The importance of tree cover for water resources in semiarid West Africa
The current paradigm in forest hydrology implies that an increase in tree cover always leads to reduced water yields as a result of increased interception and transpiration (ET) losses. This trade-off theory, in which more trees mean less water, has led to concerns that the establishment of trees in drylands may jeopardize scarce water resources. But in the seasonally dry tropics relevant studies are scarce, and few have explored the impact of intermediate tree densities on water yields in degraded soils, which greatly limits the applicability of the trade-off theory in this region.
Here, I propose an alternative optimum tree cover theory in which, under conditions typical of the seasonally dry tropics, groundwater recharge is maximized at an intermediate tree cover. At tree covers below this optimum, the gains from more trees on soil hydraulic properties exceed their additional ET losses, leading to increased groundwater recharge. The overall aim of this thesis is to test this hypothesis and to clarify the main processes influencing the relationship between tree cover and groundwater recharge. To do this, a number of measurements were taken in an agroforestry parkland in semiarid West Africa; these included soil infiltrability, soil water drainage, tree transpiration and degree of preferential flow, in combination with stable isotope data.
Results from this thesis show that deep soil water drainage was minimal near the tree stem, reached a maximum close to the canopy edge and from there decreased linearly with increasing distance to the nearest tree. This pattern is probably the result of a combination of increased ET losses next to the tree and reduced infiltrability and preferential flow with increasing distance from the nearest tree. The combined increase in infiltrability and degree of preferential flow close to trees allows for enhanced soil and groundwater recharge. Tree transpiration data were used in combination with the observed pattern in soil water drainage and data on tree water sources to model groundwater recharge as a function of tree cover. Modelling results confirm that groundwater recharge was maximized under intermediate tree cover irrespective of the scenarios considered. That trees do not always reduce water yields but can substantially improve them suggests new opportunities for tree protection and tree-based restoration in the seasonally dry tropics, benefitting hundreds of millions of people.
Bargués Tobella, A. (2016). The importance of tree cover for water resources in semiarid West Africa. Doctoral Thesis. Department of Forest Ecology and Management. Faculty of Forest Sciences. Swedish University of Agricultural Sciences (SLU). Umeå.
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