In the wet-dry tropics the supremacy climate factor is the water deficiency. By an anual rainfall of 500 to 1500mm ( = 500 to 1500 l water per m² and year), more rain evaporates than falls.
The precipitaions fall while rains, usually from 3 to 7 months. While these months, the water supply is temporarily sufficient.
The vegetation shows a downright period of activity. The productivity of biomass rises with a rainy season from 7 tons per hectare and year in desert-near regions through to 20 tons per hectare and year in equator-near evergreen forests.
In the remaining months, the dry season, there is nearly no precipitation and the middle temperatue of the month and also the evapoartion are increasing. So there are months of droughts, which can just be survived by some plants which are appointed to these special conditions.
You defferentiate the semi-humid zones depending on the quanty of humid months (humid
* = anual precipitations are higher than evaporations) and dry months ( arid
* = anual precipitation is lower tha evaporation) .