Meteorological and agricultural droughts are frequently, but erroneously, considered synonymous.
module 2
Meteorological and hydrological droughts are physical events but agricultural drought refers to
the impact of the first two on agricultural production. It is necessary to distinguish between these
types and clarify where and how they overlap (Fig.2.1). Both climate variability and climate
change influence such aspects as time (season, intra-season), location and length of drought
occurrence.
Meteorological drought occurs when the reduction in rainfall for a specified period (day, month,
season or year) is below a specified amount – usually defined as some proportion of the long-term
average. It is usually an expression of precipitation’s departure from normal over some period of
time (Fig.2.2). These definitions are region specific and presumably based on a thorough
understanding of regional climatology.
Hydrological drought refers to deficiencies in surface and subsurface water supplies based on
measurements of stream flow and lake, reservoir and groundwater levels. When precipitation is
reduced or deficient during an extended period of time, this shortage eventually will be reflected
in declining surface and subsurface water levels. However, hydrological measurements are not the
earliest indicators of drought because of the time between reduced periods of precipitation and
reduced water in streams, rivers, lakes and reservoirs.
Agricultural drought occurs when there isn’t 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.
Socioeconomic drought occurs when physical water shortage starts to affect people, individually
and collectively. In more abstract terms, most socioeconomic definitions of drought are associated
with its effect on the supply and demand of a product that has market value.