The influence of climate and dam construction on the Ibera wetlands, Argentina

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1 Reg Environ Change (2006) 6: DOI /s ORIGINAL ARTICLE Graciela A. Canziani Æ Rosana M. Ferrati Claudio Rossi Æ Diego Ruiz-Moreno The influence of climate and dam construction on the Ibera wetlands, Argentina Received: 9 March 2002 / Accepted: 31 March 2006 / Published online: 20 May 2006 Ó Springer-Verlag 2006 Abstract The purpose of this work is to assess the impact on the Esteros del Ibera wetland ecosystem caused by the Yacyreta dam, a large hydroelectric power plant on the Parana River, Argentina, in comparison to other factors of environmental change. The project of the dam started around The power plant began operating in In 1989, the neighboring Ibera wetland ecosystem showed a substantial increase in the water level for which several different causes were conceivable, including climate change and the dam construction. We analyzed all existing hydrometeorological data and studied other changes that were observed in this ecosystem. A water balance model was used to analyze different scenarios. Increased groundwater inflow, generated since the construction of the dam, appears to be affecting the wetland more than any other factor. The study has implications for the assessment of global and regional consequences of building dams. Keywords Wetlands Æ Esteros del Ibera Æ Water balance Æ Modeling tools Æ Hydroelectric dam construction Æ Impact assessment Æ Sustainable development G. A. Canziani (&) Æ R. M. Ferrati Æ D. Ruiz-Moreno Multidisciplinary Institute on Ecosystems and Sustainable Development, Facultad de Ciencias Exactas, Universidad Nacional del Centro de la Provincia de Buenos Aires, Campus Paraje Arroyo Seco, 7000 Tandil, Argentina canziani@exa.unicen.edu.ar Tel.: Fax: C. Rossi Department of Chemical and Biosystem Sciences, University of Siena, Via Aldo Moro 2, Siena, Italy D. Ruiz-Moreno Department of Ecology and Evolutionary Biology, University of Michigan, Ann Arbor, MI , USA Introduction Wetlands are transitional ecosystems between aquatic and terrestrial environments that are characterized by permanent or temporary inundation. They can also be described as a terrestrial ecosystem with water saturated soils with plants and animals adapted to life in such an environment (National Research Council 1995). Most wetlands are fragile ecosystems. They cover approximately 6% of the world s land surface, contain about 12% of the global carbon stock, and hence play an important role in the global carbon cycle (Watson et al. 1996; Sahagian and Melack 1998). Coastal wetlands and peatlands included, wetlands represent the largest component of the terrestrial biological carbon pool (Dixon and Krankina 1995). Globally, reservoirs may be either sources or sink of the two greenhouse gases, carbon dioxide and methane (Kelly et al. 1994). Wetland conversion, through the decomposition of peat, leads to the emission of large quantities of carbon dioxide. At the same time, the increase water flow causes erosion. In addition to direct conversion, wetlands are affected by changes in water levels, not only in the magnitude of their increase or decrease but also in the frequency of occurrence or, as it is also called, pulses regime (Neiff 1999). Therefore, conserving, maintaining, or rehabilitating wetlands may be a viable part of a climatic change mitigation strategy (Bergkamp and Orlando 1999). An important wetland conservation strategy is the prevention of stress that can reduce their adaptive capacity. Reducing chemical pollution, avoiding vegetation removal, and protecting wetland biological diversity and integrity are, therefore, viable activities to maintain and improve the resilience of wetland ecosystems so that they continue to provide important services also under changed climatic conditions. Another conservation strategy is to prevent the fragmentation of wetlands, which occurs, for example, as a consequence of the construction of navigation channels. Connectivity

2 182 between ecosystems allows migration of species to occur in response to climate change and therefore the maintenance of migration routes is important (Dugan 1990; Carter 1997; Bergkamp and Orlando 1999). The Esteros del Ibera system Esteros del Ibera is a freshwater wetland ecosystem located in Corrientes Province, NE of Argentina. It is a part of the Del Plata Basin, which covers some 3.5 million km 2 within the five countries: Brazil, Paraguay, Bolivia, Uruguay and Argentina. Many hydroelectric dams have been built along its rivers. The Parana River has the largest number of dams, basically due to its great length (4,000 km), and an average flow of 19,000 m 3 /s. The climatic and geomorphologic features of this basin are ideal for the formation of large inland wetlands, such as the Pantanal in Brazil, N eembucu in Paraguay and the Esteros del Ibera in Argentina (Fig. 1). The catchment area of the Ibera wetland (Fig. 2) has a size of about 14,000 km 2 (Ferrati et al. 2005). It has a triangular shape of around 250 km in length and a width of km. The difference in water level along the entire length of the triangle is only 20 m, with an average slope of 1/10,000. A slow secondary slope is observed from East to West. The Ibera is a depression located on top of ancient beds of the Parana. It covers more than 50 m of permeable sediments, mostly deposited fluvial sand, which take the form of a pool of superficial storage of pluvial water. These sediments belong to the Ituzaingo formation and rest over the impermeable formation of Fray Bentos (basalt) (Herbst and Santa Cruz 1985). The paleolithic river bed of the ancient Parana River runs perpendicular to the separation of the present Parana River and the Ibera system, precisely at Puerto Valle, under the thinnest strip (4 km) of land beside the Fig. 1 Map showing the location of the Esteros del Ibera macrosystem within the Del Plata Basin

3 183 Fig. 2 Map of the Esteros del Ibera system and position of stations 1 Newspaper El Libertador August 30, 2000, Corrientes, Argentina. Yacyreta dam reservoir, and towards the Ibera system. Nowadays, the average depth of the basalt at the left margin of the Parana River is 50 m a.s.l., under some 40 m of sandy sediments, and plunges down to 20 m a.s.l. over a stretch of 400 m in the paleolithic river bed (Ferrari Bono et al. 1965). In the 1970s, by the time of the planning of the dam, this was a well-known fact, and probably led the planners to think of the Esteros del Ibera as a natural site for an alternative reservoir in case of conflict with Argentina s up-river neighbor Brazil. Geologists and hydro-geologists believe that there are more than one paleolithic-river bed in the contact area between the Ibera system and the Parana River. The complexity of the underlying structure can be observed in the fact that the city of Ituzaingo has a well reaching down to a depth of 80 m, (i.e., 38 m into the basalt layer), which yields between 20 and 25 m 3 h 1 with a piezometric head at 20 m below surface level 1. This means that the basalt is not impermeable and that the water pressure inside the basalt layer is two atmospheres. The studies carried out over the contact area on the left margin, between the city of Ituzaingo and Puerto Valle, have been insufficient in estimating the magnitude of the flow of water from the reservoir to the Esteros del Ibera. The isolation and the inaccessibility of the wetland region have, up to now, helped to its preservation. The Esteros de Ibera wetlands contain several plant and animal endangered species. In spite of the fact that a large portion of these wetlands is made up of floating vegetation islands or embalsados (Neiff 1981, 2004), their response to raising water levels is not instantaneous. Hence, changes in water level affect the nests of reptiles and birds, reducing their reproduction and disturbing the population dynamics. The endangered marsh deer population is suffering a decrease in the quality of their habitat by the reduction of foraging and dry areas for resting and the increasing transmission of parasites (M. Becaccessi, personal communication). Another potential problem is the introduction of nutrients into the oligotrophic Ibera system. If the sediment-rich water from the Parana

4 184 River enters Ibera without being properly filtered, then a change in nutrient concentration or ratios could alter the vegetation composition and drastically change the structure of the wetlands. Nowadays, most of its surface is covered by permanent or temporary floods, depending on the balance of atmospheric input and output, surface runoff, and groundwater balance, as well as on the antecedent water level. The long delay in the hydrologic response of the system is due to its morphology, the type of soil, and the presence of vegetation, which retard the flow. Floating vegetation creates natural spillways that retain the sheetflow and release it slowly towards the Corriente River, which is the natural channel of evacuation. Given the vastness and inaccessibility of the region, remote sensing is one of only a few tools to detect changes. Nevertheless, it is difficult to use satellite images in a direct way for the determination of monthly or seasonal variations between flooded versus dry areas because of its unique characteristics. For example, the islands of floating vegetation rise with floodwater although not instantly rather than being submersed. In addition, due to high water transparency, it is sometimes impossible to distinguish vegetated flooded areas from humid emergent soil. The water level records taken at Laguna Ibera, the only site in the Esteros del Ibera system with more than 30 years of data (EVARSA 2000), show a change of regime starting in The mean values before and after 1990 differ in more than 80 cm, and the pulses regime has dampened. These alterations could be produced by changes in atmospheric conditions, such as in rainfall and/or in evaporation, or by variations in groundwater inflow or outflow. The main input variable in the Ibera system is precipitation, while the surface-water inflow is negligible. Hence, the Esteros del Ibera may be considered a closed system, at least superficially. The variables taken into account for a mass balance are precipitation, evapotranspiration, outflow and storage. Comparing the results with historical records, we hypothesize a water input and analyze the possibility that it may be a groundwater input from the Parana River and contemporaneous with the construction of the Yacyreta Dam. This work proposes a model as a tool to justify such hypothesis. The Yacyreta dam Only a few kilometers North of the Ibera system, and separated by a narrow ridge of sand and clay, is the Yacyreta dam on the Parana River. The Dam is anchored on the Yacyreta Island, taking advantage of the gap created by the Apipe rapids and the possibility of using the two branches of the Parana, the Main branch and the An a-cua branch. The civil works of the dam started in 1983 with the construction on the Northern or right margin of the river, on Paraguayan territory, of the Right Lateral Dam (25.7 km long). This dam, as well as the Yacyreta Island Lateral Dam (18.7 km long), has been constructed with zoned sections, a clay core and sand shoulders, including a water-proof cut-off wall of bentonite cement running from the core to the rock 2. Curiously, the Left Lateral Dam (12 km long), on Argentine territory, with homogeneous sections, built on watertight ground, and the Main Left Dam (1.8 km long), with zoned sections, a central clay core and sand shoulders, were not provided with a cut-off wall of bentonite as the Northern dams were. The deviation work for the Yacyreta dam was executed during the period April September The main construction work was completed and the reservoir was filled in 1994, reaching 76 m a.s.l. The design of the dam requires an optimal reservoir level of 83 m a.s.l. that has not been implemented yet. Plans for alternative uses of the contiguous paleolithic river beds (Ibera wetlands and Aguapey and Mirin ay Rivers), either as level-pool reservoirs 4 or as a cascade of hydroelectric flatland dams, had been considered for a long time (De Alba and Vera Morinigo 1964; INCyTH 1974). Even the construction of a navigation channel uniting Paraguay, Brazil, Argentina, and Uruguay, across the Paraguay, Parana, and Uruguay Rivers, was considered feasible until 1998, when the Brazilian Government decided to cancel its participation in the project because its execution would endanger the Pantanal wetlands 5. Impacts on the regional economy The problems generated by the excess of water extend beyond the Ibera system to the surrounding lands that are being used basically for cattle rising and forestry. Since 1995, Fucosa (Fundacion Correntina de Sanidad Animal) has recorded the reduction in about 10,000 heads of cattle per year in herds in the department of Ituzaingo, due to the annual loss of 20 25,000 ha of productive land. The cattle owners in the neighboring departments of San Miguel and Concepcion are experiencing similar losses. The Rural Society of Ituzaingo pointed out that this process started to be noticed in 1990 and that, even in periods of drought, the water does not recede. A comparable situation is observed in the adjacent wetland area of Batel-Batelito, West of the Ibera system, separated from the latter by a distinct elevation, but connected by groundwater flows, as mentioned earlier 2 Information published by EBY, Datos y descripcion del Emprendimiento Binacional. Relaciones Publicas, 10 pp; and EBY- ERIDAY, Yacyreta Project, Contract Y-C.1. Main Civil Works, 20 pp. 3 Newspaper Clarin, May 12, Buenos Aires, Argentina. 4 Newspaper El Litoral, October 13, 1999, Corrientes, Argentina. 5 Newspapers El Litoral, October 13, 1999, Corrientes, Argentina; El Cronista Comercial, December 21, 2000, Buenos Aires, Argentina.

5 185 (Paoli and Santa Cruz 1981). Landowners, alarmed by the steady increase of losses in productive lands due to seepage and water covering thousands of ha, have gathered into a society that represents their interests. As an example, some 200,000 ha of productive lands have been lost to water in the Batel-Batelito Basin, while other 180,000 ha are reported lost in the Ituzaingo district (ICA, D. Acevedo, personal communication). As the phreatic layers continue to rise, an excess of water could damage the forestry exploitations, producing enormous losses to investment companies. It is unfortunately too often that large civil engineering works in developing countries are carried out without previous thorough, complete, and seriously integrated assessment of the full consequences of the disturbances introduced. The attractive socio-economic hook involving the creation of new jobs, even if they happen only for a brief period, and the partial reference on the economic benefits which could be generated for the national administrations, are often the determinant factors when decisions are taken. This becomes painfully evident in the lack of integrated assessment covering all the issues involved, including the value of the ecosystems lost, which is not well understood by authorities. The impact of the environmental modifications comes out only when the civil work is completed, leaving only few possibilities for remedial actions. Very little attention is usually paid to the costs produced by accumulated losses in the long term. It is necessary not only to be capable of estimating the costs due to short and long-term damages produced in the environment and the socio-economic costs, but also to compare them to the benefits obtained. Materials and methods Water balance assessment The wetland water balance is the total of inflows and outflows of water from the wetland. We estimate the water balance in order to determine the cause of the regime shift recorded in We assume that the Ibera system as a whole behaves as a response function whose main characteristic is storage. Water balances were estimated at a regional level on yearly intervals and at a system level on monthly basis. Furthermore, we analyzed historical data for the period ending in Based on this analysis, we developed a mathematical model for the Ibera system. As mentioned earlier, in the Ibera system the main input variable is precipitation, while the surface-water inflow is considered negligible. The main surface outputs are the outgoing flow of the Corriente River and the evapotranspiration demand. As a first hypothesis, and based on earlier studies (Popolizio 1981), the watershed was considered hydrologically closed and the groundwater balance was assumed to be contained in the storage compartment. For this case, the mass balance equation applied over a particular time interval is ðv þ DSÞ P E ¼ 0 ðmmþ; ð1þ A with P net precipitation input per unit area; E net evapotranspiration loss per unit area; DS change in water stored both above and below the ground surface; V volume of water leaving the system as surface flow; and A effective area. The regional annual water balance was calculated from the annual areal precipitation and evapotranspiration values for the catchment obtained from Posadas, Mercedes and Corrientes meteorological stations (National Meteorological Service SMN and National Institute for Agriculture Technology INTA) and mean annual discharge from the Corriente River at Paso Lucero and Los Laureles gauging stations (EVARSA 2000). The calculated variable was the change in water storage over the period The local monthly water balance was calculated from the precipitation data obtained at Ituzaingo, Yacyreta, Concepcion, Galarza, Pay Ubre, Chavarria, and Colonia Carlos Pellegrini and weighed using Thiessen polygons. The evapotranspiration was computed taking monthly mean temperature data measured at Posadas, Mercedes and Corrientes meteorological stations (Thornthwaite and Holzman 1942). Monthly mean flows for the Corrientes River were calculated from data at Paso Lucero and Los Laureles gauging stations. Available data allowed us to calculate the local water balance in the periods July 1968 December 1970; January 1977 October 1979; August 1986 September 1989; and May 1990 May 1999 (Ferrati and Canziani 2005). All available data, and the model, were used to answer the following questions related to the possible causes of the increase in the storage capacity of the Ibera system: Could the increment in storage be explained by the increase in precipitation recorded on the Del Plata Basin during the last century (Canziani and Diaz 1997) with further positive trends since the 1970s, such as the enhanced precipitation during ENSO events? Is it possible that an accumulation of vegetation or debris may have obstructed the outflow through the Corriente River towards the Parana River? Or, is it that the floods recorded in the Middle Parana River may have reversed the flow of the Corriente River, keeping back the natural outflow from the Ibera system? Is there a change in the groundwater flow system? The period in which the increases were recorded coincides with the closing of the main branch of the Parana River and the use of the derivation channel next to the northern border of the Esteros del Ibera. Could a change in groundwater flow have been originated after the construction and filling of the Yacyreta dam?

6 186 Ibera Lagoon Water Level (m a.s.i.) /06/68 17/11/72 25/04/77 01/10/81 09/03/86 15/08/90 21/01/95 29/06/99 Results Average : m Recent changes in the Ibera wetlands Average : m Fig. 3 Hydrologic data of the Ibera system: water level data recorded at the Ibera Lagoon Relative Water Level (cm) Ibera Lagoon Water Balance Fig. 4 Water balance output (medium grey line) as compared to recorded data at the Ibera Lagoon (light grey line) and Water balance output (black line) when missing data at Paso Lucero are replaced with a null discharge The Del Plata basin as a whole has suffered changes due to both natural processes and anthropic actions. These have been attributed to climatic change (since 1970), El Nin o Southern Oscillation (ENSO) events in , 1986, 1992 and 1998, deforestation of natural tropical forests in Brazil and Paraguay and changes in the regime of the main rivers due to the construction of more than 40 dams (Garcia and Vargas 1996, 1998; Genta et al. 1998; Barros et al. 1999; Grimm et al. 2000; Camilloni and Barros 2003). The Ibera wetland has experienced significant change in water levels, affecting plants and animals, as well as human activities in the region. The gauge installed in the Ibera Lagoon is the only stage station located inside the borders of the system that has been measuring water levels since its installation in Its records show a rapid increase in water levels over the period of a few months between 1989 and These levels have been sustained until today, bringing the mean level from m a.s.l. over the period , to m a.s.l. over the period (Fig. 3). This increase is equivalent to the average depth of the system in previous years. The wetlands also exhibit an increase in water flow and the dragging of sediments, as well as changes in vegetation dynamics and habitat quality that affect the persistence and structure of native species. On one hand, the Ibera system has been able to maintain its equilibrium through hydro-biological regulation mechanisms favored by the hydraulic characteristics of the embalsados and the morphology of the submersed soils. On the other hand, the Ibera has shown a prolonged response delay, making it likely that the ultimate consequences of the sudden increase in water levels are not yet fully manifested within the ecosystem. The output of the storage compartment from the water balance equation (Fig. 4, medium grey line) show a good fit when compared to water level data recorded at Ibera Lagoon. Therefore, the water balance equation appears to be a good first approximation to the behavior of the system. Nevertheless, there is a gap in the data over the most important period, the years , when the increase in water levels was detected. We therefore used data from the meteorological stations at Posadas, Corrientes and Mercedes to estimate regional precipitation and potential evapotranspiration consistently (Thornthwaite and Holzman 1942). Looking into the accumulated decadic values, we observe that in the period total rainfall amounted to 15,093 mm, and potential evapotranspiration to 11,029 mm (73% of rainfall). This yields an excess of 4,064 mm in the vertical balance over 10 years. The outflow through the Corriente River was calculated to be m 3, yielding only a negligible excess of m 3 in the calculated balance (Table 1). The water levels at the Ibera Lagoon measured at the beginning and at the end of the decade coincide and are close to the average over the period. Hence, no change in storage occurred during this decade. This confirms the result obtained from the balance equation, yielding less than 0.3% in error, which is significantly less than the standard error in the measurement of evapotranspiration (Maidment 1993). In the period , two ENSO events occurred, in and , providing alone 25% of the total rainfall recorded in this 10-year period. Although initial conditions were similar (low water levels), the response of the Ibera system was very different in each case. During the event, a precipitation input of 2,228 mm yielded an excess of 790 mm, while in a precipitation input of 2,210 mm yielded a deficit of 345 mm. For the entire decade, the accumulated values of rainfall and evapotranspiration were 17,104 and 11,058 mm (65% of rainfall) yielding an excess of 6,046 mm in the vertical balance. The accu-

7 Table 1 Cumulative values of rainfall, discharge, potential evapotranspiration, and resulting balance for each decade between the years In parentheses, it is shown the percentage of the cumulative value of the variable relative to total precipitation 187 Period Precipitation (mm) Discharge (hm 3 ) Evapotranspiration (mm) Balance (hm 3 ) ,093 49,692 (24%) 11,029 (73%) ,104 77,510 (33%) 11,058 (65%) ,429 69,609 (40%) 8,560 (69%) 15,835 mulated discharge is m 3, some 56% higher than in the previous decade, the balance yielding once more a negligible excess of m 3 over the period. For the period , the accumulated values of evapotranspiration and discharge show a distribution of 69 and 40% with respect to the accumulated precipitation. Hence, a negative balance is obtained for this decade, clearly revealing a water input not previously considered. The time series (Table 1) indicates a steady state lasting two decades ( ), without any excess to be carried on to 1990, followed by a sudden increase in water level Comparing the negative balance obtained for this last decade and the increase in the mean level observed in Ibera Lagoon water level records reflects and confirms the occurrence of an extra input not considered in the balance. Changes in outflow The analysis of runoff data showed that the outflow in Paso Lucero had changed along side the storage in the Ibera Lagoon. We first studied the typical response of outgoing flow to individual rainfall events, finding that the system shows a clear change in the hydrologic regime, starting in During the period , an average 24% of total rainfall left the system through the Corrientes River. During the following period , the average increases to 33% of the total rainfall. Since 1990, the average has risen to 40% of the total rainfall (Table 1). This means that, assuming constant precipitation, the relative discharge has at least increased in 55% as compared to records of the decade This analysis does not assume the absence of obstructions preventing the outflow, but rather that, even if there had been, they could not explain the accumulation of water in the system. Comparison of the periods to shows that the system is retaining a much larger volume of water. To assess the stability of the new conditions, we assumed the worst possible situation in order to fill the gap of missing discharge data over the period Hence, the discharge at Paso Lucero was assumed to be zero in the period February July and October December 1989 as well as January April 1990, where data is missing. Under this assumption, the water balance results in storage levels that relate well to the Ibera Lagoon water levels as shown in Fig. 4 (black line). This indicates that, even under such strong restrictions, the increase in water levels recorded in cannot be explained by geobiophysical processes taking place at surface level. Subsurface processes As mentioned earlier, the Ibera system lays on the paleolithic-river beds of the Parana River which shifted to its present position as result of cataclysms occurring from the late Pliocene to the middle Pleistocene periods. In following one ancient course, the Parana River carved a canyon on the now underlying basalt, which was subsequently filled with sandy sediments of various degrees of coarseness of grain (Castellanos 1965; Herbst and Santa Cruz 1985; Iriondo 1987, 1991; Ramos 1999; Fulquet et al. 2002). Moreover, the Apipe rapids signal one of the fractures on the basalt layer in the vicinity of the Yacyreta Island. Beside the large fractures, the basalt bed presents many fragmentations or diaclasae into which water can flow (De Alba and Morinigo 1964; Blanco et al. 2003). At its northernmost limits, before the construction of the Yacyreta Dam, the water level at the Esteros del Ibera was approximately 12 m above the level of the Parana River (Ferrari Bono et al. 1965). At that stage the Ibera system must have contributed some underground water to the Parana River. Nowadays, the lake created by the dam rises some 4 m above the previous headwater levels of the Ibera system. Noting that the narrow (4 12 km wide) ridge that separates them is a Ibera Lagoon Water Level (m) Parana River Water Level (m) Fig. 5 Water levels at the Ibera Lagoon versus levels of the Parana River at the construction site over the period April October 1989

8 188 Fig. 6 Adjusted water balance model output compared to water level data recorded at the Ibera Lagoon (light grey line). Medium grey line shows the output of the model when an input equivalent to an increase of 10 cm month 1 is added over the period April November Black dotted line reflects the output when an additional 2 cm month 1 are added whenever the level of the Parana River exceeds 65 m a.s.l. between 1991 and Black solid line shows the output when an extra 0.25 cm month 1 are added as the reservoir level exceeds 72 m a.s.l Relative Water Levels (cm) sandy sediment formation, one may assume that groundwater flow could have been reversed in this area. The average discharge of the Parana River has increased from a historical 12,000 19,000 m 3 s 1 over the last three decades as a consequence of climatic changes (Goniadzki et al. 2002), while extreme records of more than 50,000 m 3 s 1, in 1983, during ENSO events have been recorded (Ferrati et al. 2005). As a consequence of the deviation of the flow of the Parana River into the much narrower section of the channels, the water levels quickly rose by more than 10 m (A. Fulquet, personal communication; Eriday data). At the same time water levels in the Ibera Lagoon rose with short delay. This would imply that the transfer process was so important that it blurred the effect of other possible factors during this few months between April and November In Fig 5, it is easy to observe that the level at Ibera Lagoon increased as the level of the Parana River reached thresholds at 60.5 and 63 m a.s.l., but did not decrease as the latter moved downwards. After this, the Ibera system reached a new equilibrium and maintained a pattern where the oscillations are tied to atmospheric processes. Revision of the water balance model under the hypothesis of groundwater inflow It is appropriate now to take a closer look at the water balance model. Successive groundwater inflows were considered in order to obtain a close agreement between the balance model output, and the historical stage data at Ibera Lagoon. Firstly, a theoretical effluent volume that increased the water level by 10 cm month 1 was incorporated in the groundwater balance model along a period of 8 months, from April to November of In this period the water level in Parana River reached successively 60.5 and 63 m above sea level (Fig. 6, medium grey line). If we consider the volume of water required to increase the water level in 10 cm month 1 over an extension of thousands of square kilometers, it becomes very difficult to accept that the water is entering the system through the sandy erosion front or albardon that separates the wetland from the Parana River. It is also important to note that the rise in the water level at Ibera Lagoon occurred even when the level of the Parana River laid below that of the Ibera system (72 m a.s.l.) at the headwater in its closest northernmost boundary (Fig. 7). Yet, given the general slope of 1:10,000 from the NE to the SW, the Ibera Lagoon was indeed below the level attained by the river. This observation and the fact that the response is extremely rapid seem to support the hypothesis of a groundwater inflow through the fractures and diaclasae in the basaltic beds. If this is the case, the main inflow could be entering the Ibera system at its centre and not at the headwater. Later on, a monthly theoretical effluent volume that increased the Ibera system in 2 cm month 1 was added (Fig. 6, dashed black line) during the months when the water level in Parana River at Yacyreta Dam rose to 65 m a.s.l. These months are April and June of 1991, February, April, May, June, August, October, November and December of 1992, March of 1993 and February and June of The good fit of the corrected model with the measurements recorded at Ibera Lagoon seems to be in agreement with the hypothesis of a rapid inflow of enormous magnitude through fractures and diaclasae in the basaltic bed. Finally, a groundwater inflow that increased the stage in the Ibera system in 0.25 cm month 1 was added, whenever the level of water at the Yacyreta reservoir exceeded 72 m a.s.l. (altitude at the headwater of the Ibera system). This last correction (Fig. 6, solid black line) seems to be in agreement with the estimations of 12 m 3 s 1 calculated by Lotti e Associatti 6 for the 6 Lotti and Associati, Societa di Ingegneria SpA (1999). Estudio Hidrogeologico Macroregional del Area Costanera del Rio Parana en el Tramo entre la Presa Yacyreta y el Arroyo Yabebiri. Report.

9 water level (m a.s.l.) Water Level of Parana River at Yacyreta Dam 189 The two consulting companies, Harza-CIDY, in 1989, and C. Lotti e Associati, in 1999, produced estimates 7 differing by one order of magnitude: m 3 s 1. If the largest value, 12.7 m 3 s 1,isconsidered to be true, then the time required to bring the levels of the Esteros del Ibera from to m would have been 23 years. Nevertheless, the system reached this value in 18 months. Hence the groundwater inflow is necessarily much larger than the official estimates, as the water balance model confirms. Impact assessment Abr- 89 seepage through the sandy albardon that separates the Ibera system from the reservoir. It is possible then to identify four different stages from the hydrometric data recorded at Ibera Lagoon. A first stage where the system is regulated by processes taking place at a surface level, such as precipitation, evapotranspiration and discharge through the Corriente River. A second stage, during the interval , where the system is taken to a new equilibrium by the contribution of an important groundwater inflow. Then, a third stage, from 1991 until 1994, with intermittent pulses, before the reservoir was filled. Finally, the fourth stage, when the reservoir reached a level above 72 m a.s.l., which is the headwater level of the Ibera system. Discussion 17-Nov Jul Mar Nov Nov Mar Mar- 00 Impact of Yacyreta on groundwater flows 03-Abr May- 02-Jun Fig. 7 Water level data of the Parana River at the Yacyreta Dam reservoir site, showing the two thresholds considered: 65 m a.s.l. (possible location of diaclasae deducted from data) and 72 m a.s.l. (headwater level of the Ibera system) A change in the groundwater system is the best explanation for the changes in water level. Could this have been induced by the construction and filling of the Yacyreta dam? Although the data analyses support this assumption to some extent, the full assessment of this important issue calls for additional study and research on the geology and hydrogeology of the site. The limited number of soundings performed on the left margin, the low quality of the collected data, the insufficient depth of the wells which do not reach the basalt base of the aquifer, the scant transversal lines for seismic recognition, and non-standardized water pumping, are mentioned as problems that have provoked an enormous uncertainty in the numeric results of the models intended to estimate the underground transfer flow. The existence of four stages in the evolution of the Ibera system within the last two decades seems to indicate that the underlying geology is in fact more complex than what the constructors of the dam could predict. The scarcity of meteorological and hydrological information, including data on underground water and information on geological features is also a clear indication that the required and pertinent impact assessment was incorrectly made. The underlying geology is not completely known and the hydro-geologic processes involved are still not well understood. In order to meet the requirements for optimal operation conditions of the power plant, the water level of the reservoir needs to be increased from the present 76 to 83 m a.s.l. The governments of Argentina and Paraguay have approved this measure in spite of the changes produced in the natural neighboring ecosystems, such as the Esteros del Ibera, and the extent of productive lands that are currently being inundated by seepage. Not only the current transfer flow has not been accurately measured, but also the consequences of the future increase of water level such as the response time of the system, the conditions of a new equilibrium and its effects on the ecosystems involved, the short and long term ecological, social and economic implications and the measures that should be taken to overcome and control the disturbances are ignored. Present situation In September 2001, Fundacion Vida Silvestre Argentina (FVSA), associated to WWF, and one of the leading NGOs dedicated to the preservation of natural ecosystems, published a book on the effects of the Yacyreta Dam on the Ibera ecosystem (Blanco and Parera 2001). The publication gathered the results of investigations on different areas carried on by several independent scientific research groups. The book captured the attention of the media and prompted the reaction from EBY. It was only in October 2001 that EBY called a meeting with representatives of different stakeholders (government, NGOs, landowners, and Universities). The Foro Ibera- Yacyreta was thus created, and a four-member Executive Committee, one for each sector, was named with the purpose of integrating the opinion of the stakeholder in the decision making process, in particular regarding 7 Quoted in the report produced by Lotti and Associati in 1999.

10 190 the elevation of the water level of the reservoir to 83 m a.s.l. In March 2003, the Foro Ibera-Yacyreta called for a public meeting with EBY in which both parts presented their findings. Here FVSA presented an update of their book (Blanco et al 2003). A technical meeting was held the following May in Ituzaingo during which technical representatives and researchers from both parts discussed the validity of available studies without reaching an agreement. In October 2003, the Committee on the Civil Works of the Yacyreta Dam of the National Chamber of Representatives (Honorable Camara de Diputados de la Nacion) invited EBY, the Foro Ibera-Yacyreta, as well as researchers having performed studies in the region, and various national and provincial government officials to a special meeting for the discussion of the present situation. As a result, a Consensus Board (Mesa del Consenso) was created with the mission of finding the way to a solution to the problem. This Consensus Board is formed by one representative each of EBY, of the Undersecretary of Environment and Sustainable Development, and of the Government of the Province of Corrientes, as well as one representative of each sector of stakeholders (government, NGOs, landowners, and Universities) belonging to the Foro Ibera-Yacyreta. The Nation s Attorney of the People (Defensor del Pueblo de la Nacion) acts as mediator. In 2005, as a first result of the discussions, the Board has agreed on the terms of an international call for an independent analysis of all existing documents on the possible transfer of water from the Yacyreta reservoir to the Ibera system. The Committee on the Civil Works of the Yacyreta Dam is in charge of the procedures. It will be interesting to see if this process will eventually lead to the execution of the necessary additional studies and research on the geology and hydrogeology of the site. Conclusion The data analyses performed here are strongly revealing. The water balance model was used as a first approach to the problem. Even if the limitations due to the extension of the area under study and the scarcity of local stations are not negligible, the model permitted to look into the hypotheses of an increase in precipitation and of an obstruction of the Corriente River and to obtain clear conclusions. In spite of the uncertainties intrinsic to this type of aggregated model, it was possible to calibrate it and use it as a tool to test and discard hypotheses. Moreover, it also permitted to infer other scenarios that need to be analyzed and verified using a different methodology. The feeling that little attention was paid to the implications of a groundwater transfer that could be provoked by damming a river like the Parana, with an average flow of 19,000 m 3 s 1, is unavoidable. The results of impact assessment for the construction of the dam must have been either insufficient or simply ignored. The changes at regional level, being recorded since 1989, give a picture of the situation generated by setting the Yacyreta reservoir at a level of 76 m a.s.l. The economic losses have not been officially quantified. The damages in the natural ecosystems have not been systematically assessed. Neither have been the effects at global level. Basically, it has not been studied yet what the outcome of a balance between the benefits obtained from generating electric energy and the costs of current and potential damages and losses would be in the long term. The need to undertake serious and complete studies of all the outstanding aspects, socio-economic, ecological and environmental, before the reservoir level is raised seven more meters is of primary importance and urgency. Acknowledgments We thank SERNAH, EEA INTA Mercedes, CRL-INA, NOAA, Maria Josefa Fioriti, Daniel Cielak, Marcos Garcia Rams (San Juan Poriahu), and Luis Lenzi for allowing us access to hydrological and meteorological data. We are indebted to Instituto Correntino del Agua for permitting us to consult the many volumes of Estudio del Macrosistema Ibera and to Adolfo Fulquet for his willingness to share valuable information. Our appreciation to Juan Jose Neiff, Jose Angeleri, Delia Acevedo, Margarita Melo de Vaquer, Javier Corcuera, Marcelo Acerbi, Juan Rodrigo Walsh, and Ruben Patrouilleau for generating opportunities for fruitful discussion. We thank CONAE and NOAA for providing satellite images. Special thanks to Florencia Castets (Personal de Apoyo CIC) for her collaboration in digitalization and cartography, to Helena Vayo who helped us polish the text, and to Wolfgang Cramer, Tony Patt and the two reviewers who made very valuable suggestions. 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