Trends in drought intensity and variability in the middle Ebro valley (NE of the Iberian peninsula) during the second half of the twentieth century

Here we analyse trends in drought magnitude in the middle Ebro valley, a semi-arid area of the Iberian Peninsula, between 1951 and 2000. A significant increase in the severity of drought was identified from 1951 to 2000, and principal components analysis revealed three general patterns of drought evolution. Trend analysis of these patterns indicated that trend is significant only in northern areas (p < 0.01). Trends in drought variability were also analysed; a positive trend was recorded between 1951 and 2000. However, the overall results show a high degree of spatial variability. We show that this variability is determined by several geographic/topographic factors, mainly the distance to the Mediterranean Sea and the Bay of Biscay, water bodies that regulate the origin and direction of air masses and flows. It should also be noted that spatial variability of drought was detected because we used a dense database. Our results indicate that at the sub-regional level, drought patterns should be studied using a large amount of empirical data, since spatial variability may be relevant.