By HENRY OWINO (Senior Correspondent)
Climate change projections indicate that West Africa will experience an increase in the number of extreme rainfall events in the 21st century.
Results from a study carried out by scientists suggest that tree cover is key not only to making this an opportunity to increase soil and groundwater recharge but also to avoiding escalated land degradation.
If tree cover is absent, there is a higher risk for rainfall to be lost either through evaporation or overland flow. In contrast, when trees are present, rainfall is more likely to infiltrate into the soil and contribute to deep soil and groundwater recharge.
These were research finding by scientists from World Agroforestry (ICRAF), Swedish University of Agricultural Sciences (SLU) and Université de Ouagadougou that was funded by the Swedish Research Council (Vetenskapsrådet) and the Swedish Research Council Formas.
These results suggest that maintaining or promoting appropriate tree cover in tropical African drylands may be crucial to improving deep soil and groundwater recharge under a future climate with more heavy rainfall.
Knowing how changes in tree cover, either climate or human induced will affect soil and groundwater recharge under different scenarios of rainfall intensity is vital to plan sound strategies for adaptation to climate change.
Moreover, this information should be of great interest for large scale, tree‐based landscape restoration programs in the region, such as the African Forest Landscape Restoration Initiative or the Great Green Wall of the Sahara and the Sahel.
Specific role of tree cover in enhancing deep soil-water drainage
In semi-arid West Africa, rainfalls are characterized by a high spatial, intra and inter annual variability. Annual rainfall is concentrated in a single, relatively short, rainy season that occurs between May and October.
Rainfall intensities are high and a large proportion of annual rain falls during very intense storms.
Soils in the semi-arid tropics and in semi-arid West Africa in particular, are typically sensitive and vulnerable to degradation, which is mainly a result of their low structural stability, especially when soil organic matter is low.
The prevalence of high rainfall intensities, coupled with the physical characteristics of these soils, frequently leads to the formation of crusts on the soil surface. These crusts reduce water infiltration, resulting in enhanced overland flow and limited soil and groundwater recharge, which can negatively affect primary production, local water supplies, ecosystem services and the livelihoods of local people.
In most soils, the recharge of soil and groundwater occurs via a two‐domain flow process, that is, both through the soil matrix and through macropores. The soil matrix consists of solid particles and voids filled with water and air.
Macropores are large soil pores, generally greater than 0.08 mm in diameter. Macropores drain freely by gravity and allow easy movement of water and air. They provide habitat for soil organisms and the roots of plants can grow into them.
When water flow occurs primarily via matrix flow, the recharge process is typically slow. By contrast, water flow along macropores, also known as preferential flow, is much faster and leads to deeper water drainage.
In a previous study in the same area, the researchers found that the degree of preferential flow decreased with increasing distance to the nearest tree stem and that it was higher in small as compared to large open areas among trees. They concluded that this was likely the result of trees increasing the amount of macropores through the combined effect of leaf litter, root and faunal activity, and microclimate.
Macropores serve as pathways for the preferential flow of water, therefore, the degree of preferential flow will often decrease gradually from the vicinity of a tree towards an open area. This is particularly so in the case of ‘funneled preferential flow’, which occurs around the base of tree stems where stemflow concentrates.
The radius of influence of individual trees on enhancing preferential flow will largely depend on their root system and canopy architecture, in particular, on the radial extent of their lateral roots.
For example, in Burkina Faso, roots of Sarcocephalus latifolius, a native tree of West Africa, were found up to a distance of 20 metres from the trunk. It is therefore every likely that in the area the researchers studied, tree roots extended well beyond the canopy edge of trees into the open areas, which would explain why small open areas (radius 6–13 m) had a higher degree of preferential flow compared with large ones (radius 22–30 m).
That small open areas have a higher degree of preferential flow means that a larger portion of the infiltrating soil-water moves faster through the soil profile and penetrates more rapidly to deeper soil depths. This could explain why small open areas received more soil-water drainage when rainfall intensity increased.
In contrast, in large open areas, which were further away from the influence of trees, soil-water flowed mainly through the soil matrix and penetrated more slowly.
Implications for landscape management
In conditions typical of the semi-arid tropics, macropores are needed to enable the recharge of deep soil-water under increased rainfall intensities. In the absence of macropores, more intense rainfall events could lead to increased recharge of topsoil water but because matrix flow is a slow process, a large fraction of this water would likely return to the atmosphere as evaporation and never contribute to deep recharge, especially if evaporation increases as a result of global warming.
Because trees and associated soil fauna enhance macroporosity and preferential flow, maintaining and promoting a moderate tree cover might be a good strategy to improve deep soil and groundwater recharge under a future climate with more frequent heavy rainfall events.
However, tree cover should not be too high; otherwise, transpiration and interception losses from trees would counteract any beneficial effects they might have on deep soil and groundwater recharge.
In the same study area, the researchers found that there was an optimum tree cover that maximized groundwater recharge, which reflects the balance between the positive and negative effects of trees.
The optimum tree cover represents a threshold below which increasing tree cover leads to improved groundwater recharge whereas above this threshold more trees result in reduced water yields. In water limited environments, understanding the potential thresholds in the relationship between tree cover and water availability is critical.
In a similar study in China’s semi-arid Loess Plateau, researchers have estimated the threshold at which additional revegetation in the area will cause a shortage in the water supply for human activities. Additionally, they found that this threshold could be significantly reduced in the future owing to climate change and increased water withdrawals and called for a better match of species and planting density in large‐scale restoration programs.
In line with the growing awareness of the important relationship between tree cover and groundwater recharge, more research is needed to better understand how this relationship will change in response to projected changes in rainfall intensity.