La provincia de Buenos Aires, importante productora agropecuaria argentina, ha sido escenario de eventos hidrológicos extremos (sequías e inundaciones) de distinto grado de gravedad a lo largo del tiempo. Muchos de ellos ocasionaron catástrofes mientras que otros solo alteraron las condiciones normales de algunas áreas. Investigadores de distintas ramas científicas han estudiado estos eventos, principalmente en sus impactos económico-sociales, pero no desde una mirada que contemple las variaciones que experimenta el contenido de agua en el suelo.En este trabajo, primero, se analiza la evolución de eventos húmedos y secos aproximadamente desde el año 1600 (época colonial) hasta 2008. Por otro lado, se aplicó un modelo de balance de agua en el suelo utilizando datos diarios de precipitación y constants hidrológicas de los suelos (basados en datos obtenidos in situ); con lo que se obtuvo la distribución de los excesos y deficiencias de agua en el suelo, considerados como desencadenantes de los eventos hidrológicos extremos.Por último, se analizan la suma de los excesos y las deficiencias anuales de agua en el suelo desde 1969 a 2008, en 16 sectores determinados según sus cuencas y se exponen los resultados en mapas con su distribución espacio-temporal.
Soil water surplus and deficit occur frequently in Buenos Aires province in Argentina. This paper analyses the soil water surplus in a sub-area, the Salado River basin, in the period 1968-2008. This basin is divided in seven drainage areas, delimitated according to the National Water Resources. The series of soil water surplus data were adjusted by means of the theoretical normal cubic-root probability distribution, and the mean areal soil water surplus value of 300 mm was considered as a threshold above which floods can cause severe damage. An increase in the frequency of extreme events and in their tendency exists during the recent years, coherent with the increase of precipitation recorded in the region. The statistical significance of the results was assessed using the Mann Kendall and MAKESENS tests. The results showed a relevant temporal variability, but did not show significant tendencies.
This paper presents the soil water deficit and soil water surplus obtained from soil water balance in three drainage areas of Buenos Aires province for the period from 1971 to 2010. The soil water balance had been performed using the evapotranspiration formula of Penman-Monteith and considering the soil water constants: field capacity, soil water moisture, and soil wilting point for all the different types of soils of the region. The obtained soil water deficit and surplus are considered as triggers of extreme hydrologic events. Annual threshold values of 200 mm of soil water deficit and 300 mm of soil water surplus were considered for drought and flood, respectively. It was found that almost the 25% of the floods are severe and extreme while the 50% of droughts were of these intensities.
Mann-Kendall statistical test was performed, and significance trends at level 0.1 were found for drought and for two periods, one of twenty years (1991–2010) and the other of ten years (2001–2010). As a sample of the temporal evolution of both events and their trends, the results of one locality (Junin) were deeply analyzed.
Daily soil water balances are estimated for the period 1951-2006 in the Pampean flatlands. Daily soil water surplus is the major output with their annual trends, being analysed because of their harmful effects on crop's harvest and floods. Soil water surplus trends were analysed founding positive trends, although only two stations were statistically significant at probability level of 95%. Differing phases of ENSO -using the multivariate ENSO index (MEI) -have differing impacts on rain and soil moisture conditions. The probabilities of occurrence of predetermined soil water surplus and hence flood risk are calculated and mapped.
For environmental and economic conditions, precipitation can be considered as the most relevant climatic element. Its drought and wet periods are known to change the natural water supply, river discharges, and crop yields, as well as natural vegetation. Argentine agriculture was and surely will drive the national economy and the most important agriculture region is Pampa or pampean flatlands, mainly because it allows the rain-fed crops production. The grain production increased from 23 to 90 million tones during the period 1970-2010 by means of the best available technology application and this growth had two components: a) the soybean which has now a ratio of 6 to 1 with respect to corn and b) the increase of precipitation amounts during the last decades of XX century. There is a need to maintain agricultural sustainability and for that changes in production patterns would be considered. On the other hand, the pampean flatlands experienced a succession of extreme hydrological events related to precipitation. Droughts and floods were a constant, according ancient documents during the Spanish domain, the argentine government and the installation of meteorological stations near 1870.
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