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Trees and Rain

Is it correct to say, like those wazee at the turn of the century, that trees bring rain? There is even an Ethiopian proverb that insists, "Trees pull the rain", and we recognize the truth in it. Most of us realize that there is a relationship between well-forested land and total rainfall. The question remains; which came first, the rain or the trees, because we are talking of the huge hydrological cycle on our earth, and the way it manifests itself in different places.

Recent evidence from scientific studies in the Amazon rainforest indicates that trees do have an influence on total rainfall. According to this study, 75% of all rainfall recirculates into the atmosphere via evaporation and transpiration over heavily forested areas. Only 25% runs off over the land to streams and rivers. In areas extensively cleared for cultivation or pasture, the situation reverses, as 75% runs off and only 25% is recirculated. (3) This reduces atmospheric moisture and thereby reduces rainfall downwind of the cleared zone.

But Kenya is not the Amazon. Our forests are not extensive, and the impact of recirculated moisture is much smaller. Perhaps there is some effect, but this is probably too small to measure. Kenya's rainfall pattern is governed primarily by the movement of the monsoon in combination with the highlands and mountains. Mt. Kenya, the Aberdare Range, the Cherangani Hills, Mau Escarpment, Mt. Elgon, and even the lower ranges like Mt. Marsabit, Mt. Kulal, and the Chyulu Hills affect their surrounding climate by slowing down air movement, forcing it higher, which causes condensation into clouds and then rainfall. This will happen whether the forest is there or not.

Yet there are important, water-related reasons for preserving our forests and creating new ones, the first concerns these very clouds that the mountains help create. These wrap the mountain in a cloak of moisture-laden air. It may not rain, yet the trees seem to drip as if it were. What happens is this: water droplets from the clouds drift by the trees. When a droplet touches a leaf, it sticks. Other droplets join it, eventually creating a large drop. This drop is forced by gravity to fall from the leaf to the ground. The number of drops reaching the ground depends on the surface area of leaves available for droplets to touch. A forest provides far more surface area than grass or bare ground. This kind of precipitation is called ‘occult’ precipitation and can amount to over 500mm annually if conditions are right. Some people in the Huri Hills, west of Marsabit, have succeeded in simulating this method of occult precipitation by erecting large plastic sheets to catch the fog-laden moisture. It can be a significant source of water for many uses.

Many hilly areas in Kenya are prime areas for catching occult precipitation if trees are re-established. Even now, some of the higher mountains in Turkana District are covered with this mist forest, dominated by species like Juniperus procera and Podocarpus gracilior. In Machakos, many hills of similar elevation have been cleared for agriculture. What effect this has had on streams in the area is not documented, but older people talk convincingly of times when water-courses flowed year round. The absence of occult precipitation certainly has an impact on this. Reforesting hill tops may not miraculously bring back year round streams, but it will positively influence the local environment in other ways.


3. Bayard Webster, IDRC Reports No. 4, Volume 12, International Development Research Centre, Ottawa.