Rainfall varibility and its implications for the transferability of experimental results in the semi arid areas of Tanzania

Abstract

Most of agricultural activities in semi-arid areas of Tanzania depend on direct rainfall. Conse- quently, any significant variation in the temporal and spatial distribution of rainfall usually results in serious shortage of soil-water available to plants and thus poor crop and livestock production. In this paper the variability and reliability of rainfall in four representative areas of the semi-arid lands in Tanzania namely Hombolo (Dodoma); Morogoro (Morogoro); Kisangara (Kilimanjaro) and Ngudu (Mwanza) is examined. The start arid end of the rains and the occurrence of long dry spells within the seasons are also analysed. The result showed that during the long rainy season, the seasonal rainfall for Kisangara and Morogoro were only 525 mm and 445 mm respectively. During the short rains, the seasonal average rainfall was 359 mm and 327 mm for Kisangara and Morogoro respectively. For Hombolo which has monomodal rainfall, the seasonal mean was 592 mm. Apart from seasonal mean rainfall being low, the seasonal rainfall variability expressed by the coefficient of variation (CV) was also observed to be high. Kisangara and Morogoro had CV's of 20% and 50% during long rainy season respectively. During short rainy season the CV's were 49% and 42% respectively. Hornbolo had a CV of 24%. Analysis of long-term rainfall characteristics for Morogoro and Kisangara revealed that these areas experience dry spells of various lengths during the seasons. At Kisangara for example, dry spells of 15 days with 30% probability may occur during short rainy season, while in long rainy season dry spells of more than 14 days at 30% chances do occur in the rest of the months except April. These results form a good basis for explaining the performance of soil-water conservation measures including rainwater harvesting in the semi-arid areas of Tanzania.