Jefferson County - East-West Gateway Coordinating Council

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Jefferson County – Section 2
less well understood by farmers, some farmers in the semiarid Australian climate claimed
drought assistance every few years.
Operational definitions help define the onset, severity, and end of droughts. No single
operational definition of drought works in all circumstances, and this is a big part of why
policy makers, resource planners, and others have more trouble recognizing and planning
for drought than they do for other disasters. In fact, most drought planners now rely on
mathematic indices to decide when to start implementing water conservation or drought
response measures.
In the early 1980s, research by Donald A. Wilhite, director of the National Drought
Mitigation Center, and Michael H. Glantz, of the National Center for Atmospheric
Research, uncovered more than 150 published definitions of drought. The definitions
reflect differences in regions, needs, and disciplinary approaches. Wilhite and Glantz
categorized the definitions in terms of four basic approaches to measuring drought:
meteorological, hydrological, agricultural, and socioeconomic. The first three approaches
deal with ways to measure drought as a physical phenomenon. The last approach to
measuring drought deals with drought in terms of supply and demand, tracking the effects
of water shortfall as it ripples through socioeconomic systems.
Meteorological drought is usually an expression of precipitation’s departure from normal
over some period of time. These definitions are usually region-specific, and presumably
based on a thorough understanding of regional climatology. Meteorological measurements
are the first indicators of drought.
Agricultural drought occurs when there is not enough soil moisture to meet the needs of a
particular crop at a particular time. Agricultural drought happens after meteorological
drought but before hydrological drought. Agriculture is usually the first economic sector to
be affected by drought. Agricultural drought links various characteristics of meteorological
(or hydrological) drought to agricultural impacts, focusing on precipitation shortages,
differences between actual and potential evapotranspiration, soil water deficits, reduced
ground water or reservoir levels, and so forth. Plant water demand depends on prevailing
weather conditions, biological characteristics of the specific plant, its stage of growth, and
the physical and biological properties of the soil. A good definition of agricultural drought
should be able to account for the variable susceptibility of crops during different stages of
crop development, from emergence to maturity. Deficient topsoil moisture at planting may
hinder germination, leading to low plant populations per hectare and a reduction of final
yield. However, if topsoil moisture is sufficient for early growth requirements, deficiencies
in subsoil moisture at this early stage may not affect final yield if subsoil moisture is
replenished as the growing season progresses or if rainfall meets plant water needs.
Hydrological drought refers to deficiencies in surface and subsurface water supplies. It is
measured as streamflow and as lake, reservoir, and groundwater levels. There is a time lag
between lack of rain and less water in streams, rivers, lakes, and reservoirs, so hydrological
measurements are not the earliest indicators of drought. When precipitation is reduced or