Droughts are among the costliest climate hazards with negative impacts
for agriculture, the environment, and society. There is potential for
worse droughts in the future due to changes in rainfall patterns and
increasing temperatures under a changing climate. The Queensland Future
Climate Science Program provides two indices and three metrics to
evaluate changes to future droughts and wetness.
What is a drought?
In comparison to other natural hazards, the onset and severity of a
drought are difficult to determine since droughts are characterised by a
gradual build-up. It can take many months or years for the full extent
and severity of a drought to become apparent. Definitions of a drought
can be based on 3 broad concepts of drought:
Meteorological droughts: relating to extended periods
of below average rainfall that is exacerbated through high
temperatures and evaporation.
Hydrological droughts: describing impacts on
streamflow and other water systems (e.g., reservoirs or lakes).
Agricultural droughts: focussing on agricultural
impacts, primarily through assessments of soil moisture.
What drought indices did we use?
The Queensland Future Climate Science Program focusses on meteorological
droughts and wetness and presents information based on two of the more
common meteorological drought indices, namely:
Standardized Precipitation Index (SPI; McKee et al., 1993)
Standardized Precipitation Evapotranspiration Index (SPEI;
Vicente-Serrano et al., 2010)
SPI is a rainfall-based index, calculated from accumulated rainfall over
a set number of months. SPEI is an extension of SPI, and also includes
the impacts of increased temperatures through
potential-evapotranspiration (PET). Rather than being calculated from
accumulated rainfall, SPEI is calculated from the difference between
rainfall and PET. It has been argued that SPEI better reflects changes
to the overall water budget by considering the effects of changing
atmospheric evaporative demand. The Queensland Future Climate Science
Program adopts PET derived from the Penman-Monteith equation (Allen et
al., 1998).
Both SPI and SPEI were calculated using an accumulation period of
12-months, as this was considered the timeframe most likely to have
impacts on various hydrological and agricultural systems. Drought and
wetness events are split into three different categories according to
the severity (moderate, severe or extreme) as shown in Table 1:
Table 1. SPI and SPEI classification table following
McKee et al. (1993)
SPI/SPEI values
Categories
Probability of event (%)
SPI/SPEI ≥ 2.0
Extreme wet
2.3%
1.5 < SPI/SPEI < 2.0
Severe wet
4.4%
1.0 < SPI/SPEI < 1.5
Moderate wet
9.2%
-1.0 < SPI/SPEI < 1.0
Near normal
68.2%
-1.5 < SPI/SPEI < -1.0
Moderate dry
9.2%
-2.0 < SPI/SPEI < -1.5
Severe dry
4.4%
SPI/SPEI ≤ -2
Extreme dry
2.3%
Metrics relating to the frequency of events, duration of events, and
percent time in drought are calculated for each of the drought and
wetness categories (extreme, severe, and moderate). These metrics show
how drought and wetness characteristics will likely change in the
future. A description of these metrics is presented below and an
illustrated example provided in Figure 1.
Frequency of events is the total number of events recorded over a
given period.
Duration of events is the average duration of recorded drought events
(in months).
Percent time in drought/wetness is the total time spent in a category
of drought/wetness divided by the total time.
Figure 1. Pictorial representation of categories of drought and
wetness over a hypothetical SPI time-series, highlighting the
duration and frequency of extreme drought events.
In addition to the three metrics above, timeseries of the extent of
territory in drought or wetness (as a percentage) were also calculated
for the diffferent severity categories.
Droughts and wetness on Queensland Future Climate
The Queensland Future Climate Science Program has used state-of-the-art
high-resolution downscaled climate simulations from both
CMIP5 and CMIP6 global
climate models to assess the impact of climate change on droughts and
wetness. The CMIP5 information is provided using
SPI only, while metrics based on both SPI and SPEI are available for
CMIP6.
The main tools for viewing climate projections data on the Queensland
Future Climate website are the
Queensland Future Climate Dashboard
and
Regional Explorer. Please refer to the
user guide
for detailed information on how to access and interpret information
available from these resources.
Examples of the drought information available on the Queensland Future
Climate Dashboard and Regional Explorer are shown below.
Sample map and plots from the Queensland Future Climate Dashboard
(CMIP6) showing the projected change in the
percent time in severe drought (based on SPEI) for the Central
Highlands Regional Council area in 2070 under a high emissions
scenario (SSP3-7.0). The drought information can also be downloaded
as tables or shapefiles to enable further analyses.A sample timeseries chart from the Regional Explorer (CMIP6) showing the projected extent of territory in the 3 categories of
drought (moderate, severe and extreme) for the Central Highlands
Regional Council area under a high emissions scenario (SSP3-7.0).
References
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (1998).
Crop evapotranspiration-Guidelines for computing crop water
requirements-FAO Irrigation and drainage paper 56
(FAO, Rome, Vol. 300, p. D05109).
McKee, T. B., Doesken, N. J., & Kleist, J. (1993). The Relationship
of Drought Frequency and Duration to Time Scales.
Proceedings of the 8th Conference on Applied Climatology, 17.
Vicente-Serrano, S. M., Beguería, S., & López-Moreno,
J. I. (2010). A multiscalar drought index sensitive to global warming:
The standardized precipitation evapotranspiration index.
Journal of Climate, 23(7), 1696-1718.
https://doi.org/10.1175/2009JCLI2909.1.
