摘要：To define a dry day, the most common approach is to identify a fixedthreshold below which precipitation is considered equivalent to zero. Thisfixed threshold is usually set to account for measurement errors andprecipitation losses due to the atmospheric evaporation demand. Yet,this threshold could vary in time according to the seasonal cycle andin the context of long-term trends, such as the increase in temperature dueto climate change. In this study, we compare extreme dry spells, definedeither with a fixed threshold for a dry day (1 mm) or with a time-varyingthreshold estimated from reference evapotranspiration (ET0), for a large database of 160 rain gauges covering large parts of the Mediterranean basin. Results indicated positive trends in ET0 during summer months (June, July and August) in particular. However, these trends do not imply longer dry spells since the daily precipitation intensities remain higher than the increase in the evaporative demand. Results also indicated a seasonal behavior: in winter the distribution of extreme dry spells is similar when considering a fixed threshold (1 mm) or a time-varying threshold defined with ET0. However, during summer, the extreme dry-spell durations estimated with a 1 mm threshold are strongly underestimated in comparison to extreme dry spells computed with ET0. We stress the need to account for the atmospheric evaporative demand instead of using fixed thresholds for defining a dry day when analyzing dry spells, with respect to agricultural impacts in particular.
