21 Jun 2018 - 14:15

The Plant Conservation Unit (PCU) is pleased to announce the publication of an exciting new paper by Dr Roger Diamond (Department of Geology, University of Pretoria) and Sam Jack (former Research Assistant at the PCU) in the Journal of Hydrology. The article is titled, "Evaporation and abstraction determined from stable isotopes during normal flow on the Gariep River, South Africa” and was published online in April 2018. The abstract is provided below but the full paper may be downloaded here.

"Changes in the stable isotope composition of water can, with the aid of climatic parameters, be used to calculate the quantity of evaporation from a water body. Previous workers have mostly focused on small, research catchments, with abundant data, but of limited scope. This study aimed to expand such work to a regional or sub-continental scale. The first full length isotope survey of the Gariep River quantifies evaporation on the river and the man-made reservoirs for the first time, and proposes a technique to calculate abstraction from the river (see Figure below). The theoretically determined final isotope composition for an evaporating water body in the given climate lies on the empirically determined local evaporation line, validating the assumptions and inputs to the Craig-Gordon evaporation model that was used. Evaporation from the Gariep River amounts to around 20% of flow, or 40 m3/s, of which about half is due to evaporation from the surface of the Gariep and Vanderkloof Reservoirs, showing the wastefulness of large surface water impoundments. This compares well with previous estimates based on evapotranspiration calculations, and equates to around 1300 GL/a of water, or about the annual water consumption of Johannesburg and Pretoria, where over 10 million people reside. Using similar evaporation calculations and applying existing transpiration estimates to a gauged length of river, the remaining quantity can be attributed to abstraction, amounting to 175 L/s/km in the lower middle reaches of the river. Given that high water demand and climate change are global problems, and with the challenges of maintaining water monitoring networks, stable isotopes are shown to be applicable over regional to national scales for modelling hydrological flows. Stable isotopes provide a complementary method to conventional flow gauging for understanding hydrology and management of large water resources, particularly in arid areas subject to significant evaporation."

Figure: A schematic of the Gariep River, showing geographically distinct sections of river, on which different stable isotope calculations can be applied. For a tributaried river, contributions from tributaries can be calculated with one flow gauge measurement if stable isotope data exists for upstream and downstream of the confluence on the trunk stream, and for the tributary. For a reservoir, the proportion of evaporation can be calculated with only stable isotope data and if influent flow data is available, then the actual quantity of water evaporated can be calculated. For a “singular” river, where flow gauging data is available over a reach of river, the proportion of water abstracted can be calculated as that remainder of the losses after evaporation and transpiration have been accounted for.