SECOND SYMPOSIUM ON RESOURCES OF THE CHIHUAHUAN DESERT REGION
approximately 55% of the annual precipitation falling in El Paso, Texas, was the result of tropical revolving storms
UNESCO 1979 ? this map indicate that 30% of the earth's land surface is hyperarid (5.6%), arid (12.8%), and semiarid (1 1 .gO/O). Another 10% is categorized subhumid, and subject to desertification. Only 4% of the world's hypemrid and arid zones are located in North America. The Chihuahuan Desert embedded in the center of North America's subtrop- ical latitudes, accounts for more than a third (35.7%) of the continent's total. This desert spans more than 1 lo of latitude
Mexico, where it accounts for 13% of the national territory and represents the largest desert in Mexico
Mexico where 53% of the national territory is considered arid (22%) and semiarid (31°/0) and an additional 40% of the land area expe- riences long seasonal drought
Chihuahuan Desert is situated between two significant elongated orographic bamers which parallel the coastlines. These mountains are the Sierra Madre Occidental on the west, and the Sierra Madre Oriental on the east.
Basin and range topography characterizes the Chihuahuan Desert
nearly the entire desert has a basin level of 900-1 200 m
terrain classification involving complexity, scale, and asso- ciation make it virtually impossible to make precise distinctions among the land surface form categories '(Mabbutt 1968)
any landform protruding above 1800 m could be classified as a "Desert Mountain" with a very high degree of confidence. Also, the 1800 m level is the upper limit of the Chihuahuan Desert based upon climatic analysis (Fig. 1, Schmidt 1979). Nearly 12% of the surface area in the Chihuahuan Desert is above 1800 m in altitude
Rio Grande (Rio Bravo del Norte) and the Rio Conchos are the only significant drainage systems in the Chihuahuan Desert. Historically, the Rio Conchos supplied approx- imately 18% of all the water that entered the KO Grande (Tamayo and West 1964).
Major published sources of data for Mexico are: the Servicio Meteorobgico Nacional (SMN 1976) Normales Climatologicas: Periodo 194 1-1 970; the Instituto de Geografia work which includes Garcia's 1970 Carta de Climas, Climas: Precipitacion y Probabilidad de la Lluvia en la Republica Mexicana y su Evaluation (dates vary 1975, 1977); Mosiiio and Garcia (1974) The Climate of Mexico; Garcia and Mosiiio (1 981), Cantidad de Lluvia Mas Frecuente (Moda) en la Republica Mexicana; and the Direccion General de Geografia del Temtorio Nacional(l98 I), Atlas Nacional del Medio Fisico. For additional information pertaining to the climatology of Mexico's arid zones see Schmidt (198 I).
NOAA. Four sources of infor- mation and data which are very useful are: The Climate of New Mexico (Tuan et al. 1973); The Climate of Texas and the Adjacent Gulf Waters (Onon 1964); the Monthly Data System of the Texas Water Oriented Data Bank (Texas N.R.I.S. 1983); and New Mexico Clima- tological Data: Monthly and Annual Means 1850-1975 (Gabin and Lesperance 1977). Alvarez G., Chief of Meteorology and Geography for the State of Chihuahua. Since 1957
1981 - five megabyte data [lol]
temperature conditions found in the Chi- huahuan Desert are rather mild and constant
average annual temperature for the entire desert is 18.6"C, with station averages ranging from 14"-23°C
Only a few stations have recorded extreme temperatures-higher than 50°C, or lower than - 15°C
Almost 30% of the Chihuahuan Desert lies at an altitude between 1100 m and 1500 m
northern portion of the Chihuahuan Desert are about 3"-4°C cooler than those in .he south
large diurnal and annual temperature ranges are common ? 1 5"-20°C.
There is a strong relation between the occurrence of the temperature maximum shift and the onset of the summer rainy season (Mosiiio and Garcia 1974). The highest monthly temperatures in central Mexico occur in May. By June, the heat wave moves over the southern two-thirds of the Chihuahuan Desert, and in July the northern and far-eastern portions of this arid zone achieve their maximum temperatures. It should be noted that seldom is the temperature difference between June and July more than 1°C.
largest diurnal temperature ranges generally occur in the spring, and the smallest ranges are recorded in late summer
minimum range of temperatures occur during September in the south, and during August in the north.
As might be expected, the minimum range is closely associated with the month which receives the greatest precipitation.
Nearly the entire arid zone is 400-700 krn from the nearest sources of precipitable water-the Gulf of Mexico and the eastern tropical Pacific Ocean
Aridity is more the result of orographic barriers than continentality
mean annual precipitation for the Chihuahuan Desert is 235 mm ? range of approximately 150-400 mm
most of the precipitation falls during the summer in the form of rain from the thundershowers. Hail-producing thunderstorms also occur, but they seem to be less frequent and of lower intensity than those found on the Great Plains
northern portion of this arid zone usually receives some snowfall during the cooler half of the year. An average of about two snowstorms can be expected each year, although seldom does snow remain on the ground for more than a day or two.
Nearly all locations in the Chihuahuan Desert receive more than 70% of their annual precipitation during the warmest half of the year (May through October), with maximum rainfall occumng in July and August
mean min- imum of precipitation occurs during the spring, especially in the months of February and March (Sands 1959)
does not have a winter rainy season like that of northern Sonora and the Arizona Uplands. The seasonality of precipi- tation is the major distinction between the warm deserts ofNorth America (Schmidt 1983).
zonal westerly flow of wind persists over the Chihuahuan Desert for more than two-thirds of the year (Lahey et al. 1958, 1960). In June prevailing easterly winds extend to about 25" with a westerly flow prevailing poleward of 28"N. Between the latitudes 25"N and 28"N winds exhibit no distinct pattern. In the months of July and August, an easterly flow predominates over all of the desert except the far northern region in New Mexico. In September, the easterly flow is confined to the area south of the 27"N parallel, and completely disappears from the desert by October.
It is during the summer months that circulation around a warm upper level high-pressure cell shifts westward. Clockwise circulation around the Bermuda high-pressure cell brings moist air from the Gulf of Mexico into the Chihuahuan Desert. The influx of moist tropical air is accompanied by higher relative humidity, increased cloud cover, and thunderstorms. The much smaller quantities of precipitation received during the winter are the result of mid-latitude cyclones transporting moist Pacific air into the region.
FIG. 9. Precipitation and temperature for El Paso, Texas. [pdf pg 8]
tropical revolving.storms, which sometimes reach humcane intensity
Deleting isolated thun- derstorms, which were unrelated to tropical revolving storms, it was found that approximately 45% of the precipitated moisture was derived from the eastem-tropical Pacific and 55% from the Gulf of Mexico (Schmidt 1983
strong westerly winds aloft, associated with an upper-level high-pressure system, can shear strong westerly winds aloft, associated with an upper-level high-pressure system, can shear the top off a storm and quickly dissipate its energy
Pruszka's (1983) analysis of selected tropical storms affecting the El Paso, Texas, region indicates that tropical storms which make landfall between Tampico, Tamaulipas, and Corpus Christi, Texas, on the east coast, and northward up the Gulf of California from Los Mochis, Sinaloa, on the west have the greatest effect on the northern Chihuahuan Desert
(Schmidt 1975, 1979). The aridity indices were derived using the formula: Ia = Pmm / T^C + 10
aridity value below 5 generally characterizes the true deserts; indices of approximately 10 cor- respond to dry steppes; values of about 20 to the prairies; and above 30 forest dominatesFIG. I5. Climatic zones based upon the de Martonne (1926) Aridity Index using the mean annual precipitation. [pdf pg 10]
natural resource base of the desert is fragile and its management requires careful application of climatic information in conjunction with other environmental factors.
Because arid zones are not gen- erally associated with conventional agricultural and forestry endeavors, non-traditional methods of using climatic information and applying it to the terrain must be sought.
Approximately 37% of the United States (excluding Alaska) is considered arid and semiarid. In Mexico, 53% of the land is classified as desert, while another 40% experiences long seasonal droughts
The potential for improving agriculture management of natural vegetation, and increasing the utility of these marginal lands certainly does exist, but precisely how, when, and where is not clear.
yucca (Yucca spp.) and agave (Agave spp.) fiber plants, candelilla (Euphorbia antisiphilitica) for wax, guayule (Parthenium argentatum) as a source of natural rubber, and mesquite (Prosopis spp.) for fuel
More traditional crops, such as grapes, and deciduous fruits, may prove to be productive ventures in the more moist foothill regions of the mountains in this area of basin and range topog- raphy, particularly when coupled with water conservation methods