Protection of rain forests = Protection of watershed

As the moist sea air flows from the Indian Ocean towards the continent it first encounters the East Usambara mountains plateau levelling off at about 1000 masl. Part of the moisture condenses to water and rain. The average annual rainfall in the plateau is about 2000 mm, which is about two times higher than in Tanga town. The climate is monsoonal, the rainy seasons coinciding with the Nothward and Southward passages in March to May (the long rains with usually the most rain) and October to December (the short rains). However, there tends to be occasional precipitation even during the dry seasons. Relative humidity is high.

The eastern slopes of the of the East Usambaras are drained by Muzi, Semdoe and Kihuhwi rivers which flow into the Sigi-river, which itself drains most of the Amani plateau region. The total area of watershed of Sigi is about 1100 km2. The rivers flows into the sea North of Tanga town. The Sigi is the main source of the water supply for the town population. Moreover, in the mountains a population of about 100,000 utilise water from the river. The tea factories in the mountains, sisal farms in lowlands and other industry in the area are also dependent on water supply from the Sigi. The western slopes of the East Usambaras drain into the Lwengera river, which flows into the Pangani. The Pangani river supplies water resource to a power plant situated in the downstream.

The rain forests and water

The sub-montane forests of the East Usambaras enhance the continuous supply of good quality water:

The water drops hit the twigs in the tree canopy and drop gently down to surface or run down along the trunk. The water soaks into the soil, from where part of it slowly flows to the rivers or groundwater, as the other part evaporates straight or via plants back to the air. Crop residue left on the soil surface, roots and livings organisms maintain the quality of the underground soil and keep it light and porous.

Without the vegetation cover the rain falls straight into the soil surface and flows along the surface to the rives. During the rainy seasons the floods increase and the soil does not have qualities to store water for the dry season. Due to erosion the topsoil is lost, the water gets dark and humous, and water collection ponds are filled with silt. The steepest slopes are most vulnerable to erosion. In the beginning of 1990's lots of villagers were drowned in the West Usambaras because of severe floods. Villages and houses were damaged so badly that population had to be shifted to the roots of the east Usambara mountains. Part of the cultivation fields for the new population were cleared in natural forest.

Part of the moisture captured by the trees and other vegetation in the East Usambaras is later evaporated again. This moisture may turn back to water and rain in the inland Tanzania. Thus the mountains and the sub-montane rain forests have remarkable positive effect on the climate and the living conditions also in a larger area.

The forests stabilise the climate so that the mountains are often covered by the clouds and mist. That kind of steady micro-climate is required both by the rare sub-montane vegetation and many of the food and cash crops, especially tea. Protection of forests means protection of water resources The total area of natural forests is decreasing continuously and is now less than half of the original area. Today, the clearance of small-scale cultivation fields "shambas" is the most significant reason for the decrease of natural forests. In addition to high birth rate there are lots of migrants who come to temporary or permanently to work in tea estates or who as seeking for cultivation land.

The settlements are spreading into steeper areas. Terraces and the sheltering plantations are still rare in steep slopes and the soil gets poor soon. The riversides are cleared for cultivation without buffer zones. So the conversion of the forests into the fields decrease the state of the watershed.

East Usambara Catchment Forest Project aims at maintaining the unique rain forests. Protection of the forests is complementary to watershed management. The steady micro-climate maintained by the forests favour also cultivation. The success in nature conservation improves the supply and the quality of water resources downstream in Tanga town.

Banning the industrial loggings has already improved the condition of the watershed. The next steps are: to improve and co-ordinate the land use in cultivation areas, help the local villagers to maintain the water collection areas and to increase the amount of sheltering plantations.