Trees and Water in the Soil

Trees effect on water does not stop with occult precipitation. Trees, shrubs and grasses are very important in stopping soil erosion and enhancing moisture penetration in soils. When rain falls, it builds up energy according to drop size and speed. The large drops of East African thunderstorms contain high energy that is released when it hits the ground. On bare earth this can loosen topsoil, causing it to move down a slope with runoff. This is termed sheet erosion. Sometimes this water, containing topsoil, forms rivulets that dig deeper into soils causing gullies. Trees and grasses break the energy of falling rain. Instead of loosening soil, it follows the stem down, gently reaching the ground. In a `trees only' area runoff and gullying still occurs, but not as rapidly. With trees and grasses, water would be trapped, and penetrate the soil. Many eroded sites in Kenya, especially in semi-arid areas, would look far different if a tree and grass cover remained.

Rainwater, after entering the soil, has to go somewhere. Often it follows tree root channels, even reaching through hardpans, recharging the water table. From there it may reappear in springs, or be recaptured by roots and re-enter the atmosphere via transpiration. Some evidence indicates that deforested areas may lose their water tables, causing springs to dry. Reforestation can recharge this water table by increasing occult precipitation in appropriate areas, decreasing surface runoff, and increasing soil penetration. This effect has not been confirmed conclusively but there is important evidence that this happens. Removing a forest from a city watershed supply adversely affects both quantity and quality of the supply.

On steep slopes trees play an even greater role in erosion control. Tree roots, unlike most grasses, and shrubs like tea or coffee, penetrate deeply into the soil. Their great shear strength prevents soil slippage on slopes of more than 30 percent. Some tea fields grow on slopes greater than this. Tea plants do capture all the water falling on a field, but their roots are not deep. Water builds up in the upper layers of the soil creating a situation where the upper layers can slip over the dryer, deeper layers. Trees, like Grevillea robusta , planted at 10 x 20 metre spacing in these fields, would greatly reduce this problem without affecting yields. Other steep farmed slopes should include trees for this reason, unless they are properly terraced.