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

Abstract

There is a growing demand for increased water abstraction from ephemeral rivers such as the Kuiseb in western Namibia. Studies in the 1980.s recorded mortality rates of the most common riparian tree species in a prolonged drought from 1979 to 1984. These results show that mortality for the three species differed considerably with 16 % mortality for Acacia erioloba, 39 % for Faidherbia albida and 75 % for Tamarix usnoides. Here we determine the water sources and rooting strategy of three age groups of the three most common riparian tree species growing along the Kuiseb River using stable hydrogen and oxygen isotope analysis. We do this to better understand the relationship between germination and establishment of trees along river courses in hyperarid western Namibia. A secondary objective is to determine whether the mortality rates recorded in the drought in the 1980.s may be related to rooting strategy. We use a linear mixing model approach (IsoSource) to quantify probable contributions of multiple water sources to tree water uptake. Our results show that none of the tree species in this study use fog water. Rather, all of the trees are reliant on a seasonally fluctuating combination of groundwater, shallow soil water and deep soil water. All of these water sources are directly reliant on regular recharge from annual flood events. Our results also show that the mortality rates recorded in the early 1980.s need not necessarily relate to rooting depth or water source but that there may be a combination of possible causes including root growth and structure. If predictions for increased water abstraction and global climate change are realized then the vegetation structure along ephemeral river courses in Namibia will be seriously threatened.