Causative 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 major 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).

l-Aguascalientes-l--Coahuila------1-----Chihuahua—I----Du rango-----1—-Zacatecas—I

60years data (1941-2000) from the N ational M eteorological Service l-Aguascalientes-l--Coahuila------1-----Chihuahua—I----Du rango-----1—-Zacatecas—I

60years data (1941-2000) from the N ational M eteorological Service

Figure 10.2 Variation in annual temperature for five states of Mexico, for the period from 1941 to 2000. Upper, lower, and middle points of a vertical line represent maximum, mean, and minimum temperatures, respectively.

The five north-central states are typified by hot summers and cold winters (figure 10.2) with moderate to low rainfall (García, 1981; Schmidt, 1986). As a result, the region has a potential annual water loss through evapotranspiration that far exceeds the annual precipitation. The rains occur mostly during the summer months, with extreme annual variability (figure 10.3).

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