A revision of the Combined Drought Indicator (CDI) used in the European Drought Observatory (EDO)

In: Natural hazards and earth system sciences: NHESS, Band 21, Heft 2, S. 481-495

Abstract

Abstract. Building on almost 10 years of expertise and operational application of the
Combined Drought Indicator (CDI), which is implemented within the European
Commission's European Drought Observatory (EDO) for the purposes of early
warning and monitoring of agricultural droughts in Europe, this paper
proposes a revised version of the index. The CDI conceptualizes drought as a
cascade process, where a precipitation shortage (WATCH stage) develops into
a soil water deficit (WARNING stage), which in turn leads to stress for
vegetation (ALERT stage). The main goal of the revised CDI proposed here is
to improve the indicator's performance for those events that are currently
not reliably represented, without altering either the modelling conceptual
framework or the required input datasets. This is achieved by means of two
main modifications: (a) use of the previously occurring CDI value to improve
the temporal consistency of the time series and (b) introduction of two
temporary classes – namely TEMPORARY RECOVERY for soil moisture and
vegetation greenness, respectively – to avoid brief discontinuities in a
stage. The efficacy of the modifications is tested by comparing the
performances of the revised and currently implemented versions of the
indicator for actual drought events in Europe during the last 20 years. The
revised CDI reliably reproduces the evolution of major droughts,
outperforming the current version of the indicator, especially for
long-lasting events, and reducing the overall temporal inconsistencies in
stage sequencing of about 70 %. Since the revised CDI does not need
supplementary input datasets, it is suitable for operational implementation
within the EDO drought monitoring system.