Could planned reforestation in deforested areas reduce floods? Could this ultimately reduce flood insurance premiums?
At its root, the flood equation is quite simple. If a river cannot handle the load of water it is required to carry, it must rise. With enough water, it must rise above its bank and flood. With excessive deforestation floods are more likely.
The faster water runs from the watershed into the river, the higher a flood rise. Increases in runoff speed such as excessive pavement or ditching of farmland, will contribute to floods.
Deforestation plays several roles in the flooding equation. Trees prevent sediment runoff. Forests hold and use more water than farms or grasslands.
Deforestation is key in the flooding equation.
- Rainwater that remains on leaves evaporates directly into the air.
- The shape and type of leaf minimises raindrop impact onto the underlying ground. Therefore less soil erosion. Leaves that do fall increase the nutrient content of the soil.
- Tree roots absorb water from the soil, resulting in a drier soil. This soil is able to store more rainwater.
- Root formation of trees holds soil in place, thus less movement of sediment.
In 1998 the huge Yangtze flood was a direct result of 85% loss of trees. This loss had occurred during the previous few years. Loss of wetlands and river engineering added to the dilemma.
Cropland covers about 16 million square kilometres. This is the size of South America. Global pastureland occupies more than 30 million square kilometres. This is the extent of Africa.
Agricultural land covers about 40% of the Earth’s land surface. Creating additional farmlands would require the destruction of other Ecosystems such as tropical rain forests.
We lack the strong research base necessary to accurately quantify the anti-flooding benefits of planting trees. Large scale studies cost money. Scientists have difficulty repeating experiments. No two catchment areas are the same.