Agricultural Drought Indices - US Department of Agriculture

Europe around the Iberian Peninsula, causing a dramatic decrease in precipitation. Sometimes
this synoptical situation is persistent and provokes long periods of drought.
Figures 1, 2, and 3 represent a series of annual precipitation volumes over the peninsular Spanish
basins (Cantabrian, Atlantic, and Mediterranean). They show the high variability of the annual
precipitation amount in Spain, as well as the irregular occurrence of drought cycles, the highly
variable duration of droughts, and an average periodicity of 3-5 years. As a consequence of this
high precipitation variability, numerous episodes of severe meteorological drought have occurred.
Several drought episodes affected extended areas of Spain during the first 30 years of the last
century, the late 1940s, and 1957-58. After the precipitation maximum of the 1960s, a gradual
decrease in the amount of precipitation took place, with a severe drought episode between 1981
and 1983, and another at the end of the 1980s, especially in the peninsular northern part. This
decreasing bias culminated with a long-lasting severe drought, which took place at the beginning
of the 1990s and affected the southern half of the Iberian Peninsula. Between 1990 and 1995, in
extended areas, 5 dry or very dry consecutive hydrological years followed one another (Peral et al.
2000). From 1996 to 2004, a general wet cycle occurred, although interrupted by dry years (1998-
99, 1999-2000, 2001-2002). In November 2004, a new period of meteorological drought began,
which lasted along the hydrological year 2004-2005 and resulted in the driest hydrological year
since 1947 (the year that measurement of average precipitation over the main river basins began
to be done in a systematic way) (Mestre 2005). More recently, a drought period occurred in the
first half of the hydrological year 2007-2008. Its first semester was the driest of the whole series,
although that rain deficit was later compensated by the abundant precipitation of spring 2008.
The National Water Balance of AEMET
Soil moisture estimation is an important tool for decision making in different sectors, particularly for
farmers (evaluation of irrigation needs, soil workability, drought assessment, and early warning),
forestry services (forest fire risk assessment and controlled burning authorization), and hydrology.
Hence AEMET developed a 0.2º resolution gridded national water balance in 1997 aimed at
providing a daily assessment of soil moisture conditions in order to meet the specific needs of the
different users. The main features of the water balance are described below (Navarro and
Picatoste 1998, Mestre and Moreno 2004).
Series of annual precipitation volume over the northern Spanish basins
120000
100000
Volume ( Hm3)
80000
60000
40000
20000
0
1940 1950 1960 1970 1980 1990 2000 2010 2020
Year
Figure 1. Precipitation variability in Spain; series of annual precipitation over the northern basins (period:
1947-2008).
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