Abstract:
The aboveground components of the hydrologie cycle of rain forest
are important processes that determine the hydrologie 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 79% in the Ulugurus. Stemflow was less than 2% 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 of forest harvesting on water yield are pertinent to enable adequate planning
of forest utilization.
