Rainfall Patterns as Background to Plant Phenology in Northern Tanzania

The rainfall pattern in time and space within the Masai Ecosystem of northern Tanzania was studied using rainfall data of sixteen stations within the area. Annual amounts of rainfall were found to be unpredictable, and the annual deviations of the long-term mean annual rainfall were without cyclic trends for the individual stations. However, the pooled rainfall data of the different stations show that series of wet and dry years alternate. Total annual rainfall increases with altitude and the variability in rainfall decreases with altitude. It appears that rainfall in the dry season is caused by thunderstorm activity, and no relation between rainfall and altitude exists during this time of the year. Rainfall in the wet season shows a very different spatial relationship, indicating the effects of the monsoon, and, especially during the long rains, rainfall increases with altitude. Different seasons can be discerned, and the data indicate that the amount of rain in the long rainy season is more predictable than the amount in the short rainy season. The differences are discussed briefly with regard to plant phenology patterns and grass growth.