Title:

Water sourcing by riparian tree species in ephemaral rivers

Publication Year:
2010
Abstract:

In arid southern Africa, many rivers are ephemeral, and surface flow occurs for less than 10% of the year. Floods, which occur erratically, recharge groundwater resources on which riverine vegetation depends. Trees and shrubs are the most common life forms along these river systems, because surface flows are too erratic for aquatic organisms to flourish. There is concern that alterations to the current water regime of ephemeral rivers will affect the distribution of riparian vegetation as trees provide valuable fodder, shade, firewood and construction material in otherwise inhospitable surroundings. Climate change, population growth and development needs all place growing pressure on these water-limited ecosystems. Careful catchment management is essential to meet human water needs without jeopardising the ecosystems. This is easier said than done however, as ephemeral river ecology and functioning are hardly understood. Their unpredictable hydrology, strong groundwater association, often remote location and the presence of large terrestrial life forms make ephemeral rivers difficult to understand and current ‘Environmental water allocation’ methods for wetter systems are unsuitable. Alternative methods are being developed, but to date they still have a strong emphasis on aquatic life forms and riparian vegetation is still insufficiently addressed. This study provides an ecophysiological perspective on the water sources and drought tolerance levels of four key riparian tree species in Southern Africa. The aim is to contribute to an improved understanding of the interrelationships between river flow, groundwater and use of water by trees. Stable Hydrogen and Oxygen isotope results show that the three key species along the Kuiseb River (Acacia erioloba, Faidherbia albida and Tamarix usneoides) do not take up any water from the regularly occurring fog events. Instead, they depend on a seasonally fluctuating mix of shallow and deep soil moisture as well as groundwater. All these water sources rely on regular recharge from floods. It suggests that reductions in flood frequency and especially in magnitude and duration will reduce groundwater recharge and affect species productivity and survival.

Place:
University of Cape Town
Type:
PhD Thesis
Item Type:
Thesis
Language:
en

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