1. A Beginning Period
The beginning of the growing period occurs when precipitation (PPTN) equals half PET and marks the start to the normal rainy season, shown as a in fig. 1. A value of one half PET has been chosen as germinating crops do not evapotranspire at the full rate of PET and false starts to the rainy season are eliminated. The beginning marks the transition from the dry period to the "intermediate" period when PET/2 < R < PET.
2. A wet (humid) Period
This is the period during which precipitation exceeds PET. The beginning and ending dates (shown as b and c in fig. 1, respectively) are the two points where the precipitation and PET curves cross.
3. An End to the Growing Period
The end of the growing period occurs at the point where the precipitation curve crosses the one half PET curve (labeled as d in fig. 1).