Agricultural Drought in North-Central Mexico - Monitoring and Predicting Agricultural Drought: A Global Study - page 134

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Insurance companies have adopted a different definition of drought
for the purpose of payments to farmers during drought periods. These
companies define an agricultural drought as a state of insufficient water
or rainfall availability such that plants are damaged due to any of the
following problems: rickets, dehydration, permanent withering, total or
partial desiccation of the reproductive fruit organs or grains, irregular
pollination during the embryo formation, or plant death.

Im pacts of Droughts
According to the Instituto Nacional de Estadística, Geografía e Informática
(INEGI; i.e., National Institute of Statistics, Geography and Informatics;
1999a), maize production in the state of Chihuahua declined from 880,082
tons in 1993 to 303,627 tons in 1995 due to a drought causing $168 mil-
lion in losses. Likewise, during the 1994-96 period, bean production in
Coahuila State decreased from 31,908 tons to 6,035 tons causing an eco-
nomic loss of about $16 million (INEGI, 2000a). Droughts plagued the
state of Chihuahua and other northern states during 1993-2002 and dras-
tically reduced the sowing area, even in irrigated districts. The water level
in the 10 largest Chihuahuan reservoirs was reduced to 20% of its total
capacity, from 4,259,000 to 851,800 m 3 . In Delicias irrigation district,
crops were sown only in 25% of the land during spring-summer season
due to historically low water levels in the La Boquilla Dam, which per-
mitted the cultivation of only 26,000 ha compared to 104,000 ha during
normal years.
Persistent drought also forced a high percentage of productive men of
the northwest zone to migrate to the United States in search of jobs. An
estimated 16,000 workers moved to the United States, mostly due to lack
of agricultural jobs in Mexico ( El Heraldo de Chihuahua, 2002).

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C ausative Factors of Drought
Climate in the northern central plateau is mostly arid and semiarid (García,
1981). Orographic barriers and subtropical high-pressure cells are the ma-
jor factors responsible for formation of dry climate in Mexico (Schmidt,
1989). The two mountain ranges, the Sierra Madre Occidental in the west
and the Sierra Madre Oriental in the east, both parallel to the coastlines,
are major elongated barriers to moisture flow from the Pacific Ocean and
the Gulf of Mexico (figure 10.1). In addition to the orographic barriers,
most of the year, the presence of subtropical high-pressure cells over the
landmass blocks the progress of a storm and prevents the penetration of
moist air (Schmidt, 1986). Because of these conditions that prevail in 70%
of the national territory, the northern zone receives less than 40% of the
total rainfall, while the southern part with 30% of territory receives the
remaining more than 60% of rainfall (Velasco, 1999).