Floods Increasing in Buenos Aires Salado River Basin González, Marcela Hebe and Fernández, Adriana Elsa Sea and Atmospheric Re

Floods increasing in Buenos Aires Salado River Basin González, Marcela Hebe and Fernández, Adriana Elsa Sea and Atmospheric Research Centre - CONICET Department of Atmospheric and Oceanic Sciences - University of Buenos Aires 54-011-4787-2693 [email protected] Publicado en “Environmental Change and rational water use” Eds. Scarpati y Jones, 96-113. Orientación Gráfica editora, junio 2007. Abstract The objective of this paper is to analyse the water excess in the soils during the last twenty years in order to improve the knowledge of the behaviour of the principal hydrological variables in the Salado River basin. In this region, rainfall and soil water storage have to be monitored in order to minimise the negative impacts of the intense floods that have been taking place more frequently since 1980. Rainfall, water balance and the runoff mean annual cycle during the period 1980-2000, using the Thornwaite and Matter method (1955), were analysed, detecting maximum rainfall in January and minimum in August. However, soil water storage reaches a peak in winter in a great part of Buenos Aires Province (Argentina). An example of a relevant flood event is shown in the paper. The flood in 2000, which affected the northwest of Buenos Aires and lasted from May to December, was used to compare the observed state of soils with the results derived from some hypothetical experiences. The main conclusion is that rainfall greater than normal in autumn causes important runoff and the area will probably be flooded the rest of the year because of small evaporation even though rainfall decreases in winter. Keywords Floods – water – rainfall – soils - runoff 1 1. Introduction The Salado River is located in Buenos Aires province, in the east of Argentina and runs into the Atlantic Ocean (fig 1a). It is an almost flat region with a mean gradient of 0.25 m/km from west to east, located in an extended region known as the Pampas Plain. The maximum altitude of the plain is only 108m above sea level. The steeper slope is in the north-western part of the province being 3.5m/km from northwest to southeast. Rainfall, consequently, spreads easily over lower areas. On the east side, the gradient decreases considerably. There is an area with a mean gradient of 1.1m/km where rainfall does not run freely. Finally, there is a large region with gradients under 0.35m/km, where it is very difficult for rainfall to runoff. This area also receives the water flowing from the west. There, the vertical transference (evaporation-infiltration) is greater than horizontal running making floods probable. There are only two mountain systems: Tandilia and Ventania. The former, in the central part of the province, is 300 km long and its highest peak is 500m high. The latter is 80km long, its highest peak is 1200m high and it is located in the central south of Buenos Aires from northwest to southeast. There is plenty of humus in the soil of the Pampas Plain. It is composed of animal and crop detritus. Known as “black soil”, it has great water retention capacity and is good for growing of excellent crops. It is, therefore, considered one of the richest regions in Argentina. Most of the wheat, corn and soybean production comes from this area and it is, of course, important for the economy of a country where agricultural activities are one of the major economic resources. There are many rivers and streams in the plain; some of them end in the Paraná River and others in de la Plata River. The Salado is the most important river in the area, it starts in the south of Santa Fe province, runs along 700km and ends in the Samborombón bay, in the Atlantic Ocean. Its principal tributary rivers are: Las Flores, Vallimanca and Saladillo streams and its basin is 91505 km2. An important characteristic of the basin is that it is more densely populated and land use more intense in the high basin, while the lower basin is used more extensively and thus less contaminated towards the Atlantic Ocean. 2 When the gradient becomes very flat, the river speed diminishes and more sediment settles on the river bed. The climate of the region is temperate with yearly rainfall increasing from southwest to northeast and raining more in summer (Gonzalez and Barros, 1996, 1998). Alternatively wet and dry periods are observed, as Ameghino (1954) reported more than a hundred years ago; for example, the wet period beginning in 1860 and ending in 1920 and the present one which began in 1970 and still ongoing. Although short humid periods have been appearing since 1900, the situation seems to be changing in recent times and heavy floods are detected. During the last forty years, rainfall has increased an average of 20% in the plain, yearly rainfall has risen from 600mm to more than 1000mm in the west of Buenos Aires and the agronomic limits have shifted about 200 km towards the west (Castañeda and Barros, 1994; Minetti and Vargas, 1998; Minetti et al, 2001) and associated changes in circulation were detected (Vargas et al, 1995). Some authors have studied the occurrence of wet and dry periods in the Pampas Plain. For example, Vargas (1979) evaluated the humidity excess and deficits over the period 1906-1973 in this region to make mean field of the principal hydrological variables. Vargas and Alessandro (1990) studied the rainfall and temperature extremes in Corrientes and Minetti and Sierra (1989) did the same in Cuyo. Krepper et al (1987) studied the rainfall pattern in wet and dry years in Buenos Aires Province. The term “floods” refers to extended inundations, longer than a month, when soils remain saturated of water. Floods frequency has considerably increased during the last twenty years in Buenos Aires province. Penalba and Vargas (2001) observed that positive rainfall anomalies tend to concentrate since 1960 when they studied rainfall during the last century. (6) Wet conditions with some flooded areas occurred in the north-west of Buenos Aires Province in 2000, 1999 and 1997, over the Salado river basin in 1996, 1995, 1993 and 1985, in the west of the province in 1991-1992, in the central part of the Salado River basin in 1991, in the central-south of Buenos Aires in 1982 and in the north of the province in 1986 and 1987. When these floods occur, roads are covered with water, some towns are isolated and of course, thousands of animals die and crops are damaged. Floods during wet periods form temporary lagoons and as a result the ground water rises. The province authorities then built draining canals to reduce 3 accumulated water and to lower the water table. Besides, in the Pampas Plain there is more water stored in the soil than usual when an ENSO (El Niño Southern Oscillation) is in its mature phase (Forte Lay and Spescha, 2001) and rainfall is already affected (Grimm et al, 2000). Probably these facts add to the magnitude of floods in the Salado basin but the study of all of them is beyond the scope of this paper. The problem of the increase of flood frequency in the Salado basin requires a quick and definitive solution because of the economic importance and the population density of this area. As the last and most important wet period began in 1970 approximately and continues, there is a great social need for improving the knowledge of flood development, but data are still scarce. As a matter of fact, it is very difficult to determine the most convenient structural and non-structural decisions in order to minimise the negative impact of this climate variability effect. This paper is a first approach to understand the situation in the Salado basin during the last twenty years of the twentieth century. The principal objectives are to analyse mean rainfall and water balance in Buenos Aires province and to study the principal features of the flood occurred in 2000. Therefore some experiences are supposed in order to confirm that the flood risk increases when positive rainfall anomalies take place in autumn. The data and methodology will be described in section 2. The mean evolution of precipitation and water balance will be discussed and the analysis of relevant flood in 2000 is presented in section 3. 2. Materials and Methods Monthly rainfall and temperature from 27 selected stations (fig.1b) from the National Meteorological Service (SMN) gauge, during the period 1980-2000 were used to carry out this study. The station data were considered if the record had only few missing data. In some cases, missing data could be completed using an homogeneous station and the rates traditional methodology (World Meteorological Organisation, 1990). 4 To investigate the economic and social impact of flood occurred in 2000, a detailed search was carried out, especially from local newspapers and magazines. Informative news generated by institutions like National Agropecuary Technology Institute (INTA) or National Water Institute (INA) WebPages and Climatic Bulletins from SMN, were used to enlarge the knowledge of the situation. Mean monthly temperature and monthly-accumulated rainfall during the period 1980-2000 were calculated in order to evaluate the mean water balance. It consisted in the monthly evolution of the water storage in the soil and the deficit or excess of water, using the methodology proposed by Thornwaite and Matter (1955) (TM, hereinafter). The mean field capacity of soils in the Pampas Plain used to perform the study was taken as 200mm (Lamas, 2005) for the entire region. This is an adequate value for the kind of soil of that plain. Following TM (1955) methodology, potential evapoperspiration was first calculated for each month using the mean temperature and sunlight period.

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