Detection and attribution of trends in magnitude, frequency and timing of floods in Spain

An analysis to detect trends in magnitude, frequency and timing of floods was conducted in Spain through nine flood indicators. A data set of gauging stations where the effect of dam regulation on flow series is negligible was obtained. Annual maximum and peaks-over-threshold series were extracted in three periods: 1942–2009, 1949–2009 and 1959–2009. A pre-whitening procedure was applied to remove serial correlation and the Mann–Kendall test selected to detect trends. A general decreasing trend in magnitude and frequency of floods was detected in the three periods, with more notable evidence in 1959–2009. An increasing trend in the timing (i.e. towards later floods) was also found in the northwest of Spain. In addition, a study to relate detected flood trends to a set of potential drivers was also conducted. Most such trends in flood series could be explained by corresponding and increasing trends in evapotranspiration that increase water losses in soils and decrease soil moisture content before the occurrence of floods.