Agricultural Drought Indices - US Department of Agriculture

very nature of the requirements of drought relief assessment policy and implementation in
Australia and the need for very detailed and specific assessments depending on the commodity in
question and the region in question.
However, it is also noted that for certain specific purposes, such as, for example, climate change
studies, standard agricultural drought indices hold promise as an additional measure that can
assist analyses and understanding of extreme drought likelihood in a country with such extreme
variability in climate, soils, water holding capacity, and farming systems as Australia.
The Value of Standard Agricultural Drought Indices, Especially the PDSI, in Australia in
Application for Drought and Climate Change Research
Standard, published agricultural drought indices have been identified as being potentially suitable
for drought assessment in Australia. The PDSI, developed by Palmer (1965) (also see Alley 1984)
to provide information on the “cumulative departure of moisture supply” from normal, may have
potential for application in Australia, despite the well-known high levels of interannual climate
variability in this country, an issue most often quoted as being counter-productive in the utilization
of the PDSI. Sivakumar and Wilhite (2002) note “the Palmer Index” is more effective in
determining long-term drought rather than shorter-term events. Nevertheless, they point out that
the PDSI “is popular” in many world regions, especially the United States, where it is most effective
in measuring agricultural impacts sensitive to soil moisture (Willeke et al. 1994).
As noted by Burke et al. (2006), the potential evaporation required as input to the PDSI was
calculated using the Penman-Monteith equation instead of the Thornthwaite (1948) equation.
Analysis shows that the PDSI has a memory of ~12 months, resulting in effective use of this index
in many world regions. It is also noteworthy that this index has such wide application in a country
such as the United States, where modified values of the PDSI appear regularly in the Weekly
Weather and Crop Bulletin, suggesting considerable value and application for policy makers,
regulators, and general users from such an index if it were to prove an effective agricultural
drought index for a country such as Australia. A useful summary of the “Palmer Index” may be
obtained in Hayes (2006), who noted the problems in applying the PDSI in a region such as
Australia if the index is used for ongoing assessment and associated regulatory purposes.
Conversely, the PDSI has been successfully applied in Australia when used in a research
framework. For example, the relationships between severe drought and the El Niño-Southern
Oscillation (ENSO) have been effectively explored by Dai et al. (1998) through utilizing the PDSI
for analyses in regions and countries that include Australia. These analyses using the PDSI
suggest an increase in the combined percentage areas in severe drought—and also severe
moisture surplus—since the late 1970s.
In a further research application, it is noteworthy the PDSI has been effectively applied as an index
in estimates of future drought frequency when utilizing climate change scenarios for Australia and
elsewhere. Comparisons between PDSI and soil moisture (Sheffield et al. 2004) suggest that the
PDSI might also provide useful indication of future agricultural drought in many world regions
(Burke and Brown 2008). It has been noted that although there are uncertainties in these types of
regional drought projections because of some uncertainty in the distributions of precipitation, “it is
possible to show that there are major increases in potential evaporation and percent changes in
area associated with PDSI” (Burke and Brown 2008). Burke et al. (2006) and Burke and Brown
(2008) demonstrated that in utilizing climate change scenarios (Hadley Centre Model), the
Standardized Precipitation Index (SPI) showed little change in the proportion of land surface in
drought. However, agricultural drought indices “which included a measure of the atmospheric
demand for moisture” showed a significant increase in the proportion of land surface in future
drought. This was especially demonstrated to be the case where the PDSI was employed and
which could provide information such as potential evaporation in Australia (Dai et al. 2004, Burke
and Brown 2008).
In a similar vein, Hobbins et al. (2008) utilized the simple water balance model underpinning the
PDSI together with other attributes of the PDSI to estimate likely ecohydrologic impacts of climate
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