Thursday, 27 January 2011
Washington State Convention Center
Global mean temperatures began an upward trend from 1976, which could be attributed to increase in greenhouse gas concentrations in the atmosphere. In the future years a heating, due to anthropogenic greenhouse gas emissions, is very likely to happen. Higher atmospheric temperatures affect the hydrological cycle, leading, at global scale, to higher moisture content and an increased evapotranspiration. Regionally the relation between the different involved factors, thermodynamics and dynamics, is not so direct. This interaction leads to a geographically complex response of mean precipitation to global warming. For example, surface run-off depends strongly on the rainfall intensity and frequency. Therefore, changes in intense rainfall events rather than mean rainfall will impact more strongly on floods and also in soil erosion. By the other hand, drought and wind erosion processes enhance the effect of water erosion on soil. Within this complex problematic, the natural resources and specifically the water availability occupies an important place. The different human activities are highly dependent on water and exists a rising demand on surface water resources. Assess the temporal fluctuation of hydrological processes and most especially rainfall, which constitutes the main source of water input and estimation of the regional water balance, is of great importance. The region under study, southern South America, is part of the La Plata Basin. This region is a densely populated one where agriculture and hydrology have serious social and economic ramifications. The aim of this study is to examine the inter-annual, inter-decadal and low frequency variability of rainfall in different time scale jointly with the water balance in the soil, with special emphasis in extreme events. Daily rainfall and temperature data used in this study were provided by different national institutions or Weather Services, in the longest period 1930-2007. This paper analyzed three characteristic of hydrological cycle: rainfall; dry condition and deficit and surplus water. The definition of an extreme event will largely depend on the activity and region affected. In the particular case of extreme rainfall events, their definition depends, moreover, on the nature of the rainfall in the region under study. In this research, we define extreme event when surpass a defined threshold (tails of the distribution). To assess the different rainfall characteristic in the region, the following indices or variables are analyzed a) total monthly rainfall; b) percentage of extreme events of rain; c) percentage of daily intensity of extreme rainfall; d) lack of daily rainfall and e) percentage of months with deficit and surplus, estimated by water balance. The indices were calculated per each austral season: summer (December, January and February, DJF), autumn (March, April and May, MAM); winter (June, July and August, JJA), spring (September, October and November, SON), and the year as a whole. Different statistical methodologies were applied to analyze the temporal variability of the indices, such as a non-parametric Kendall-Tau test and an 11-year running mean with distributed weights. The results of the annual indices show a regional variability highly non-stationary. The most outstanding feature is the difference before and after the 1950s or 1960s. The interdecadal variability is particularly well defined in the west, with a jump or discontinuity around the mention decades. In the eastern zones, a gradual increase can be observed starting in the 1950s. The interdecadal and interannual variations affect the behavior of extreme rainfall in the annual scale and during the months with maximum rainfall in the region. In summer, autumn and spring, the region has a marked spatial coherence in the positive sign trends of rainfall indices. Winter is the exception, with negative trends. The temporal variability of lack of rainfall is in accordance with these results. In addition, this index identified the development and evolution of the most important dry and wet periods throughout the study region. The temporal analysis of this index was performed, accumulating the index during periods of 1, 2 and 5 years, by means of severity-duration plots. This analysis shows that an increase in the amount of dry days dominated much of the country during the recent years. The decadal analysis of deficit and surplus reveals the complexity of the different factors involved. Moreover, the results show the different hydrological behavior in the region. The most outstanding results were that 1960s and 2000s were the decades with greater amount of deficit for all seasons.
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