A diminuiçào da precipitaçào em épocas do año como indício de mudança climática: casos estudiados em Portugal continental

Abstract

Resumo:Como manifestação da mudança climática que se julga estar a ocorrer no território português, têm sido referidas a maior irregularidade da distribuição da precipitação ao longo do ano e a sua apreciável diminuição em algumas épocas do mesmo. Para constituir indício de mudança climática, tal diminuição deverá corresponder a uma tendência que se tenha vindo a acentuar ao longo dos anos. Com o objectivo de detectar variações com carácter tendencial em séries temporais da precipitação registadas no território nacional, aplicaram-se a tais séries dois métodos: o método clássico de médias móveis e um método, expressamente desenvolvido no âmbito do estudo que deu origem a este artigo, que utiliza a comparação, em termos estatísticos, das médias da precipitação em determinados intervalos de tempo em que se dividiram os períodos de registos. As aplicações efectuadas a séries de precipitação em onze postos do território nacional evidenciam que as precipitações no 2º Trimestre do ano hidrológico (de Janeiro a Março) e, de modo mais acentuado, no mês de Março, exibem uma tendência nítida de redução. Palavras-chave: mudança climática, diminuição da precipitação, persistência, média móvel, quebra de homogeneidade.

Trends in hydrologic time series

“…For each one of the first 11 rain gages of Figure 3, Table 1 allows comparing the averages of the rainfall series in the recording periods and in the last 15 years analysed by Portela and Quintela [6]. It shows that all the rain gages exhibit notorious decreases in the rainfall in March.…”

“…The moving average technique and the trend detection based on the identification of persistent non-homogeneities between consecutive pairs of anterior and posterior subsets were firstly applied to the rainfall series in the rain gages 1 to 11 schematically located in Figure 3 (Portela and Quintela [6] . The minimum length of each moving average was fixed at n=15 years which was also the minimum length of any anterior and posterior subset.…”

“…Statistical tools, as well as moving average techniques and specific procedures to detect non-homogeneities, were applied (Portela and Quintela [6]). Afterwards not only the rainfall series at a much larger number of rain gages were studied (Santos and Portela [11]), but also the performance of irrigation reservoirs under changing hydrologic constraints was analyzed (Portela et al [7] and Santos [10]).…”

Trends in rainfall and streamflow series: portuguese case studies

“…For Mainland Portugal and as far as the rainfall is concerned, although a decrease is generally pointed out, the results from different climate scenarios denote substantial differences, meaning that there is a considerable uncertainty regarding future projections [9,10]. However, at the annual level, it is almost consensual that the decrease over the full time span of records is not statistically signifi cant [2,3,[11][12][13][14][15][16][17], although there are some sub-periods with signifi cant trends (either increasing or decreasing) that often alternate over time. At the monthly level, a decreasing trend in the rainfall in March, statistically signifi cant at the 5% signifi cance level, is almost always mentioned [2,3,[11][12][13][14][15][16].…”

“…However, at the annual level, it is almost consensual that the decrease over the full time span of records is not statistically signifi cant [2,3,[11][12][13][14][15][16][17], although there are some sub-periods with signifi cant trends (either increasing or decreasing) that often alternate over time. At the monthly level, a decreasing trend in the rainfall in March, statistically signifi cant at the 5% signifi cance level, is almost always mentioned [2,3,[11][12][13][14][15][16]. For the period from 1976 to 2002, Boken et al [18] also identifi ed a signifi cant decrease in the precipitation during the spring season.…”