Trend analysis in Icelandic discharge, temperature and precipitation series by parametric methods Jo ´ na Finndı ´s Jo ´ nsdo ´ ttir, Cintia B. Uvo and Robin T. Clarke ABSTRACT Jo ´ na Finndı ´s Jo ´ nsdo ´ ttir (corresponding author) Hydrological Service, National Energy Authority, Grensa ´ svegur 9 108 Reykyavik, Iceland Tel.: +354 569 6066 Fax: +354 568 8896 E-mail: jonafinndis@gmail.com Jo ´ na Finndı ´s Jo ´ nsdo ´ ttir Cintia B. Uvo Department of Water Resources Engineering, LTH, Box 118, 22100 Lund, Sweden Robin T. Clarke Instituto De Pesquisas Hidra ´ ulicas -UFRGS, Porto Alegre, RS Brazil This paper presents results of analyses by parametric methods of annual means of temperature, precipitation and discharge, and of seasonal maximum precipitation at 17, 28 and 10 Icelandic stations, respectively, for the period 1961–2000. Trends in mean seasonal temperature and precipitation are in broad agreement with results found by other authors using other methods. A positive trend appears in both mean annual temperature and mean temperatures in most seasons. Annual mean precipitation trends are positive in most seasons except for negative trends in the September–November season in the south. Additionally, positive trends appear in maximum one-, three- and five-day precipitation, both during the spring and autumn, except at a group of stations in central Iceland. Some of the positive trends in mean annual and seasonal precipitation may, however, be attributed to the positive trend in temperature which may have influenced gauge catch. Trends in mean annual and seasonal discharge are small and statistically insignificant; the trends found in temperature and precipitation do not all relate directly to trends in discharge but suggest hypotheses for further study of the relationships between them. Key words | discharge, Iceland, parametric methods, precipitation, temperature, trend INTRODUCTION This paper presents results of analyses to detect trends in Icelandic records of precipitation, temperature and dis- charge. These results complement those reported earlier by Jo ´ nsdo ´ ttir et al. (2006) who looked for trends in seasonal and annual mean temperatures, in seasonal and annual precipitation totals, and in seasonal maximum discharges and their times of occurrence. The present paper differs from their work in that (a) the analyses for trends in the present paper use parametric methods, instead of the non- parametric tests used by Jo ´ nsdo ´ ttir et al. (2006) and (b) additional results are given for trends in annual and seasonal precipitation extremes of one-, three- and five-day durations. The period taken for analysis was the period 1961 – 2000, and the variables analysed were temperature, precipitation and discharges at both annual and seasonal timescales, using four three-month seasons. Times and magnitudes of seasonal flood occurrences were also analysed using two within-year periods: namely spring (1 March to 16 July) and autumn (17 July to 30 November). Additionally, trends in spring and autumn mean tempera- ture and maximum precipitation were explored as part of an attempt to relate them to trends in spring and autumn floods, in terms of both the timing of maximum daily discharge and its magnitude. The study by Jo ´ nsdo ´ ttir et al. (2006), which reviewed earlier studies of trends in Icelandic meteorological and discharge series, reported trends in temperature, precipi- tation and discharge for the periods 1941–2002 and 1961–2000, with a positive trend in mean annual tempera- ture for the period 1961 – 2000 but a slightly negative trend during 1941 – 2002. They found trends in both annual and seasonal (three-monthly) precipitation for both periods but concluded that the trends in precipitation might be at least partially explained by precipitation gauge losses, which may doi: 10.2166/nh.2008.002 425 Q IWA Publishing 2008 Hydrology Research | 39.5–6 | 2008 Downloaded from http://iwaponline.com/hr/article-pdf/39/5-6/425/364567/425.pdf by guest on 03 June 2022