Crop distribution is delimited by climate factors worldwide, either by a lack or by an excess of the individuals vital needs and the specific requirements of different biotypes. The plants are submitted to climate components with asynchronous variations, turned into the most determinant factors for the success of any given crop [10] - [15].

The particular needs and requirements are most commonly described in ranges, usually reported for each species or even genotypes.

The result of the diagnosis shall rely on the intervals taken into account; as a result, if optimal values are considered, the resultant potential areas may indeed provide a better yield and crop profitability.

The following criteria were considered to determine the potential areas for V. unguiculata under rainfed conditions: 1) when all of the variables interact properly with each other the area was considered as the most suitable for an optimal development; 2) when there are some restricted variables and most of the climatic and soil characteristics interact properly the area was ranked as suboptimal; finally 3) those areas with all variables considered as inadequate for cowpea cultivations were reported as marginal.

Potential areas were determined taking into account only those actual open cultivation areas; as a result, their potential is even higher nationwide [16] [17] [18] [19]. Beside climate, the altitude, soil, temperature, and annual rainfall levels were considered, as defined in Table 1.

The Arc/View version 3.3., software developed by the American company ESRI,

was the program used for data processing and analysis. Geo-referenced data was also represented including characteristics and distribution patterns [20]. All of the Arc/View activity considered project parameters, which were a collection of related documents controlled during the Arc/View session.

Projects contained as many as five different views, tables, charts, layouts (or printouts) and scripts. The documents in the Arc/View project were displayed through the Project window; properly organized and stored.

The project decided how and where such documents were meant displayed, keeping the document selection active in window program in a well-defined picture. The information was further stored in an ASCII format file, always with an *. Apr. Extension.

Arc/View was mainly a maps vector generator performed by the Arc/View 3.3, according to Figure 1, in order to take the variables intersection to generate optimal, suboptimal and unsuitable potential areas.

V. unguiculata, is an annual legume, native from Africa and widely cultivated in Africa, Latin America, Southeast Asia, and the Southern United States. It is relatively well adapted to various agroecosystems, especially semi-arid regions, humid tropics, and temperate zones. It tolerates heat and drought conditions, but does not tolerate frost [21] [22].

The semi-humid and semi-dry climate favors the growth, with temperatures from 19˚C to 34˚C [23]. However, other studies [24], suggests that this crop can survive to temperatures above 45˚C with an adequate level of soil humidity. However, flower abortion occurs and temperatures below 25˚C negatively influence

growth.

Ustimenko and BakumovskI (1982) [25], mention that flowering and ripening phase require high temperature, while low temperatures between 15˚C - 18˚C prevent the seed maturation. The optimal temperatures for its cultivation in Mexico are shown in Table 1. The temperatures considered optimal are from 25˚C to 35˚C and suboptimal from 18˚C to 24˚C and from 30˚C to 35˚C. The Mexican states that stand out because of their higher yield potential are the next: Tamaulipas, Tabasco, Guerrero, Campeche, Michoacán, Jalisco, Sinaloa, Nayarit, Veracruz, Yucatán and Quintana Roo (Figure 2).

The water requirements of this crop depend on its stage of development [26]. It is distinguished by its resistance to drought and tolerant to abundant rains, but it does not tolerate waterlogging; however, the lack of water in specific stages of development can affect germination, flowering, formation of pods, stem diameter, leaf area, number of leaves, number of pods per plant, weight of pods, weight of seeds and yield [27] - [32]. Bezerra et al. (2003) [33] report that the water deficit during the entire growth period can reduce yield up to 59%. This crop thrives in places with rainfall between 650 to 2000 mm. It is moderately tolerant to

drought, since it can grow with less than 300 mm [34] [35]. For Mexican conditions, the range of 1000 to 2000 mm was considered optimal and, suboptimal two ranges from 650 to 1000 mm and from 2000 to 2500 mm; less than 650 mm and greater than 2500 mm were ranked as not suitable areas (Figure 3).

Pandey (1990) [36], indicates that cowpea grows in many kinds of soils such as sandy, clayey, in compact soils with low water infiltration capacity, tolerating acid and neutral soil pH’s from 4.3 to 7.5 and it is little tolerant to salinity.

It is a drought-tolerant species with good atmospheric nitrogen fixation capacity, adapted to different cropping systems [37] and to a great diversity of soils but not to akalinity or salinity. It thrives well in light, well-drained, deep soils [27] - [31] [38].

In Mexico, the most appropriate soils, with loam and sandy loam textures, are the Fluvisols, Regosols, Luvisols, Nitisols, Andosols, Phaeozem and Castañozems; the Cambisols (Figure 4) represents the suboptimal production potential areas.

When cultivating cowpea, enough light hours for each growth stage are required.

The photoperiod can have great effects on reproductive development, although some genotypes are insensitive [39] [40]. ASPROMOR (2012) [41] reports that a good light favors flowering and filling of pods. The optimal photoperiod for flowering induction is 8 - 14 hours. The reduction of light favors a stunted or creeping development of the plant, with a negative effect on yields [42]. On the other hand, White (1999) [43] mentioned the important role of light in regulating plant development, mainly through photoperiod effects; this crop responds better to short days than the long ones, which delays flowering and maturity. For Mexico, the optimal conditions were encountered in areas with more than 3000 light hours; equivalent to 125 days of light during crop development; the suboptimal areas are those receiving from 2500 to 3000 light hours; areas with less than 2500 light hours ranked as not suitable (Table 1).

The crop develops favorably from sea level to 1200 meters above, demonstrating a wide range of adaptation [44]. Díaz (1997) [45] mentions that cowpea grows well at altitudes of up to 1500 meters above sea level (masl) while Pandey (1990) [36] reports ranges from 0 to 1500 masl, but optimal from 0 to 800 masl. For Mexico, altitudes of 0 to 1500 masl are recommended as the most optimal planting areas; suboptimal from 1500 to 2000; and those greater than 2000 are not suitable (Figure 5).

There were found more than 13 million hectares of high yield potential under rainfed Mexican conditions distributed in 17 states: Guerrero, Oaxaca, Campeche, Michoacán, Jalisco, Sinaloa, Nayarit, Veracruz, Yucatán and Quintana Roo (Figure 6 and Figure 7) being the most important ones, and with lower potential the states of Chiapas, Tamaulipas, Tabasco, Puebla, State of Mexico, Colima and Durango. The loamy and sandy loamy soils are more suitable for cowpea production and for Mexican conditions the Fluvisols, Regosols, Luvisols, Nitisols, Andosols, Phaeozem, Castañozems and Cambisols are considered as optimal ones (Figure 4).

Based on the agro-ecological requirements, a large surface presents favorable altitudes for cowpea production and, only those regions (central and north-central Mexico) with more than 2000 masl were considered unsuitable. Regarding the number of light hours per year, only small areas of the states of Veracruz, Puebla and Oaxaca, with less than 2500 light hours per year were not suitable (Figure 8).

On the other hand, and based on the average annual temperature, the optimal and suboptimal conditions are mainly found in the states closed to the Pacific

Sea, the Caribbean Sea, and the Gulf of Mexico, with tropical and subtropical climates and an average annual temperature between 25˚C and 30˚C. Regarding to the soils, practically all the states have optimal conditions, but the states bordering the Pacific Sea, from Baja California to Chiapas stand out for their quantity.

The average annual rainfall is one of the most influential variables to delimit the high and medium potential areas so the outstanding states are: Yucatán, Campeche, Quintana Roo, Veracruz, Guerrero, Oaxaca, Jalisco, Michoacán, Nayarit, and Sinaloa.