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Discussion papers
https://doi.org/10.5194/nhess-2019-31
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/nhess-2019-31
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 20 Feb 2019

Research article | 20 Feb 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Natural Hazards and Earth System Sciences (NHESS).

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

Pauline Rivoire1, Yves Tramblay1, Luc Neppel1, Elke Hertig2, and Sergio M. Vicente-Serrano3 Pauline Rivoire et al.
  • 1HSM (Univ. Montpellier, CNRS, IRD), Montpellier, France
  • 2Institute of Geography, University of Augsburg, Germany
  • 3Instituto Pirenaico de Ecologia (IPE-CSIC), Campus de Aula Dei, Zaragoza, Spain

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 (ET0) for a large data base of 160 rain gauges covering large parts of the Mediterranean basin. Results indicated positives trends in ET0 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 ET0. 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 ET0. 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.

Pauline Rivoire et al.
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Short summary
In order to define a dry period, a threshold for wet days is usually considered to account for measurement errors and evaporation. In the present study we compare the threshold of 1 mm / day, the most commonly used, to a time varying threshold describing evapotranspiration to compare how the risk of extreme dry spells is estimated with both thresholds. Results indicate that considering a fixed threshold can underestimate extreme dry spells during the extended summer.
In order to define a dry period, a threshold for wet days is usually considered to account for...
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