A study of isotopic composition of xylem water of woody vegetation and groundwater along a precipitation gradient in Namibia

A mini thesis submitted in partial fulfilment of the requirements for the Degree of Master of Science (Applied Geology)

An understanding of the water used by vegetation in water limited environment is critical to fully understand water relations of natural areas with vegetation. Such information can be integrated in water management plans to estimate the influence of groundwater abstraction on the vegetation. Trees and shrubs are able to access water from: the upper unsaturated soil profile, the capillary zone of a groundwater store, from nearby streams and rivers. Previous studies have proven that uptake of water by roots is not associated with isotope fractionation. Stable isotope ratios of oxygen and hydrogen were analyzed in groundwater, surface water and plant xylem water. Groundwater samples from the north east part of the country (Enyana and Fair constantia) were most depleted, while samples from the southern part (Guruchas) were enriched. A very weak negative correlation (R2=0.07), statistically insignificant (P>0.05) relationship has been noted between δ18O values of precipitation and altitude. The estimated depletion rate was 1.3‰ δ18O per km. The correlation between distance from the coast and δ18O composition of precipitation was negatively weak (R2=0.29) and statistically significant (P<0.05) with an estimated depletion rate of 0.31 ‰ per 100 km inland. The correlation between longitude and δ18O composition of precipitation was very weak (R2=0.016) and statistically insignificant (P>0.05) at roughly 0.09‰ δ18O depletion per degree. A strong negative correlation (R2=0.64) existed between δ18O of precipitation and latitude and it was statistically insignificant (P>0.005). The observed high variability in δ18O and δ2H values of groundwater at different sampling sites was attributed mainly to continental, elevation and amount effect. Both groundwater and xylem water samples plotted below the GMWL, however plant xylem were more depleted in comparison to groundwater. Throughout all the sampling sites there was a considerable variation in average isotope ratio of xylem water of different species examined, but there were still noticeable patterns regarding their main water source. The results displayed that P. juliflora, A. hebeclada and V. erioloba relied mainly on soil water for transpiration. S. mellifera mostly utilized a mixture of groundwater and soil water while B. albitrunca, C. mopane, C. imberbe and T. sericea and R. trichotomum predominantly utilized groundwater. The water uptake pattern is highly attributed to the rooting morphology. These vegetation uptake groundwater and soil water in a hierarchical manner at different depths to avoid competition between species.