Agricultural Drought Indices - US Department of Agriculture

In Australia, an approach was developed that would capture a multitude of agricultural drought
assessment methods to assist drought declarations, especially drought exceptional circumstances.
This approach was pursued through the National Agricultural Monitoring System (NAMS) and
appeared to contain all the requirements that would deliver scientific objectivity into the agricultural
drought assessment process. Indeed, a science advisory group was included which would
oversee that the “best indices and production models in Australia” (whether for rainfed cereals,
pastures, or irrigated systems) were employed in order to help overcome disputes within the
scientific community and between various scientific institutions regarding drought status. It is not
known why NAMS has recently been discontinued.
Valuable assessments have been made in Australia that show that when various drought indices
are compared, especially for grazing enterprise purposes, in respect to rainfall, soil moisture,
pasture growth, animal liveweight, and generated income, they will generally all identify the major
droughts in the relevant regions of Australia (Stafford-Smith and McKeon 1998).
However, Stafford-Smith and McKeon (1998) point out these measures may well differ in respect
to ranking the more contentious droughts in which affected farmers may be very sensitive in
eligibility for associated “exceptional drought” assistance (severe droughts of a 1 in 20 to 25 year
occurrence). As individual grazing properties in Australia can be relatively large, these
assessments on drought status will be also “dependent on assumptions about soil types and
pasture condition which are quite specific to individual properties” (Stafford-Smith 2003).
On the other hand, more straightforward, easily calculated drought indices and measures that are
based on just rainfall or soil moisture and which can be calculated on a more universal manner
appear, unfortunately, to be less closely aligned with issues of farmer hardship in Australia and
maybe less relevant to application for assessment of exceptional drought assistance, to which
drought indices in Australia are inexorably applied.
Indeed, it has been noted that a single national measure (index) of exceptional and extreme
drought is “likely to create inequity between regions” (Stafford-Smith 2003). This is especially the
case of a country as large as Australia in that the timing and pattern of drought declarations is
likely to differ between regions and types of agricultural land use (Stafford- Smith 2003, White and
Walcott 2009).
In a similar manner to that employed by Keating and Meinke (1998) in respect to assessment of
agricultural drought for cereal cropping enterprises, Stafford-Smith and McKeon (1998) emphasize
the need for an integrated index for application but focused on pastoral enterprise drought
assessment. They applied 100-year pasture growth and animal liveweight-gain simulations (using
the GRASP simulation model for pasture production) in relation to the monitoring, assessment, and
declaration of droughts in Australia and recommended a form of an integrative index since
information on soil moisture and pasture growth noticeably give differing results from a rainfall
index alone (Stafford-Smith and McKeon 1998, White and Walcott 2009).
Utilization of “Common” Single Agricultural Drought Indices
To an Australian agronomist, drought tends to be defined as below-average rainfall that restricts
typical plant growth for agricultural production. On the other hand, measurements of amounts of
rainfall expressed as deciles are rarely questioned by drought relief agencies in terms of
recognition of meteorological drought, unless a dispute on the density of rainfall gauges is raised.
However, because of the widely varying nature of soils, crops, and pasture species, disputes may
be initiated by farmers or different assessment agencies (usually state agencies) with other
drought assessment agencies (usually federal agencies) over “which is the more reliable crop or
pasture model to apply as an index in agricultural drought assessment,” especially if state
governments provide their own scientific inputs, which may be at odds or even in competition for
notoriety with federally produced scientific inputs—such has been the nature of state-federal
politics in Australia and competition or even jealousy between scientific agencies and individuals.
77