Potential impacts of climate change in the Plata basin

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1 Regional Hydrological Impacts of Climatic Variability and Change (Proceedings of symposium S6 held during the Seventh IAHS Scientific Assembly at Foz do Iguaçu, Brazil, April 2005). IAHS Publ. 295, Potential impacts of climate change in the Plata basin VICENTE BARROS 1, MOIRA DOYLE 2 & INÉS CAMILLONI 3 1 University of Buenos Aires and CONICET, C. Universitaria, Pab. II 2, 1428 Buenos Aires; Argentina barros@at.fcen.uba.ar 2 University of Buenos Aires, C. Universitaria, Pab. II 2, 1428 Buenos Aires; Argentina doyle@at.fcen.uba.ar 3 University of Buenos Aires and CIMA, C. Universitaria, Pab. II 2, 1428 Buenos Aires; Argentina inesc@at.fcen.uba.ar Abstract Significant climate change effects on river discharges of the Plata basin cannot be discounted, as the regional precipitation has already had a substantial positive trend in the past. Besides, rivers discharges, when expressed as percentages, considerably amplify the precipitation variability. Trends during the last 40 years were principally caused by the intensification of the El Niño signal on precipitation, leading to an increased frequency of large floods in the middle and lower Paraná River, where four out of the five largest floods of the 20th century occurred during the last 20 years of that century. In addition, episodes of heavy rainfall are also becoming more frequent, contributing to increased vulnerability to climate change. There is currently too much uncertainty in future precipitation and hydrological scenarios for the next 50 years over most of the Plata basin, because outputs of global climate models underestimate considerably the present mean precipitation, and also differ among themselves. Key words climate change; flood; La Plata basin; Paraná River; vulnerability INTRODUCTION The water resources of the Plata basin sustain the more developed region of South America, where agriculture is not only one of the region s most valuable assets, but also the basis of exports of four of the five countries that share the basin. In addition, the regional hydro-electric power is the source of more than 80% of the power of these countries. The most important tributaries of the Plata River are the Paraná and the Uruguay rivers. In turn, the Paraná receives an important tributary, the Paraguay River. Along the Paraguay River and the middle and lower Paraná, there are large flood plains, which are being increasingly occupied by agriculture and settlements. The ongoing climate change may have impacts on the hydrology of the Plata basin, which could modify its vulnerability to the present hydrological variability. This issue is explored in this paper by considering present precipitation and streamflow trends, and future climate scenarios.

2 2 Vicente Barros et al. VULNERABILITY TO CLIMATE CHANGE The Plata basin has two features that have made its river discharges highly susceptible to climate change. The first is that the percentage rates of change in most of the river discharges are amplified, by a factor generally larger than two, when compared to the corresponding rates of change in precipitation (Tucci, 2000). At the decadal timescale, this feature could be attributed partially to deforestation and changes in the use of soil, which increase runoff. However, when the precipitation rate averaged north of 27 S is compared between two consecutive years (1998 and 1999) or between all El Niño and La Niña years of the period , its percentage change was lower than half the respective change in the discharges of the Paraná and Uruguay rivers about that latitude (Berbery & Barros, 2002). The amplification rate of discharges with respect to precipitation was about the same, no matter if two consecutive years, two groups of years according to ENSO phase, or two 20-year periods were considered (Table 1). This result suggests that the amplification of the hydrological response to precipitation does not depend only on the change in the use of soil, but is an intrinsic feature depending on the slope and other characteristics of the basin. In the context of a changing climate, this feature increases the vulnerability to hydrological changes, because no matter the sign of the precipitation change, the discharge responses will be considerably larger. Table 1 Average rainfall rates and streamflows in the Plata basin. Precipitation (m 3 s -1 ) Streamflow (m 3 s -1 ) Amplification factor Difference 31% 79% % El Niño La Niña Difference 7% 17% 2.4 3% Difference 16% 35% 2.2 9% Evaporation plus infiltration (m 3 s -1 ) The second feature that has made water resources in the Plata basin potentially vulnerable to climate change is that significant changes in the long-term precipitation means are possible, as was already observed in many sub-basins, and in the Plata basin as a whole. During the last century, southeastern South America, which includes the Plata basin, had the largest positive trend in precipitation among all the sub-continental regions of the world (Giorgi, 2003). Over almost all the huge area of the Plata basin, km 2, there has been a rapid positive trend in precipitation of more than 15% during the last 40 years, resulting in a 35% increase in the basin s discharge (Fig. 1). Except in the northern part of the upper Paraná, all over southeastern South America to the east of the Andes, there was a positive trend that reached 5 mm year -1 in the northeast of Argentina and the neighbouring regions of Brazil and Paraguay.

3 Potential impacts of climate change in the Plata basin Fig. 1 Precipitation trends in subtropical South America, east of the Andes, (mm year -1 ). PRECIPITATION AND STREAMFLOW TRENDS Precipitation tendencies, calculated since the beginning of the 20th century, were positive in most of southeastern South America, but with lower values overall than those for the last 40 years (IPCC, 2001). On the other hand, west of the Andes over central Chile, there was a pronounced negative trend. This pattern of positive trends in the annual precipitation over the east of the continent and negative ones in the west appears also in the case of Australia (IPCC. 2001). The strong positive drive to higher precipitation started during the 1970s in the subtropical part of southeastern South America, and a decade before in central Argentina north of 40 S (Barros et al., 2000). Consistent with this change in precipitation, discharges of those rivers with catchment zones in the subtropical part of the Plata basin had a positive trend since the early 1970s (Fig. 2). In the case of the three largest rivers, the mean values after that date are considerably greater than the average of the whole record and the average of any other 30-year period before 1970 (Fig. 2). ENSO IMPACT Trends in discharges in the main rivers of the Plata basin over the last 30 years have been caused largely by the intensification of the El Niño signal on the precipitation field. Almost two thirds of the Plata discharge comes from the contribution of the region between 22 and 27 S, where most of the precipitation trend was from El Niño periods (Fig. 3). Note that, since El Niño periods were approximately evenly distributed in the 40-year period, the precipitation trend can be roughly split into two trends corresponding to El Niño and no Niño periods. The positive precipitation trend of the last years was accompanied by an increased frequency of the largest discharges in the middle and lower Paraná River (Camilloni & Barros, 2003). Four out of the five largest discharges of the 20th century occurred

4 4 Vicente Barros et al (a) (b) (c) Fig. 2 Annual discharge in m 3 s -1 for (a) the Paraná River at Corrientes, (b) the Uruguay River at Paso de los Libres and (c) the Paraguay River at Asunción (----- indicates the annual average and the linear trend since 1972) Fig. 3 Precipitation trends in subtropical South America, east of the Andes corresponding to El Niño events: (mm year -1 ).

5 Potential impacts of climate change in the Plata basin 5 Table 2 The 10 largest monthly discharge anomalies of the Paraná River for Date Season and ENSO phase Discharge anomaly (m 3 s -1 ) June 1983 Autumn Niño (+) June 1992 Autumn Niño (+) December 1982 Summer Niño (0) March 1983 Autumn Niño (+) June 1905 Autumn Niño (+) May 1998 Autumn Niño (+) October 1998 Spring neutral October 1983 Spring neutral July 1992 Winter Niño (0) February 1997 Summer neutral during the last 20 years (Table 2). These great flows flooded large surfaces along the Paraguay and the middle and lower Paraná. In the case of the 1998 episode, the flooded area was nearly km 2 in Argentina alone. The increased frequency of these large discharges has augmented considerably the vulnerability of the flood plains, which were occupied before the changes in hydrological conditions. As a response, infrastructure defences were built in various key locations in Argentina and an alert system was successfully implemented. However, the toll of these events remains high. In the case of the 1998 flood, the economic cost was above US$700 million. Although not all of the major discharges were related to El Niño, the six highest ones occurred during ENSO phases, and none of the major 20 occurred during a La Niña phase. In addition, five of the six highest monthly flood peaks occurred in the autumn of the year following the onset of El Niño (autumn +) (Camilloni & Barros, 2003). These features could be used as indicators for early warnings of these floods. The major discharges in the Paraguay River share most of the Paraná relationships with the ENSO, but with less strength. For instance, some of the 20 largest discharges occurred also during the La Niña phase (Barros et al., 2004) HEAVY RAINFALLS Episodes of heavy rainfall are becoming more frequent. The frequency of heavy daily precipitation events exceeding thresholds over 50 mm increased over central and eastern Argentina. For example, the annual frequency of precipitation events over 150 mm in 48 h increased by a factor of four in most of northeastern Argentina, but similar trends were also observed as far away as in São Paulo in Brazil and in the Buenos Aires province in Argentina. Positive trends in heavy rainfall frequency have been observed in other regions of the world, and are consistent with what is expected under global warming conditions (IPCC, 2001). In certain cases, these events caused severe life and property losses, increasing the social vulnerability. In Argentina, the positive trend in heavy precipitation events started in the 1970s and was very rapid. Thus, much of the hydraulic infrastructure has become obsolete as it was built under the assumption of return periods for extreme events corresponding to past climate statistics, which no longer represents present conditions.

6 6 Vicente Barros et al. FUTURE CLIMATE SCENARIOS Global climate models (GCM) using socio-economic scenarios as input for estimating the emissions of greenhouse gases are the most common tools for assessing future climate scenarios. In South America, a few studies have made evaluations of GCMs that included the Plata basin (Hofstadter & Bidegain, 1997; Carril et al., 1997). The models that best simulate the current regional climate in their control experiments are likely to better represent future regional climate. Therefore, Bidegain & Camilloni (2002) calculated the differences between the observed precipitation and GCM simulations of the available model outputs at the Data Distribution Centre web page of IPCC. The observed data considered for the evaluation of the GCM outputs were the monthly averaged rainfall from the data set assembled by Willmott & Matsura (2001). The error field has the same general features in the HADMC3, ECHAM4/OPYC3, CSIRO-Mk2 and GFLD-R30 experiments, namely, overestimation of rainfall in the west near the Andes, and underestimation in the east of southeastern South America. In the case of the Plata basin, the best and more reliable models underestimate present mean precipitation by more than 35% (Fig. 4). There are many possible reasons for this underestimation shared by all models, one of the more likely being the low resolution of the GCMs. However, there are other possible causes, such as the lack of skill of atmospheric models to simulate mesoconvective systems, which in the warm part of the year are responsible for a considerable proportion of the precipitation in the Plata basin. Other possible causes could be the underestimation by the GCM of the frequency and intensity of the transient perturbations (Hewitson, 2003), and in most of the cases, the lack of an adequate simulation of the ENSO cycle. Under the assumption that model errors will be the same in present climate simulation and in future scenarios, we can assess the incremental change. This method is a valuable tool to figure out future climate scenarios when differences between -15 HADCM3 Annual rainfall differences ECHAM4/OPYC3 Annual rainfall differences Fig. 4 Annual rainfall differences between observed data and model outputs (mm day -1 ).

7 Potential impacts of climate change in the Plata basin Fig. 5 Annual precipitation difference (mm day -1 ) between the 2050s (decade) and the baseline period ( ) for the HADCM3 model outputs. present climate and its simulation by a GCM are not too large. However, in the case of precipitation in the Plata basin, and due to the physical shortcomings described above, climate scenarios developed with this technique should be regarded with extreme caution. Even with this consideration in mind, it could be considered that the common features of models that best reproduce mean precipitation and circulation features over subtropical South America provide hints useful in the estimation of future precipitation scenarios. In addition, although present trends in precipitation do not necessarily persist in a climate change scenario forced by increasing greenhouse gases, the coincidence between present trends and projections in the next decades based on MCG outputs using the incremental method could add some confidence to these outputs. Figure 5 shows the increment in mean annual precipitation of the Hadley Centre, HADMC3 experiment between the periods and for the IPCC SRES-A2 socio-economic scenario. The respective ECHAM4/OPYC3 experiment has similar incremental features in the regional precipitation (not shown). Scenarios from both models show a similar negative trend over central Chile and a positive one over central Argentina. Both models maintain the general features of their incremental precipitation fields of the 2050s for the scenarios of the 2020s and 2080s, but with increased magnitude as time progresses, except in the case of the middle of the Plata basin where the HADMC3 experiment increments do not have a definite trend while ECHAM4 has a positive trend. Although the changes in both experiments are very small, less than 70 mm in most of the region, these could be due to the underestimation of rainfall by the models. Therefore, larger trends than those shown by the models cannot be precluded, and thus, a considerable uncertainty over future precipitation scenarios over the region still remains.

8 8 Vicente Barros et al. CONCLUDING REMARKS During the last century, southeastern South America had the largest positive trend in precipitation among all the sub-continental regions of the world. This trend has become even greater during the last 40 years, averaging 15% over the upper and middle parts of the Plata basin and forcing a 35% increase in the total discharge. Floods and short-term extreme rainfalls have become more frequent, augmenting the vulnerability to climate driven events. Given the importance of the water resources and climate for the region of the Plata basin, it is important to assess their future changes. For southern South America, south of 30 S, the HADMC3 and ECHAM4/OPYC3 experiments produce precipitation scenarios for the 21st century consistent between them and with the latest 40-year trends. However, this is not the case over most of the Plata basin, where uncertainty about future precipitation trends remains too great. To reduce this uncertainty, it is necessary to improve the skill of the GCM to reproduce the mechanisms that force precipitation in the region. In particular, it is necessary to develop knowledge about how the ENSO and other known forcings of the Plata basin climate will evolve in the future. Acknowledgements This work was supported by the IAI Project CRN 056 and the CONICET Project PIP REFERENCES Barros, V., Castañeda, E. & Doyle, M. (2000) Recent precipitation trends in Southern South America, east of the Andes: An indication of climate variability. In: Southern Hemisphere Paleo-and Neoclimates: Key Sites, Methods, Data and Model (ed. by P. Smolka & W. Volkheimer), Springer-Verlag, Berlin, Germany. Barros, V., Chamorro, L., Coronel, G. & Baez, J. (2004) The major discharge events in the Paraguay River: magnitudes, source regions, and climate forcings. J. Hydromet. 5, Berbery, H. & Barros, V. (2002) The hydrological cycle of the Plata basin in South America. J. Hydromet. 5, Bidegain, M. & Camilloni, I. (2002) Regional climate baselines scenarios for the Rio de la Plata basin. In: Assessments of Impacts and Adaptation to Climate Change in Multiple Regions and Sectors (AIACC) Rio de la Plata Workshop (Montevideo, Uruguay). (CD-ROM). AIACC, Washington, USA. Camilloni, I. & Barros, V. (2003) Extreme discharge events in the Paraná River and their climate forcing. J. Hydrol. 278, Carril, A., Menéndez, C. & Nuñez, M. (1997) Climate change scenarios over South American region: an intercomparison of coupled general atmosphere ocean circulation models. Int. J. Climatol. 17, Giorgi, F. (2003) Variability and trends of sub-continental scale surface climate in the twentieth century. Part I: Observations. Climate Dynamics 18, Hewitson, B. (2003) Developing perturbations for climate change impact assessments EOS 84, Hofstadter, R. & Bidegain, M.(1997) Performance of general circulation models in southeastern South America. Climate Res. 9, IPCC (Intergovernmental Panel on Climate Change) (2001) Climate Change 2001: The Scientific Basis, Ch. 2. Cambridge University Press, Cambridge, UK. Tucci, C. (2003) Climate Variability and soil use in the brazilian Plata Basin In: Clima e Recursos Hídricos do Brasil (ed. by C. Tucci & B. Braga), Brazilian Water Resources Association, Porto Alegre, Brazil. Willmott, C. J. & Matsura,. K. (2001) Global climate data. Monthly total precipitation. Time series