Agricultural Drought in North-Central Mexico - Monitoring and Predicting Agricultural Drought: A Global Study - page 136

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2003; Tiscareño et al., 2003). Magaña et al. (2003) also established that
during La Niña years, rainfall was normal or little bit above normal during
summers, but rainfall showed a declining pattern during winters.
Tiscareño et al. (2003) used variables related to the El Ni no/Southern
Oscillation (ENSO; sea-surface temperatures and pressure differences in
the Pacific Ocean; chapter 3) and correlated them to rainfall data. This al-
lowed the identification of wet and dry periods that coincided with hot and
cold periods of the Pacific Ocean waters. Based on this analysis, Tiscareño
et al. arrived at the conclusion that the drought of 1982-83, which affected
almost the entire country, was a result of the ENSO.
Agricultural production is also affected by the ENSO. This phenomenon
has caused a decrease in national grain production since the 1960s
(Tiscareño-López, 1999). Tiscareño et al. (2003) also showed considerable
interannual yield variations as a result of climate anomalies in Mexico.

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D rought Monitoring
Currently drought monitoring in Mexico is in its early stage of develop-
ment. There are only two institutions that monitor droughts in the coun-
try. One of them is the SMN, and the other is the Centro de Investigaciones
Sobre la Sequía (CEISS; i.e., Drought Research Center). The SMN moni-
tors droughts by determining precipitation deviations from the normal, and
these deviations are regularly published (http://smn.cna.gob.mx/productos/
map-lluv/tabla.gif). Essentially, the historical monthly mean and annual
mean are compared with the actual monthly and annual precipitation; the
deviation is presented as a percentage of mean values. A negative deviation
represents precipitation deficit (dry condition or drought), while a positive
one indicates surplus (humid condition or wet period).
Recent cooperation efforts among the Mexican Comisión Nacional del
Agua, the U.S. National Oceanic and Atmospheric Administration, and
the Meteorological Service of Canada have produced the North Ameri-
can Drought Monitor, which presents a monthly drought map of southern
Canada, the United States, and Mexico. The drought monitor is the result
of a process that synthesizes multiple indices, outlooks, and local impacts
into an assessment that best represents current drought conditions. The
final outcome of each drought monitor is a consensus of federal, state, and
academic scientists (NCDC, 2003). An advantage of this small scale is that
the climatic phenomena can be visualized in all its extension; a disadvan-
tage is that the scale is not practical for important drought-affected areas.
The Mexican Drought Research Center (CEISS) applies the standardized
precipitation index (SPI; chapter 9; McKee, 1993, 1995). Total monthly
precipitation data recorded by the SMN in Chihuahua State is obtained by
the CEISS during the first week of each month. The data are processed us-
ing specially designed software to compute the monthly SPI. The software
stores the state historical precipitation database since 1970, which is auto-
matically updated every month. SPI values are computed on an interval of

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