RAINFALL INTERCEPTION AND PARTITIONING IN AFROMONTANE RAIN FORESTS OF THE EASTERN ARC MOUNTAINS, TANZANIA: IMPLICATIONS FOR WATER CONSERVATION

Abstract

The aboveground components of the hydrologic cycle of rain forest
are important processes that determine the hydrologic behaviour and dynamics of these ecosystems. Precipitation, throughfall, stem flow, canopy interception and streamflow were measured and modelled in two afromontane rain forests of the Eastern Arc Mountains of Tanzania. Measurements were made daily for 50 months in the Ulugurus and 56 months in the Usambaras. Throughfall was 76% of the gross rainfall in the Usambaras and 79Vo in the Ulugurus. Stemflow was less than 2Vo of rainfall. Both parameters were correlated with gross rainfall (r² = 0.97, 0.99, 0.86 and 0.94).
Canopy interception was 22% of rainfall for the Usambaras and 20% for the Ulugurus. Streamflow was best modelled using five months running mean rainfall in the Ulugurus (r = 0.67) and three to four months in the Usambaras (r = 0.72). A high proportion of the gross rainfall is delivered as net precipitation. There is effective partitioning of rainfall on the forest canopy providing reasonable dumping effect of rainstorms. The slow response in streamflow to rainfall events shows the efficiency of the forests to mitigate storm water impacts and thus ensuring continuous water supply. Further studies in effects offorest harvesting on water yield are pertinent to enable adequate planning
of forest utilization.