The relationship between fog, floods, groundwater and tree growth along the lower Kuiseb River in the hyperarid Namib

At our study site in the western half of the Namib Desert rainfall is extremely low averaging between 0 and 12 mm a year. Fog events are, however, more frequent, occurring on average 60–200 days per annum. Research has demonstrated that where there are frequent fog events trees have adapted to intercept and utilise the moisture from these events. In hyperarid environments such as at our study site trees are confined to the banks of ephemeral rivers. Here we determine the extent to which these trees are able to utilise both deep groundwater as well as fog water. The results of our model show that none of the trees in this study are able to utilise fog water even though fog events may contain considerable amounts of water. Rather, all of the trees are reliant on a seasonally fluctuating combination of groundwater, shallow soil water and deep soil water. These water sources directly depend on Kuiseb aquifer recharge resulting from flood water infiltration. If predictions for increased water abstraction and global climate change are realized and water levels of the aquifer decline substantially then the vegetation structure along ephemeral rivers such as the Kuiseb will be seriously threatened.

Research highlights
► Riparian trees in Namib do not use fog water
► Riparian trees use a mix of soil and groundwater
► Soil and groundwater along Kuiseb is recharged during floods
► Regular flood events essential for survival of riparian species in Kuiseb