Effective Precipitation

Rainfall in the savanna ecosystem is characterized by its seasonality (Goldstein & Sarmiento, 1987).  This is true for most of Namibia, including the dry savannah woodland areas.

The average rainfall for the dry savannah woodlands in Namibia ranges from 400mm (just north of Grootfontein) to about 750mm (in the Caprivi) per year, falling from the end of September to the beginning of May.  Much of this precipitation is in the form of thunderstorms, sometimes with extended dry periods in between.  In other periods, thunderstorms are very regular every day, towards evening (pers obs).

Rainfall should not only be viewed in terms of quantity but also its temporal distribution.  While some consider rainfall to occur in cyclic patterns of low-high-low rainfall Chivell & Mostert (1991) were unable to confirm such patterns for an area bordering the dry savannah woodlands.

The effect of rainfall lies in the amount of water that is able to infiltrate the soil, to becomes available to the plants in the area.  Very short, low intensity showers may not be effective since much of the water is intercepted by overstory vegetation or evaporates from the soil surface and in the upper soil layer before it reaches the root zones of the plants.  Heavier showers have more effect.

It is important, however, that the effectiveness of rainfall is regarded in relation to the soils.

Infiltration rates are higher when the soil is already slightly moist (Foth 1990).  It can be observed that raindrops falling on very dry soil remain there as a drop, only slowly draining into the soil.  This of course increases the potential for evaporation.
See also:
Plants and soil moisture
The soil effects on water availability

Interception by Woody Plants

The characteristic umbrella shaped crown of many woodland trees (such as Pterocarpus angolensis, Burkea africana, Schinziophyton rautanenii, Terminalia sericea and some Commiphora species) serves to intercept rainfall and concentrate it around the roots of such trees ().