The arid zones of Mexico: climatic extremes and conceptualization of the Sonoran Desert

Journal o/Arid Emnronments (1989) 16, 241-256

The arid zones of Mexico: climatic extremes and conceptualization of the Sonoran Desert Robert H. Schmidt Jr* Accepted 7 July 1988 Arid and semi-arid climates occupy more than half (52%) of Mexico's national territory. Although this in itself presents substantial problems for land-use development, some of the hottest (57°C) and driest conditions (~32 mm/year) found anywhere have occurred in the arid zones of Mexico. Also, and perhaps surprisingly,the lowestofficial temperature (- 20°C) in Mexico was recorded in the Chihuahuan Desert. Close examination of the climatic data indicates that caution is in order particularlypertaining to maximumtemperature extremes. In order to establishrelatively firm boundariesfor the SonoranDesert, climaticdata from more than 250 weather stations were utilized to delineate this arid zone based upon the de Martonne Aridity Index. In addition, 17 conceptualizations (12 presented here) of the Sonoran Desert were mapped at the same scale to determine their relevance and consistency. Introduction Mexico has two major deserts named after the largest and second largest states, Chihuahua and Sonora. Both the Chihuahuan and Sonoran deserts extend northward across the international boundary into the United States and retain their Mexican names. Because more than half of the national territory of Mexico is considered arid and semi-arid, these zones represent substantial barriers and special problems for land-use development. In addition, some of the hottest and driest conditions on the continent have occurred in these zones, and, perhaps surprisingly, the lowest official temperature in Mexico was recorded in the Chihuahuan Desert. Close examination of the climatic data indicates that caution is in order, particularly pertaining to some of the superlatives regarding maximum temperature extremes. The study of Mexico's arid zones seems particularly important, not only for the reasons stated above, but because for many U. S. scientists, serious research work has a tendency to stop at the international boundary with Mexico. This rather narrow view, lacking a much needed larger regional focus, is very limiting, leading to unneeded vagueness, ambiguities, and sometimes questionable generalities. Also, given the considerable interest in the author's previous study on the Chihuahuan Desert (Schmidt, 1979), it seems appropriate to advance the topic. Therefore, the main purpose of this paper is to present a detailed discussion on the climatic characteristics and spatial distribution of Mexico's arid zones. Literature and data sources One of the major problems in the study of arid lands and other extreme climates is a general lack of basic data. Fortunately, this situation is no longer entirely valid for climatic data in Mexico's arid zones. Instrumental observations of precipitation and temperature data are available for a large number of weather stations spread throughout Mexico. In areas of * Department of Geological Sciences, University of Texas at El Paso, Texas 79968 U.S.A. 0140-1963/89/030241 + 16 $03.00/0

© 1989 Academic Press Limited

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comparable human population, the present density of Mexican climatological stations is greater than that maintained by the United States National Weather Service. This relatively dense network of climatological stations has resulted from the recognition of the great significance of climatic-based resources, such as ranching, farming, non-traditional agriculture, and hydrologic conditions to the development of the nation. Traditionally, there have been two major sources of climatic data in Mexico: the Servicio Meteorologico Nacional (SMN), formerly the Servicio Meteorologico Mexicano (SMM), and the Secretaria de Agricultura y Recursos Hidraulicos (SARH), formerly the Secretaria de Recursos Hidraulicos (SRH) and the Secretaria de Agricultura y Ganaderia (SAG). In addition, the Comision Federal de Electricidad (CFE), the railroads, some lumber companies, and the Comision Internacional de Limites y Aguas (the International Boundary and Water Commission) have collected climatic data. In the state of Chihuahua, climatological data are collected by an association of ranchers (Union Ganadera Regional de Chihuahua) in cooperation with the state weather service. Most of these data are available through the Sistema Nacional de Informacion, Departamento de Climatologia in Mexico City. The very useful publication entitled Informacion Sobre Informacion: 'ClimatoIogia' (Instituto Nacional de Estadistica, Geografia e Informatica, 1980; hereafter INEGI) lists more than 50 sources of Mexican climatic data and information. For a discussion and bibliography of published material pertaining specifically to the climatology of arid regions in Mexico, the reader should consult Schmidt, 1981. Several recent publications by Durrenberger & Xicotencatl (1978), MacMahon (1979), McGinnies (1981), Mosifio & Garcia (1981a & b, 1979), Brown (Ed.) (1982), Direcion General de Geografia del Territorio Nacional (1983, 1981, hereafter DGGTN), Ezcurra & Rodriguez (1986), Schmidt (1986, 1983), and the Chihuahuan Desert Research Institute (Barlow et aI., 1986) provide substantial contributions to the study of arid zone climatology and environmental characteristics in Mexico. One of the most detailed, systematic, and useful contributions to arid land climatology in Mexico is the 'Carta de Climas de la Republica' of Mexico produced at a scale of 1:500,000 (Instituto de Geografia, 1970). The 46 maps, compiled by the Institute de Geografia at the Universidad Naciona1 Autonoma de Mexico (UN AM) show isohyets, isotherms, climographs, and the nation's climates based upon the Garcia (1964) modification of the Koppen classification. The climate classification of Koppen was modified by Garcia to identify particular conditions in Mexico, especially discrete differences in rainfall. This map source is in approximate agreement with previous delineations (Contreras, 1955) and with the more detailed studies of individual deserts (Hastings & Humphrey, 1969a; Schmidt, 1979). More recently the DGGTN (1983) produced a climatic map at a scale of 1:4 million, which shows all of Mexico and one map sheet. Another valuable map series is the study of rainfall frequency in Mexico by Mosifio and Garcia (1981b). Their work shows the geographical distribution of gamma and beta parameters, coefficients of variation, and the precipitation mode for the most important rainy months in Mexico.

The deserts of Mexico Based upon detailed planimeter measurements made by the author from the Instituto de Geografia (1970) climatic maps at a scale of 1:500,000, it was found that 53% of Mexico is considered arid (BW=22%) and semi-arid (BS=31 %), and an additional 40% of the land area experiences long seasonal drought (Aw and Cw). These climatic factors coupled with high rugged terrain (approximately 50% of surface is higher than 1000m) present Mexico with substantial developmental problems. The total area designated arid and semi-arid in Mexico by Contreras (52%,1955), and d' Albe (59%, 1960)is consistent with the 'Carta de Climas' (Instituto de Geografia, 1970). The major discrepancy is in the proportion of arid and semi-arid climates. Contreras (1955) assigned 33% of Mexico as arid and nearly 19% semi-arid. Similarly, d'Able (1960)

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designated 36% as arid and extremely arid, and 23% semi-arid. In reference to Mexico's arid zones, d' Able stated that the climatological classifications by Koppen, Thornthwaite, and Emberger are in approximate agreement when suitably modified to take into account the effects of altitude. Nearly all of the arid zones of Mexico are classified as hot deserts (18% are BWh). The less than 4% of the arid zones considered temperate (BWk), are found in the north or on the higher slopes surrounding the hotter deserts. For the most part, colder winter temperatures are responsible for the temperate designation. The Chihuahuan Desert

Three-fourths of the Chihuahuan Desert is in Mexico where it occupies 13% of the national territory (Fig. 1). The Chihuahuan Desert is the largest desert in Mexico, but it is not extremely hot or dry for an arid zone. As a result of the central desert's environmental homogeneity, sub-regional designations are generally inappropriate or at least they are not widely used or recognized. Only the term 'Bolson de Mapimi' (Mapimian), or 'Grande Bolsa' as it was first called, is occasionally used to identify the purse-shaped series of basins extending southward from the Rio Grande into Chihuahua and Coahuila. Tertiary level designations, such as the Samalayuca or the Cuatro Cienegas desert, are also used. Although some earlier researchers have recognized discrete subdivisions within the Chihuahuan Desert, MacMahon (1979) stated that part of the confusion is the result of complex soil mosaics. Major differences stem from the pedogensis of soils derived from

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Figure 1. The Sonoran(left)and Chihuahuan (right) deserts. The boundariesof the Sonoran Desert are by Schmidt following the de Martonne Index of Aridity (1926), and the DGGTN (1983) climatic map for the north-western boundary in B.C.N. Toe Chihuahuan Desert boundaries are from Schmidt (1979). The delineationsof both deserts are in verycloseagreementwith the DGGTN map. The 'islands' within the deserts are highlands which are too high, too cool, and too humid to be considered arid. The numbers on the map are climatic stations and place names referred to in the text: 1. BahiaMagdalena,B.C.S; 2. BahiaSebastianVizcaino,B.C.S; 3. Bataques, B.C.N; 4. Delta, B.C.N; 5. Maclovio Herrera, Chih; 6. EI Major, B.C.N; 7. Mexicali, B.C.N; 8. Nazas, Dgo; 9. Puerto Cortes, B.C.S; 10. EI Riito, Son; 11.San Luis RioColorado,Son; 12.Sierrade La Giganta, B.C.N; 13. Sierra La Laguna, B.C.S; 14. Topolobampo, Sin., Higuera de Zaragoza, Sin., Ahome, Sin; 15. VillaAhumada, Chih; 16. Sierra Juarez, B.C.N; 17. Sierra de San Pedro Martir, B.C.N.

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Cretaceous limestones, tertiary volcanic rocks, or alluvial deposition. Based upon the physiognomic characteristics of native plants, MacMahon divides the Chihuahuan Desert into three zones. These are as follows: (1) microphyllous desert scrub (creosotebush desert of Leopold, 1950), (2) rosettophyllous desert scrub dominated by agave lechugilla (cactus zone of Leopold), and (3) crassicaulescent formation dominated by large cacti. Although the closest sources of moisture are 400 to 700 km away, and substantial orographic barriers intervene coupled with the domination of subtropical high pressure, the Chihuahuan Desert still receives an average annual rainfall of 235 mm with a range of 150 to 400 mm (Schmidt, 1979). Approximately two-thirds of the climatic stations have annual precipitation totals between 225 and 275 mm. No portion of this arid zone has experienced a year without recording precipitation, although Maclovio Herrera (#5 on Fig. 1) in eastern Chihuahua received only 5·5 mm in 1965. Normally the driest region in this desert is in southern Coahuila, particularly those lowland localities sheltered by surrounding mountains. Temperature conditions are relatively mild and consistent from year to year. This is largely the result of latitudinal location and the fact that 90% of the desert lies at an altitude between 1100 and 1500 m. Annual temperatures average 18·6°C with a range of 14° to 23°C(Schmidt, 1986). Nearly half of the mean annual temperatures are within 2°Cof the average. Temperature extremes higher than 50°Cor lower than -15°C are very rare. The hottest monthly temperatures are very similar throughout the Desert, ranging from 25° to 30°C. It is largely the colder winter months (e.g. January <10°C) in the northern Chihuahuan Desert that account for the cooler average annual temperatures. The general weakening and equatorward shift of the subtropical high pressure cells in the winter allows the more well-developed portion of cold fronts to pass through the northern part of the region, especially poleward of 29°N latitude. Hill (1969) found that 80-95% of all negative temperature changes during the winter months in north-central and northeastern Mexico were the result of synoptic cold fronts. These cold fronts cause the temperature to drop an average of 6°C. The coldest temperature officiallyrecorded in Mexico was apparently at Villa Ahumada (#15 of Fig. 1) in Chihuahua (INEGI, 1984). During a 6-day period, when the daily minimum temperatures reached below -20°C, this town located about U5 km south of EI Paso, Texas-Ciudad Juarez, Chihuahua (U.S.-Mexico border), recorded a -30°C on 11 January 1962 (Schmidt, 1975). This extreme minimum temperature is certainly the coldest recorded in the Chihuahuan Desert. The official daily temperature data for Villa Ahumada were inspected and verified by the author at the Departamento de Climatologia, Sistema Nacional de Informacion in Mexico City. Other very low temperatures were recorded in the general area during this period. the highest official extreme temperature for Mexico is credited to Nazas, Durango (#8 on Fig. 1; near Torreon, Coahuila at an altitude of 1264 m) where the temperature allegedly attained 59°C on 19 May 1969(INEGI, 1984). Inspection of the daily climatic data for that station indicates that the thermometer reached 39'0°C, and not 59°C. Probably the handwritten '39°C' was misread as '59°C'. Thirty-nine degrees Celcius was recorded for that date, and also listed as the maximum for that month. The preceding day registered 36°C, and the day following recorded 38·5°C. The low was 19°C with light westerly winds, and the mean monthly maximum temperature was 36·5°C. .

TheSonoran Desert Mexico's second largest desert is centered on the second largest state, Sonora, with the remainder on the Baja Peninsula, and a narrow coastal belt extending into north-western Sinaloa. More than two-thirds of the Sonoran Desert is in Mexico and more than 90% (Ia = :0;::; 10)of the entire Baja peninsula is considered arid. Approximately half of the state of Sonora is classified arid (Fig. 1). Shreve & Wiggins (1964), and more recently Brown & Lowe (1980) and McGinnies

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(1981) have divided the desert into subregions based upon varying floral and environmental characteristics. These subregions are the deserts of the Plains, and the Foothills of Sonora, the Arizona Upland, the Lower Colorado River Valley, the Central Gulf Coast, the Viscaino, and the Magdalena. In addition, local designations are used, such as the Gran Desierto, and Desierto de Alta.

Conceptualizations of the Sonoran Desert In order to establish a more representative delineation of the Sonoran Desert, 17 conceptualizations were mapped at the same scale. Because several of the delineations were very general or for the most part copies of previous work, 11 maps of the Sonoran Desert are presented here (Figs 3-13). The similarities and differences between the 110·

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Figure 2. The greatest (dashed lines) and least (solidlines)expanseof the SonoranDesert based upon composites of the delineations shown on Figs 1, and 3-13.

Figure 4. The Sonoran Desert: delineation by Axelrod(1979) basedupon Shreve (l937a), Leopold (1950), and personal observations.

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Figure 3. The Sonoran Desert: delineation by Arbingast et al. (1975). Basedupon the Koppen climaticclassification.

Figure 5. The Sonoran Desert: delineation by Brown, (Ed.) (1982) using subdivision boundary from Shreve (l95 1)and modified by Brown and Lowe (1980).

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Figure 6. The Sonoran Desert: delineation by Dunbier (1968) based primarily upon Shreve (195I), and personal observations.

Figure 8. The Sonoran Desert: delineation by Instituto de Geografia (1970) based upon the Koppen-Garcia climatic classification.

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Figure 7. The Sonoran Desert: delineation by Garcia and de Byres (1984)following the Koppen climatic classification.

Figure 9. The Sonoran Desert: delineation by Leopold (1950) based upon natural vegetation.

various delineations can be made by comparing Figs 3 through 13 with the composites of all the Sonoran Desert maps (Fig. 2). The solid lines on Fig. 2 show the areas every delineation considers to be part of this region or the core area. The arid zones of the Baja Peninsula were included as part of the Sonoran Desert on all conceptualizations with the exception of the map by Dunbier (1968, Fig. 6). The dashed line represents the area considered to be the greatest expanse of the Sonoran Desert. In order to present a regional map of the Sonoran Desert based upon a consistent, uniform and comprehensive system, the author selected the de Martonne Index of Aridity (de Martonne, 1926) where indices are derived from the formula: Ia =

Pmm Toe + 10

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Figure 10. The Sonoran Desert: delineation by MacMahon (1979) using plant and animal distribution.

Figure 12. The Sonoran Desert: delineation by Shreve and Wiggins (1964) based upon plant distribution.

Figure 11. The Sonoran Desert: delineation by Nelson (1966) using the arid tropical and lower Sonoran life zones.

Figure 13. The Sonoran Desert: delineation by Thornwaite (1931) using his climatic classification.

Where 'Pmm' is the mean annual precipitation in millimeters, "T'C' is the mean annual temperature in Celsius, and 'la' the Index of Aridity. According to de Martonne, an index of aridity value below five generally characterizes the true deserts; indices of approximately 10 correspond with dry steppes; values of about 20 to the prairies; and above 30 forest dominates. Aridity indices of,;;;10 are considered desert and are presented here to delineate the Chihuahuan and Sonoran deserts (Fig. 1). These isoaridities are based upon recorded weather observations, and the extrapolation of these data to areas having similar altitude and latitude. As stated by the author in 1979, the decision to select the de Martonne Index of Aridity over other possible methods of regionalization was based upon the close parallel between the indices and the visual appearance of the natural environment. In addition, numerical values have an advantage over the letter symbols used in many climatic classifications, because the indices clearly portray the intensity of the precipitation-temperature ratios,

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and in areas where long-term climatic records are not available, it can be based solely upon annual values which stabilize in a much shorter time than monthly values. The de Martonne aridity index has considerable value for explaining drainage conditions, and the relation of climate and land-use potential. Meigs (1963) pointed out that it agrees more closely with the 1948Thornwaite system than any other widely used scheme; thus, it is very useful for agricultural purposes. These factors, coupled with the generally wide acceptance of previous works (Schmidt, 1975, 1978b, 1979, 1986) with climatic classifications applied to northern Mexico, and a large quantity of empiricism based upon the relation of climatic data to altitude and vegetation distribution, comprise in aggregate, the basis for using the de Martonne aridity index to delineate the arid zones of Mexico. The major criticisms of the de Martonne index include its inability to show seasonal differences in temperature and precipitation, its implication that evaporation appears to be a function of temperature alone, and its inability to classify properly areas having mean annual temperatures which approach -10°C. In Mexico, the Sierra Madre Occidental clearly separates the Sonoran and Chihuahuan deserts. Therefore, the boundary between the two deserts or the SonChih corridor found north of the Sierra Madre, which is based upon the seasonality of precipitation and altitude (discussed in Schmidt, 1979and here), does not create a formidable problem of delineating the deserts in Mexico. Although done independently, and originally at a larger scale, my delineation of the Sonoran Desert using the de Martonne Aridity Index of ~ 10 was very similar to the boundaries appearing on the climatic map by the DGGTN (1983) using the Koppen-Garcia classification. Because the two delineations are very similar, the DGGTN map (1983) does not appear with the other conceptualizations (Figs 2-13). The DGGTN map represents a refinement of the 1970 Carta de Climas de la Republica series discussed earlier in this paper. As previously stated, the de Martonne index does not consider the seasonality of precipitation. In the north-west portion of Baja California Norte, indices are ~1O, but the area has a pronounced winter maximum regime of precipitation. In that area, the author followed the DGGTN delineation of the dry summer subtropical climate. Generally, most conceptualizations of the Sonoran and Chihuahuan deserts include a larger regional expanse than can be justified on the basis of utilizing consistent methodologies and quantitative data. A comparison of Figs 1-13 indicates there are obviously several unsettled questions regarding the boundaries of the Sonoran Desert that need to be addressed. These are (1) the interior boundary on the mainland in the state of Sonora, (2) the southern coastal boundary on the mainland, and (3) the so-called 'non-desert' between the Sierra La Giganta (#12 on Fig. 1) and the Sierra La Laguna (#13 on Fig. 1) in the southern portion of the Baja Peninsula. The inland penetration of the desert in the state of Sonora, as indicated by Shreve & Wiggins (Fig. 12, 1964), Dunbier (Fig. 6, 1968), and more recent modifications of Shreve's map by Hastings & Turner (1965), McGinnies (1981) and Brown (Ed.) (1982), followed the 3000 ft (915 m) contour as the interior boundary. This boundary is based upon the statement by Shreve (1937b) and Shreve & Wiggins (1964), and repeated elsewhere (Humphrey, 1974; McGinnies, 1981) that plants on both the mainland and the Cape Region on the peninsula below 1000 m (also 1050 m and 915 m or 3000 ft) are distinctly xeric. More recently Turner & Brown (1982) pointed out that parts of Shreve's 'foothills of Sonora' as well as some of his other subdivisions are considered Sinaloan thornscrub and not part of the Sonoran desertscrub, This boundary more or less corresponds with an aridity index of 15which is the middle value for the semi-arid climates. It seems incongruous that areas on the interior mainland below 1000m are considered part of the Sonoran Desert, but on the peninsula areas below 1000m are considered non-desert. A similar conflict is the acceptance of mainland areas as desert which receive more than 400 mm of precipitation annually, and yet on the Baja Peninsula areas receiving less than 200 mm per year are delineated as non-desert. Although it is recognized that there are a variety of environmental conditions which can be used to characterize arid zones, any basis

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for formulating a map of a region should have a reasonable consistency. Based upon climatic data, it is appropriate to extend and expand the arid zone at the southern end of the Baja Peninsula. An overly large portion of this area has been labeled as the 'non-desert' Giganta-Laguna province as delineated by Shreve & Wiggins (1964), and more recent modifications by Hastings & Turner (1965), McGinnies (1981), Brown (Ed.) (1982), and Axelrod (1979). Although the vegetation of the region north of the Sierra La Laguna and south of the Sierra La Giganta may not fit every botanists' concept ofaxeric or desert environment, climatically this area must be considered arid (Hastings, 1964a & b; Hastings & Humphrey, 1969a & b; Instituto de Geografia 1970; SMN 1976; DGGTN 1981 & 1983). These two mountain ranges, whose peaks reach a maximum of 2090 m and 1490m respectively are separated by a substantial gap of about 150 km where the altitude is lower than 1000 m (DGGTN, 1982, and Fig. 1). Other than several relatively isolated peaks just slightly above 1000 m, the zone under discussion is separated by approximately 250 km before a more or less continuous high (> 1000 m) mountainous mass occurs. Although local relief is impressive, the total area of these mountain ranges above 1000 m is very small. From a climatological perspective the alleged non-desert or perhaps assemblage of non-traditional vegetation in this arid zone is probably the result of (1) relatively high rainfall totals (although less than 300 mm) in a zone that is generally free of killing frost. The precipitation mode is between 100 and 200 mm (Mosino & Garcia, 1981b); (2) the variation in rainfall is about the lowest on the Peninsula based upon coefficients of variation (Mosino & Garcia, 1981a & b). This is especially true if variation during the rainy season is considered (Hastings & Turner, 1965), and if proper allowances are made for deluge rainfall; and (3) the Giganta-Laguna province receives the greatest percentage of annual precipitation falling in the summer (32%) and fall (52%) seasons on the Peninsula (Hastings & Turner, 1965). In the Magdalena Desert immediately to the west of Giganta-Laguna, the seasonality of rainfall shifts to a fall (37%) and winter (34%) regime. Further north on the west coast, a distinct winter dominance of precipitation prevails. This characteristic also coincides with the rarity of frontal activity south of 27°N (Markham, 1972). To the south, rains are associated with air mass and tropical storms (locally called 'chubascos'), some of which reach hurricane or 'ciclon' intensity. Traditionally, the southern boundary of the Sonoran Desert on the mainland is placed somewhere between the Rio Yaqui and the Rio Mayo (Shreve, 1942; MacMahon, 1979). The slightly more southerly boundary chosen by MacMahon was prompted by the distribution of reptiles and amphibians (after Bogert & Oliver, 1945), and the occurrence of certain mammals. MacMahon's data indicate that the southern Sonoran boundary is a very gradual transition zone (MacMahon, 1979). Based upon a detailed climatic study of Sinaloa (Schmidt 1978a & b; Instituto de Geografia, 1970), it appears appropriate to extend the Sonoran Desert's southern boundary further south to include the narrow coastal strip along Topolobampo (# 14on Fig. 1)and on to the area just south of the mouth of the Rio Culiacan at about 24°N (or about the same latitude as La Paz, B.C.S.). The climatic delineation presented here roughly corresponds to Dunbier's (1968) southerly limit, but it is more confined to the coastal zone (Fig. 1). In addition, the southern extension of the Sonoran Desert is not in direct conflict with Shreve's conceptualization of this arid zone. Shreve (1934) stated that the desert plants of the Sonoran Desert in southwestern Arizona are both drought and cold resistant, while Sinaloa's plants are drought resistant only. Cold temperature conditions north of Sinaloa probably limit the number of drought-resistant plants that have been able to enter the desert. Further, the transition from desert vegetation to the arid thornscrub of Sinaloacorresponds to the 375-mm isohyet (Shreve, 1934), and warm winters free from killing frost. Climatic data for stations in this coastal zone of NNW Sinaloa generally support this observation. The three Sinaloan stations: Higuera de Zaragoza (# 14 on Fig. 1) with a median rainfall of334 mm, Ahome (#140n Fig. 1) with 348 mm, and coastal Topolobampo (#140n Fig. 1) with 270 mm, all have mean annual temperatures in excess of 24°C, and thus qualify as desert (Fig. 1; Instituto de Geografia, 1970; Schmidt 1978a & b; DGGTN, 1983).

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Minimum precipitation extremes The lowest precipitation totals in North America, and the highest temperatures in Mexico occur in the Sonoran Desert. Bataques in the delta of the Rio Colorado is consistently cited as the station having the lowest average precipitation in North America (Riordan, 1970; Riordan & Bourget, 1985), although the exact quantity of rainfall varies slightly from source to source. The range of 22 (INEGI, 1984) to 33 mm (SMN, 1976) is probably the result of the record period used. Other sources place the annual average at 30 to 32 mm (Hastings & Turner, 1965; Institutode Geografia, 1970; Markham, 1972; DGGTN, 1981; Brown, (Ed), 1982; Riordan & Bourget, 1985). Annual rainfall totals for nearby Delta and E1 Riito are approximately 35 to 40 mm respectively. In keeping with the dubious superlative attached to the Colorado River delta, Shreve (1934) stated that parts of this area have the thinnest plant cover to be found in North America. To add confusion to the topic, the author obtained the complete monthly rainfall data for Bataques (# 3 on Fig. 1) for the period 1948 through 1984 from the files of the national Department of Climatology in Mexico City. The rainfall average for Bataques has increased to 49 mm if computed by the monthly averages, and 52 mm if the yearly totals are used. Prior to 1972, annual rainfall totals had never exceeded 91 mm. After and including 1972, 6 years recorded more than 100 mm. 1976was the 'wettest' year on record receiving 143 mm, followed by 1983 with 132 mm, and 1982 with 125 mm. These higher rainfall totals were probably the result of the unusually warm ocean water in the eastern Pacificor 'E1Nino' effect, and possibly the relocation of the instrument shelter 6 km to the east. The west coast of Baja California Sur from Bahia Sebastian Vizcaino (#2 on Fig. 1) to south of Bahia Magdalena (# 1 on Fig. 1) is probably the second driest area in Mexico and the third driest area in North America after second place Death Valley, California. Here in the western portions of the Vizcaino and Magdalena deserts, annual rainfall totals range from about 50 to 80 mm. Although none of the sources cited in the foregoing discussion indicate any notable discrepancies, the SMN's (1976) published data shows that the Observatorio at Puerto Cortes (#9 on Fig. 1; 24°26'N-111052'W, and listed as Isla Margarita before 1964) receives only 31'8 mm annually (1953 to 1970). Nearby Bahia Magdalena (#1 on Fig. 1; 24°38'N-111035'W) averages only 35·8 mm each year (1941 to 1966). The climatic and isohyetal maps of the Instituto de Geografia (1970), and the DGGTN (1981), which actually state the average annual precipitation for each station on the maps, are in general agreement with the data sets published by (Hastings, 1964a & b; Hastings & Humphrey 1969a & b); and examined in detail, especially the data for Bahia Magdalena, by Hastings & Turner (1965), and Markham (1972). Not only are rainfall totals low along this area of the Pacific coast, but this arid zone apparently has the largest number of rainless months in Mexico. Between 1949 and 1966Bahia Magdalena averaged 9·6 rainless months each year while Bataques averages only 8·8 months annually (computed from Markham, 1972). Although the Sonoran Desert is honer and drier than the Chihuahuan Desert, the two most important differences between these arid zones are the distribution of precipitation during the year, and the length of the growing season. In the Sonoran Desert the rainfall is more evenly distributed throughout the year especially in the northern and western portion. The circulation pattern around the subtropical high pressure cell in the Atlantic Ocean (Bermuda or Azores high) coupled with the south-west cut-off low pressure cell brings high sun rainfall to both deserts, but during the winter the Sonoran Desert feels the impact of the passage of relatively moist cold fronts to a greater extent than the Chihuahuan Desert. Frontal activity is normally confined to the area north of about nON latitude, which is often designated as north of Guaymas (c. 28°N) on the mainland. By the time these fronts penetrate the continent's interior, they have been drained of nearly all their moisture and this results in a high concentration (e.g. 65%) of summer rainfall in the Chihuahuan Desert. Although both deserts span approximately the same latitudinal

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range, the Sonoran Desert with its much lower altitude (generally less than 750 m depending upon the latitude) results in a substantially longer frost-free season (e.g. 9 to 12 months). In addition, the beginning and end of the growing season is much more variable in the Chihuahuan Desert. As a result of the more even distribution of moisture throughout the year, coupled with a longer and generally more settled growing season, the vegetation in the Sonoran Desert is relatively verdant in comparison with areas of similar annual precipitation totals in the Chihuahuan Desert.

Maximum temperature extremes An attempt to resolve the occurrence and magnitude of the extremely high maximum temperatures reported for stations in the delta region has proved to be no easy task. Climatic data published by the SMN (1976) indicated that maximum extreme temperatures of 57°C occurred at Bataques (#3 on Fig. 1) and Delta (#4 on Fig. 1) on several occasions. These temperatures would qualify these two stations for the dubious distinction of being one of the very few places in the world officiallyto record temperatures this high. El Azizia, Libya is generally acknowledged as having recorded the world's highest extreme temperature of 58°C(Riordan & Bourget, 1985; Schmidli, 1968). Death Valley, California has recorded 5rC, and this is usually regarded as the world's second highest temperature, and the highest temperature in the Americas. Analysis of both of these temperature extremes indicates that they may be unreliable, especially the Death Valley record (Court, 1949; Riordan, 1970; Riordan & Bourget, 1985). Unfortunately, the extreme high temperatures recorded in the Colorado delta are also suspect. Personal inspection of the individual monthly and daily records was made for the delta stations of Bataques (#3 on Fig. 1), Delta (#4 on Fig. 1), El Riito (# 10 on Fig. I), EI Major (#6 on Fig. 1), Mexicali (#7 on Fig. 1), and San Luis Rio Colorado (#11 on Fig. 1). Based upon data from these stations, the following observations were made: (1) Bataques has the highest monthly mean temperatures of 32'9°C (July) with the highest average maximum temperature (42'3°C, July), and the highest average minimum temperature (23'6°C, July). During the period 1948-1985, Bataques recorded maximum temperature extremes of 57°Con three occasions. This station is generally regarded as having the highest temperatures in Mexico. Other monthly extreme temperatures of ~55°C occurred eight times, and temperatures of 49°C and over occurred on 46 occasions (Fig. 14a & b). (2) Delta's highest monthly mean of 32·5°C and highest average maximum and minimum extremes of 42'1"C and 23'QoC respectively also occurs in July. During the same record period as Bataques, temperature extremes of 57°Coccurred three times, ~55°C on six dates, and ~49°C on 31 occasions (Fig. 14a & b). (3) The other delta stations recorded maximum extreme temperatures of ~49°C on 28 occasions, including downward adjustments on the data sheets. El Major (1949-1976) reported an extreme high temperature of 55·5°C in August 1953, and El Riito (1949-1980) recorded a 59·5°Ctemperature in July 1958. These extremes of temperature represent an increase of 5·5°Cand 9°C, respectively over anyother extremely high temperature ever recorded at these stations and is highly unlikely. It should be pointed out that the Colorado River (Rio Colorado) delta region of Baja California Norte is certainly a logical locality to record very high summer temperatures. This near sea-level plain is surrounded by continental deserts and orographic barriers. Based upon a perusal of the individual station records, wind does not appear to be an important factor on the day when the extremely high temperatures were recorded. Apparently calm conditions or very light breezes were typical on record days. Although regional wind conditions were not studied for the days of record occurrences, air flowing

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over the high mountains to the west or from the continental interior and heated adiabatically by descent cannot be ruled out as the major causal factor for the record high temperatures. This area of northwest Mexico is dominated by the prevailing westerly winds. The delta stations are located on the leeside or eastern side of two major mountain ranges where impressive local relief is created by the Sierra Juarez (# 16 on Fig. 1) and the Sierra de San Pedro Martir (# 17on Fig. 1)where peaks reach 3000 m. In addition, two lower mountain ranges, the Sierra Cucapa (l080 m) and the Sierra EI Major, (660 m) are located immediately to the east ofthe major ranges. Although Delta and Bataques are located 18 and 43 km respectively to the east of the lower mountains, it is very possible that these topographic features create a situation where the air is heated substantially by the adiabatic process and the flowis channeled through wind gaps to produce a venturi effect as it flows out onto the plain. Another possibility for creating very high maximum temperatures in the delta area is associated with the south-west cut-offlow pressure found in this area during the summer. Continental air from northern interior localities may be pulled southward into a deepening low pressure, heating adiabatically as it flows onto the lowlands. The presence of low

THE ARID ZONES OF MEXICO

253

pressure would account for the calm air on the record days. Further, it is conceivable that the increased irrigation activity in this area with its attending higher humidities and reduced insolation, and the relocation of the instrument shelters at Bataques and Delta would account for lower maximum temperatures since the last ;;oSsoC temperatures were recorded in 1966.

Summary and conclusions The thrust of this study has been to properly characterize the climatic features of the Sonoran and Chihuahuan deserts of Mexico, and to produce a better map of the Sonoran Desert. As it is obvious from the foregoing discussion of the various delineations of the Sonoran Desert (Figs 1-13), there is a general lack of consistent, uniform, and comprehensive methodology being applied to delineating this arid zone. The Sonoran Desert map presented here (Fig. 1) is an attempt to rectify this situation and is very consistent with the recent national climatic map of Mexico (DGGTN, 1983). The climatic maximum temperature extremes recorded in Mexico's arid zones, as is often the case elsewhere, are suspect and have not been universally accepted. It is probably incorrect to dismiss the extreme maximum temperatures recorded in the delta region of the lower Colorado River. This locality certainly has the physical criteria to record extremely high temperatures, especially before the expansion of irrigation. Given the somewhat questionable validity of the maximum extreme temperatures recorded elsewhere, it seems reasonably prudent to include the delta region as one of those areas having the dubious distinction as a world leader in maximum temperature extremes. Climatic data compilation and analysis for the northern arid region (RAN) of Mexico was largely made possibleby the author's involvement with the Systems Analysisof Arid Zones (ASZA) project working jointly with the Research Center in Applied Chemistry (CIQA) in Saltillo, Coahuila, and as part of a grant to study the ecologyof the arid and semi-arid zone in the state of Chihuahua. Initial research funding was provided by the Consejo Nacional de Ciencia y Tecnologia (CONACYT) of Mexico for both projects, and from the InterAmerican Bank for ASZA. More recently the Center for InterAmerican and Border Studies at the University of Texas at El Paso provided financial assistance for the formulation of this project in the early stages of development. The author is especiallyappreciative for the assistance of Lie. Ramon Sierra Morales, [efe de la Oficinade CalculoClimatologico,SistemaNacional de Informacion in Mexico, D.F., and of Vicente Guerrero Olvera, [efe de la Oficina de Residencia de Obras de Analisis Hidrologico in Ensenada, B.C.N.

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