Trends in extreme precipitation indices derived from a daily rainfall database for the South of Portugal

Abstract

ABSTRACT:The rainfall regime of the South of Portugal is Mediterranean with Atlantic influence. Long-term series of reliable precipitation records are essential for land and water resources management, climate-change monitoring, modelling of erosion and run-off, among other applications for ecosystem and hydrological impact modelling. This study provides a qualitative classification of 106 daily rainfall series from stations located in the South of Portugal and evaluates temporal patterns in extreme precipitation by calculating a number of indicators at stations with homogeneous data within the 1955/1999 period. The methodology includes both absolute and relative approaches and a new homogeneity testing procedure, besides the application of other statistical tests. The proposed technique is an extension of the Ellipse test that takes into account the contemporaneous relationship between several candidate series from the same climatic area (SUR+Ellipse test). The results indicate that this technique is a valuable tool for the detection of non-climatic irregularities in climate time series if the station network is dense enough. The existence of trends and other temporal patterns in extreme precipitation indices was investigated and uncertainty about rainfall patterns evolution was assessed. Three indices describing wet events and another three indicators characterizing dry conditions were analysed through regression models and smoothing techniques. The simple aridity intensity index (AII) reflects increases in the magnitude of dryness. Especially pronounced trends are found over most of southern Portugal in the 1955/1999 period, highlighting the fact that large areas are threatened by drought and desertification. The trend signals of the wetness indices are not significant at the majority of stations, but there is evidence of increasing short-term precipitation intensity over the region during the last three decades of the twentieth century. Finally, the results also indicate that extreme precipitation variability and climate uncertainty are greater in recent times.

Daily precipitation concentration across Europe 1971–2010

“…Klein-Tank and Konnen (2003) found that there has been a general increase in annual extreme precipitation across Europe over the period 1946-1999, while Haylock and Goodess (2004 showed that the trends of the number of days above the 1961-1990 90th percentile of wet-day amounts have different signs, due to the heterogeneous nature of the region. In southern Europe, mostly across the Mediterranean basin, it has been suggested that the heaviest rainfall events, or the contribution of heaviest events to the annual total, have increased in western areas (Brunetti et al, 2000(Brunetti et al, , 2001(Brunetti et al, , 2004Alpert et al, 2002;Gonzalez-Hidalgo et al, 2003;Costa and Soares, 2009), but this result is strongly dependent on the period examined Brugnara et al, 2012); on the contrary, in the eastern areas, no significant trend has been detected in the number of intense or extreme events (Zang et al, 2009;Kioutsioukis et al, 2010).…”

Changes in the Intensity and Variability of Precipitation in the Central Region of Argentina between 1960 and 2012

“…Costa and Soares (20009) [11] confirmed that using absolute approaches without metadata or complementary information makes it difficult to determine if changes in a station's time series result from inhomogeneities or simply from abrupt changes in the regional climate.…”

“…Different methods have been introduced to test the homogeneity of hydroclimatic variables [11,25,28]. In this study, two methods were applied.…”

“…[25] used a set of four tests to evaluate homogeneity in daily temperature and precipitation series, while [11] developed an integral procedure to classify the homogeneity of extreme rainfall indicators, which includes both absolute and relative procedures (i.e., with respect to another station in the network).…”

“…Detecting a lack of homogeneity in climatic series, it is important to identify spurious variability and thus establish a standard on the quality of the information being analyzed [11]. Based on different homogeneity test results, Costa and Soares (2009) [11] in Portugal proposed a procedure to select only those rainfall time-series suitable for studying their temporal trend.…”