Agricultural Drought Indices - US Department of Agriculture

Drought Monitoring in AEMET
At present, a monitoring program to watch and evaluate drought is being operationally carried out
at AEMET on a national level. SPI maps (Mckee et al. 1993) for Spain are produced monthly
(Mestre and Moreno 2008). The program to calculate SPI uses climatic data from the National
Climatic Data Base, as well as daily rainfall data obtained in quasi-real time from the synoptical
stations network. The resulting products are SPI tables and graphics representing sites and
basins over several accumulation periods ranging from one month to one year, with the aim of
evaluating drought at different time scales. The drought module has been developed as an
independent module in the framework of the National Water Balance. Climate data as well as daily
precipitation data entering in quasi-real time are its input data.
The operational application allows the monthly generation of a set of graphics and tabulated
products related to the SPI for periods ranging from 1 month to 3 years. Input data for this
application are local data from AEMET’s synoptic stations plus precipitation volumes calculated for
the main hydrographic basins. Monthly series of precipitation for a total of 90 synoptic stations and
monthly series of mean areal precipitation for the 12 geographical areas defined by the broader
Spanish water basins are used as basic information for the system. For the stations, the data are
obtained from the National Climate Database, and the gaps have been filled by interpolation in the
corresponding monthly grid. For each water basin, data are obtained from the precipitation volume
series that started in 1947. Recent data come from the AEMET National Water Balance.
Precipitation volumes are estimated by integrating precipitation data into the 0.16º grid.
Subsequently, these provisional volumes are replaced by more precise estimations once
precipitation data from the whole set of AEMET’s climatological stations are available in the
National Climate Database (usually with a delay of 1-2 months).
From monthly precipitation series, the accumulated values for periods ranging from 1 month to 36
months are assessed. Data for every month, station, or water basin are fitted to a gamma
distribution to get the corresponding series of SPI values. On the AEMET web page (internal
page), the SPI values appear separately for every station and water basin. For accumulation times
of 1, 3, and 6 months (representative of a short temporal scale) and 1, 2, and 3 years
(representative of a long temporal scale), there are graphical presentations.
The output of this monitoring system is available at the beginning of every month on the AEMET
intranet web page as a set of products, including SPI maps for different accumulation periods,
calculated from station data (Figure 6) and from averaged precipitation estimated over whole river
basins (Figure 7). Time series with recent index values for stations and basins are also available,
thus allowing a monitoring of recent values of SPI for different time scales (Figures 8 and 9).
Application of the Meteorological Drought Monitoring and Analysis System to Two
Outstanding Drought Episodes of the Last 20 Years
In all regions of Spain, there is a high recurrence of drought events, which in some cases have
greatly affected the activity of various productive sectors and caused important economic losses
and severe environmental damage. The longest drought spell of the last 75 years was from 1991
to 1995, and it affected the southern half of Spain. The drought monitoring system of AEMET has
been applied to analyze the characteristics of this drought event as well as the impacts and
response measures.
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