3.3 Relationships with climate
To estimate the relationships between dry spells and climatic drivers,
namely precipitation and evapotranspiration, the correlation between the
annual and seasonal sum of zero flow days and the maximum length of
zero-flow days with the Standardized Precipitation-Evapotranspiration
Index (SPEI, Vicente-Serrano et al., 2010) was analyzed with the
Spearman correlation coefficient (rho). The SPEI uses the monthly
difference between precipitation and potential evapotranspiration; thus
it represents a simple climatic water balance, which can be calculated
at different time scales similarly to the Standardized Precipitation
Index (SPI, McKee et al., 1993). The SPEI with 6, 12, 18 and 24 months
aggregation time have been downloaded from the CSIC Global SPEI database
( https://spei.csic.es/database.html ). The SPEI values were
computed using monthly sum of precipitation and potential
evapotranspiration at 0.5 degrees spatial resolution and a monthly time
resolution from the Climatic Research Unit (CRU) of the University of
East Anglia. Version 3.23 of the CRU dataset has been used to compute
the SPEI. The SPEI computation is based on the FAO-56 Penman-Monteith
estimation of potential evapotranspiration. The details of the SPEI
computation were presented by Beguería et al. (2014). For each station,
the value was extracted from the SPEI grid cell covering the station,
since the size of the basins considered is small (<2000km²)
compared to the CRU mesh (approximately 2500km²) used as a basis for the
calculation of the SPEI.
In addition, different climate indices describing large scale
atmospheric circulation patterns have been selected: the North Atlantic
Oscillation (NAO), the Atlantic Multi-decadal Oscillation (AMO), the
Mediterranean Index (MOI), the East Atlantic Western Russia (EAWR), the
Pacific Decadal Oscillation (PDO), and the Scandinavian Index (SCAND).
The time series for these indices have been retrieved from the CPC
database available online at:https://www.cpc.ncep.noaa.gov.
The annual, winter and summer mean values were used. For NAO, the
seasonal three-month DJF, MAM, JJA, and SON values were also included to
account for its within-year variability.