Time and Length of Growing season – Tropical Regions
In tropical regions where low temperature does not limit growth, the time and length of the growing season for sorghum is determined by the seasonal precipitation pattern. Kassam et.al. (1978) and Kassam (1979) used precipitation data and computations of potential Evapotranspiration (PET) (Thorntwaite, 1948) to determine the growing seasons for crops in tropical Africa. This procedure is illustrated in Figure 1 and was used to determine the time and length of growing season for sorghum.
The first day (a) wren the normal precipitation becomes equal to or greater than half the normal PET is the beginning of the growing season and earliest planting time. The last day of the growing season (c) is the day when the normal daily precipitation becomes less than half normal PET plus time required to evaporate 100 mm of stored moisture from the period when precipitation exceeds PET.
The sorghum growing seasons for different tropical areas in eastern end western Mexico are shown is Table 1. The growing season at Villahermosa which receives 1902 mm. of rainfall is 333 days. At Apatzingan (716 mm rainfall) the season is only 125 days. A study by Kassm (1979) shows that a creed relationship exists between the amount of annual rainfall and the length as growing season in Africa.
Table: Growing season as related to precipitation at two locations to tropical Mexico.
Villahermosa (170 59 N 920 55 W)
Apatzingan 190 05 N 1020 15 W)