Impact of the dry day definition on Mediterranean extreme dry spells analysis

Abstract. To define a dry day, the most common approach is to identify a fixed threshold below which precipitation is considered equivalent to zero. This fixed threshold is usually set to account for measurements errors and also for precipitation losses due to the atmospheric evaporation demand. Yet, this threshold could vary in time according to the seasonal cycle but also in the context of long-term trends such as the increase of temperature due to climate change. In this study, we compare extreme dry spells defined either with a fixed threshold for a dry day (1 mm) or with a time-varying threshold estimated from reference evapotranspiration (ET 0 ) for a large data base of 160 rain gauges covering large parts of the Mediterranean basin. Results indicated positives trends in ET 0 in particular during summer months (June, July and August). However, these trends do not imply longer dry spells since the daily precipitation intensities remains 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 ET 0 . However, during summer, the extreme dry spell durations estimated with a 1 mm threshold are strongly underestimated by comparison with extreme dry spells computed with ET 0 . We stress the need to account for the atmospheric evaporative demand instead of using fixed thresholds to define a dry day when analyzing dry spells, in particular with respect to agricultural impacts.