic heads in different wells used for different purposes(see also Fig. 12). The starting date of the simulation is 19 February 1995; comparison is made with monthly values

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  • Albegna sector (almost 3105 m3). The situation is madeworse by sea water that ascends the Albegna River; duringcertain periods of the year (when the piezometric level islower than the hydrometric level) this water recharges theaquifer (about 6104 m3) for a total of about 3.5105 m3

    (over 900 m3/day).Figure 15 shows water ow variations in time for the

    various components of the water budget; note that meteoricinltration occurs mostly in January and February, and thatthe withdrawals are strongly inuenced by precipitation inthe irrigation period. Figure 15b shows the coastal owdivided according to zone and aquifer layer. An outow ofgroundwater (for a total of about 8.5105 m3) alwaysoccurs in the southern sector of the Albegna plain,whereas seawater intrusion affects the northern sectorthroughout the investigated period, except in December

    and January, when increased precipitation brings about anoutow towards the sea. In accordance with eld data,seawater intrusion mostly affects the lower aquifer layer(GL), whereas the upper one (SE) is always characterizedby an outow towards the sea; this difference is in partdue to the fact that during simulations, the pumping wells,especially those withdrawing substantial amounts ofwater, almost all extract water from the underlying layers.The older wells located in the upper aquifer layer areexploited very little. Figure 15c shows the variations inspace and time of relationships between the aquifer andthe river within the modelled area; the ow towards theaquifer occurs not only upstream (with a balance of about2105 m3) but also downstream, where in June and Julythe aquifer receives the above-mentioned 6104 m3 ofsaltwater that ascends the river along the bottom.

    Fig. 14 Computation grid for the three-dimensional numerical model and water budget of the Plio-Pleistocene aquifer of the AlbegnaRiver coastal plain in the 19 February 199528 February 1996 period obtained for transient conditions: balance of inows (positive value)and outows (negative value) in the modelled area expressed in m3/day

    Table 1 Water budget of the Plio-Pleistocene aquifer of the Albegna River coastal plain (19 February 199528 February 1996) undertransient conditions

    Item Inow (m3) Outow (m3) Balance (m3)

    Meteoric inltration 1,646,577 1,646,577Lateral ows 747,347 747,347Flows from the cavernous limestone 785,432 785,432Withdrawals 2,932,010 2,932,010Albegna upstream 595,924 394,246 201,678Albegna downstream 59,784 578,413 518,629Total Albegna 655,708 972,659 316,951Sea Osa-Albegna sector 73,425,320 73,138,660 286,660Sea opposite lagoon 69,015,580 69,864,140 848,560Total sea 142,440,900 143,002,800 561,900Aquifer system 146,275,964 146,907,469 631,505

    Albegna: upstream from La Marsiliana to the conuence with the Magione-Radicata stream; downstream from the conuence with theMagione-Radicata stream to the mouth. Mare:Osa-Albegna sector from the mouth of the Osa R. to the mouth of the Albegna R.;oppositelagoon from the mouth of the Albegna R. to the southern end of the model

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  • ValidationA calibrated model is validated when it can closelyreproduce a different set of eld data, i.e. predicted valuesapproximate measured ones. The model can be built evenwhen time series data are lacking, since it is enough tohave data on a single hydrological cycle (as in this case);however, the model can only be effectively used after along period of experimentation, during which it will likely

    need to be rened, because the true relationships betweenthe developed sequence and system trends in the mediumand long term are not known (Anderson and Woessner2002; Oude Essink 2004).

    Since the calibration was performed with reference to the19951996 period, one characterized by below averagemeteoric inltration, the model was validated against elddata acquired in 2003, a year in whichmeteoric inltrationwas

    Fig. 15 Water ows in transient conditions for the 19 February 199528 February 1996 period (positive values are inows, negativevalues are outows): a ow of the main components of the water budget (values relative to the sea and to the Albegna River are thedifference between inows and outows) with respect to local precipitation (expressed in mm); b balance of ows crossing the coastline,divided according to zone and aquifer layer; c balance of ows exchanged with the Albegna River divided according to zone. For thedenition of zones and symbols, see text and Table 1

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  • close to average. This validation aimed to assess the modelsability to correctly reproduce the response of the aquifersystem to different areal recharge conditions. Starting from1996, monthly precipitation data were input into the model(the rest remained unchanged), so that it was possible tocalculate the piezometric surface for February 2003. Validationresults were tested by comparing calculated values againstmeasurements in 51 observation wells; the two sets of valuesare highly correlated and plot near the perfect correspondenceline (the absolute difference is about 1.00 m). The sameconsiderations hold for the comparison between computed andmeasured piezometric surfaces for February 2003.

    Conclusions and future researchA conceptual and numerical model was developed forsimulating the hydrodynamics of the Plio-Pleistoceneaquifer of the Albegna River coastal plain. The modelcan be used to determine the most important componentsof the water budget and to identify river/aquifer/searelationships. At present, inows and outows appear toensure the equilibrium of the aquifer system; however,seawater intrusion along the coastline is responsible for

    the substantial deterioration of the chemical quality ofgroundwater. The model helped establish that waterwithdrawal near the coast during the irrigation season isthe main cause of seawater intrusion, especially in theOsa-Albegna sector; sea water also inows along the riverwhich at times inltrates the aquifer. The deterioration ofgroundwater quality therefore appears to be due to theway in which withdrawals are carried out (where andwhen) rather than to the substantial quantity of waterbeing pumped (about 3106 m3/year). Groundwaterdegradation is currently a limiting factor for localeconomic growth, above all for agriculture, and isproducing conicts among the different users; there istherefore urgent need to improve the state of the aquiferthrough a plan for its rational exploitation based on acorrect conceptual representation of physical conditions.

    In this framework, the presented model represents thebasis for future hydrochemical simulations that will help tobetter manage local freshwater resources. This model is stillnot completely reliable because of the incomplete knowl-edge of the system and the few opportunities to fully checkresults; however, it can already provide important informa-tion on the general evolution of the system under differentstress conditions. In particular, the model can be used toquantitatively assess the impact of variations in the amountof water withdrawn and/or in the position of wells thatcould help remediate saltwater intrusion.

    The effects of hypothetical aquifer exploitations wereassessed in terms of water budget and the evolutionhydraulic head starting from February 2003; these fore-casts concern the next 10 years and take into considerationthe local climate trends, i.e. a decrease in effectiveprecipitation of about 2 mm/year (Barazzuoli et al.2002). In particular, two exploitation scenarios wereexamined: the E scenario considers the relocation of allcoastal wells to at least 3 km inland from the shore and themaintenance of the current overall withdrawal; in contrast,the B scenario considers halving withdrawal from all wellsin the plain but no relocation. The results of thesesimulations are compared with those obtained maintainingunchanged the current aquifer exploitation, i.e. scenario A(Fig. 16 and Table 2).

    Analysis of water budget variations reveals that halvingthe discharge (B scenario) determines both an increase in

    Fig. 16 Water budget in scenarios A,B and E for the February2012February 2013 period (positive values are inows into theaquifer system, negative values are outows). For the denition ofcomponents and scenarios, see text and Table 2

    Table 2 Difference between the water budget in scenarios B and A and in scenarios E and A for the Plio-Pleistocene aquifer of theAlbegna River coastal plain (February 2012February 2013)

    Item Scenario Bscenario A Scenario Escenario A

    Inow (m3) Outow (m3) Balance (m3) Inow (m3) Outow (m3) Balance (m3)

    Withdrawals 1,406,000 0Albegna upstream 203,700 284,700 488,400 81,400 167,300 248,700Albegna downstream 58,166 358,000 416,166 35,390 22,900 12,490Sea Osa-Albegna sector 90,000 90,000 180,000 80,000 80,000 160,000Sea opposite lagoon 40,000 40,000 80,000 50,000 60,000 110,000Balance aquifer system 222,860 33,200

    All scenarios consider a 1.72 mm/year decrease in effective precipitation (according to Barazzuoli et al. 2002). The different scenarios areas follows: A exploitation of the aquifer continues under the present conditions; B halving the discharge from all wells in the plain, and norelocation of wells;E relocation of all coastal wells up to 3 km inland from the shore and maintenance of the current overall discharge. Forthe denition of components, see Table 1

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  • the quantity of water drained by the river and a decrease inthe quantity of water from the river, for an overall decreaseof about 9105 m3 in the ow of water from the AlbegnaRiver to the aquifer; note that the quantity of water whichinltrates in the saline portion of the river decreases bynearly 6104 m3. The opposite effect is obtained byrelocating the coastal wells further inland (scenario E):the Albegna River recharges the aquifer more (for a total ofover 2.5105 m3) and increases by about 3.5104 m3 thequantity of seawater penetrating the aquifer as it ascendsthe river. As for the coastal limit, in both scenarios there isan increase in aquifer discharge to the sea, especially in theOsa-Albegna sector, where extraction is greatest; therelocation of coastal wells therefore determines a decreasein saltwater intrusion, and can effectively contrast thedegradation of groundwater quality.

    Figure 17 shows the piezometric surface at the end ofthe simulation (February 2013, when it has an averageheight of 5.34 m asl), and its variations with respect tocalculated values in scenarios B and E; note that the B

    scenario produces a rather uniform increase in thepiezometric level, with an average value of 0.56 m (thebalance of the aquifer system increases by over 2105 m3,see Table 2), whereas the E scenario leads to a decrease inthe piezometric level in the northern sector and an increasein the southern sector near the lagoon, for an averagereduction of 0.40 m (the amount of water in the aquiferdecreases by about 3.5104 m3).

    In conclusion, a considerable decrease in extractionfrom the aquifer would lead to the reactivation of netgroundwater discharge to the sea and especially to theAlbegna River. When wells are only relocated, with nosignicant decrease in water extraction, there is a decreasein seawater intrusion but a lowering of the groundwaterlevel (a greater share of freshwater is extracted).

    Future research will improve the geological andhydrogeological reconstruction of the aquifer systemthrough the implementation of new lithostratigraphic datafrom wells and boreholes, and through further piezomet-ric, hydrometric (height of the river bottom and of the free

    Fig. 17 a Simulated groundwater level contour map for February 2013 (A scenario). b Piezometric difference between scenarios B and Afor February 2013. c Piezometric difference between scenarios E and A for February 2013. For the denition of scenarios, see text andTable 2

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  • water surface), topographic (GPS measurement of groundelevation in areas with a low hydraulic gradient) andphysicalchemical (electrical conductivity, temperature,major ion and isotope contents) surveys. Pumping andtracer tests will improve knowledge of the hydraulic anddispersive properties of the aquifer. In particular, perfo-rations planned in coastal areas will provide data on thevertical distribution of aquifer salinity (and therefore ofuid densities), thereby reducing the aforementioneduncertainties in the local distribution of hydraulic heads,which currently represent the greatest weakness of themodel. Lastly, public bodies will monitor temperaturesand precipitation rates and will complete a census of themost important wells and keep a record of water dischargerates. These data will be used to signicantly improve theconceptual and hydrodynamic models of the aquifer,thereby allowing the construction of a robust numericaltransport model essential for monitoring saltwater intru-sion and managing local water resources.

    Acknowledgements Research was supported by a grant from theUniversity of Siena to M. Salleolini. The manuscript was reviewedby P. Renard, G.H.P. Oude Essink, M. Antonelli and an anonymousreviewer, who are gratefully acknowledged for their constructivecriticism and suggestions that signicantly improved the quality ofthe manuscript.

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    Hydrogeology Journal (2008) 16: 15571576 DOI 10.1007/s10040-008-0324-z

    A conceptual and numerical model for groundwater management: a case study on a coastal aquifer in southern Tuscany, ItalyAbstractIntroductionThe study areaConceptualizationGeometry and structurePiezometric surface and hydraulic propertiesGroundwater qualityRole of surface water

    Numerical modellingSoftwareDiscretization and boundary conditionsCalibrationWater budgetValidationConclusions and future research

    References

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