Investigation of the trends in rainfall data in Slovakia, Portugal and Libya

One of the goals of the hydrological risk assessment is to reduce the impacts of droughts and floods. the objective of study is to investigate rainfall trends in climatic stations in Slovakia, Portugal and Libya. Annual and seasonal precipitation trends were detected by the Mann-kendall non-parametric statistical test. The annual rainfall series at the Portuguese and Libyan climatic stations show mostly downwards trends (decreasing rainfall) while the series at the Slovak climatic stations show upwards trends (increasing rainfall). The results are in accordance with the IPCC forecasts. They also indicate that the variability of extreme rainfall and the climate uncertainty are greater in recent times. keywords: precipitation; trend; Mann-kendall; climatic change. Introduction Observations show that changes are occurring in the amount, intensity, frequency and type of precipitation. These aspects of precipitation generally exhibit large natural variability, and El Niño and changes in atmospheric circulation patterns such as the North Atlantic Oscillation have a substantial influence. Pronounced long-term trends from 1900 to 2005 have been observed in precipitation amount in some places: significantly wetter in eastern north and South America, northern Europe and northern and central Asia, but drier in the Sahel, southern Africa, the Mediterranean and southern Asia. More precipitation now falls as rain rather than snow in northern regions. Widespread increases in heavy precipitation events have been observed, even in places where total amounts have decreased. These changes are associated with increased water vapour in the atmosphere arising from the warming of the world’s oceans, especially at lower latitudes. There are also increases in some regions in the occurrences of both droughts and floods (IPCC, 2007). In recent years, a number of studies have been published focusing on the evolution of precipitation, drought conditions and moisture availability in the Mediterranean during the 20th century (e.g., Paulig and Paeth, 2006; Lopez-Moreno et al., 2009; Sousa et al., 2011). El kenawy et al. (2009) investigated spatial variability of temperature trends over Libya in the second half of the 20th century. One of the most extensive analyses of the spatial and temporal patterns of precipitation in Portugal was developed by Portela et al. (2013), Santos et al. http://dx.doi. org/10.14195/978-989-96253-3-4_44 266 Capítulo 3.2: Riscos climáticos e hidRológicos (2010), Santos and Portela (2008). trends in precipitation in Libya were studied in Zeleňáková et al. (2013), Zeleňáková et al. (2014). The detection of trends in meteorological data, in particular rainfall is essential for the assessment of the impacts of climate variability and change on the water resources of a region. This paper deals with the research of monthly precipitation trends in climatic stations in Libya and Slovakia in different time series – annual, half year, seasons. The Mann-kendall test is example of non-parametric tests and was applied to detect trends and to assess the significance of the trends in the time series in presented paper. Material and methods Monthly rainfall data recorded at four stations in eastern Slovakia provided by Slovak Hydrometeorological Institute with length from 1980 to 2010 were collected for this study. In Portugal monthly rainfall data from 1910 to 2004 at four stations were used for statistical trend analysis. In Libya four stations and 40 years of data observation operated by Libya’s Meteorological Service from 1970 to 2010 were set up to study precipitation trends. The evaluation was done for the time period from November to October in Slovakia and Libya and from October to September in Portugal. We have selected four stations in each country exactly with the altitude around 400 m asl. to exclude the effect of altitude to precipitation trends. In Libya: Ghadames, Sabha, Al kufrah are situated in the desert (in the south) and Shahat is situated near the seaside. The Mann–kendall (Mk) test (kendall, 1975; Mann 1945) is a rank-based nonparametric test for assessing the significance of a trend, and has been widely used in hydroclimatological trend detection in many studies (Burn and elnur, 2002). the significance level is chosen as α = 0.05 and Zα/2 is the value of normal distribution function, in this case Zα/2 = 1.95996. Hypothesis H0 no trend is if (Z < Zα/2) and H1 there is a trend if Z > Zα/2. The magnitude of the trend was determined using Sen`s estimator (Sen, 1968). A positive value indicates an upward (increasing) trend and a negative value indicates a downward (decreasing) trend in the time series. Results The results of trend analysis for the precipitation data are discussed in the following. Results of precipitation analysis are presented for annual, half year, seasons in Table I for climatic stations in Slovakia, in Table II for stations in Portugal and in Table III for climatic stations in Libya. Two signs (++ or --) indicate statistical significance at 95% confidence level as per the Mann–Kendall test (+ for increasing and − for decreasing).