Authors: Rahim Foroughbakhch Pournavab, Jorge Luis Hernández-Piñero, Marco Antonio Alvarado-Vázquez, Alejandra Rocha Estrada
Abstract: According to Lacoste and Salonon (1978) we may consider as arid regions those areas where shortage and irregularity of precipitations, and thermal oscillations, considerably reduce the development of life forms. Rzedowski (1959) defines them as all those regions with deficient water provision with precipitation and atmospheric humidity values usually well below the world-wide average. On the other hand Zerecero (1981) and Velasco (1991) make a greater precision about the arid and semi-arid zones defining the former as those areas with annual pluvial precipitation lower than 350 mm with an irregular distribution during the year, the average temperature oscillate between 15 and 25 °C and frequent drought periods of 7 to 12 months and with a vegetal cover smaller than 70%. On the other hand, in semiarid regions precipitation varies between 350 and 600 mm per year with a medium temperature of 18 to 25 °C, 6 to 8 moths drought and a vegetal cover higher than 70% dominated by thornscrub and grass vegetation. Despite the existing divergences between the different interpretations for the arid concept, the definition of the limits between different degrees of dryness is still more difficult to fix (Rzedowski, 1959). In this respect, Mc Ginnies (1983) mentions that it is difficult, if not impossible, to arrive at a satisfactory terminology universally accepted for the arid regions besides the differences among scientists with respect to the names used since the literature mention terms such as subhumid, semi-arid, subdesert, semidesert, steppe, desert, extremely arid and hyperarid among others. Lacoste and Salonon (1978) distinguished the following hierarchy in the arid zones on the basis of the humidity index: semi-arid or semidesert regions (rainfall generally smaller to 500 mm annual), arid regions (annual rainfall generally smaller to 200 mm) and hyperarid regions (territories where precipitation may not occur during a whole year). It is important to consider that the precipitation is not the only determining factor of the humidity degree, so numerous indices of efficiency of the precipitation have been proposed using other climatologic data for their correction, such as the temperature, the evaporation or the deficit of saturation. Another type of criterion used to establish the limits of the arid zones is based on the agricultural possibilities of a certain region. Many geographers settle down the limit based on the environment adversity for the human species. Vegetal ecologists generally look for limits based on the physiognomy and abundance of living organisms (Rzedowski, 1959).