Shoopala Uugulu
@unam.edu.na
Geology
University of Namibia
Scopus Publications
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Scopus Publications
Shoopala Uugulu, Heike Wanke, and Paul Koeniger
Springer Science and Business Media LLC
AbstractWoody plants play a significant role in the global water cycle through water uptake by roots and evapotranspiration. A deuterium tracer was used to assess the active root depths for Salvia mellifera and Boscia albitrunca in the Ebenhaezer area (western Namibia). The tracer was inserted at different soil depths in December 2016. Xylem cores were obtained using an increment borer, and transpired water was collected using transpiration bags zipped around the plants’ leaves. Groundwater was collected from boreholes. Soil samples were collected after the rainy season using a hand auger. Xylem and soil water were extracted using a cryogenic vacuum extraction method and analysed for stable water isotopes. Only one S. mellifera transpiration sample showed a high deuterium content (516‰) where the tracer was inserted at 2.5-m soil depth. Elevated deuterium contents were observed in two S. mellifera xylem samples; tracer had been applied at 2.5 and 3 m depth (yielding 35 and 31‰ deuterium, respectively), which constitutes a possible active-root depth range for S. mellifera. At the end of the study period (May 2017), the average δ18O value for B. albitrunca xylem samples was similar to that of groundwater. The δ18O value for S. mellifera was between that of soil water and groundwater, indicating that this species uses groundwater and soil water available for groundwater recharge. Determination of the active root depth and source water for these species would help improve hydrological modelling by incorporating the influence of woody plants on groundwater recharge.
S. Uugulu and H. Wanke
Elsevier BV
© 2020 Elsevier Ltd The quantification of groundwater resources is essential especially in water scarce countries like Namibia. The chloride mass balance (CMB) method and isotopic composition were used in determining groundwater recharge along a precipitation gradient at three sites, namely: Tsumeb (600 mm/a precipitation); Waterberg (450 mm/a precipitation) and Kuzikus/Ebenhaezer (240 mm/a precipitation). Groundwater and rainwater were collected from year 2016–2017. Rainwater was collected monthly while groundwater was collected before, during and after rainy seasons. Rainwater isotopic values for δ18O and δ2H range from −10.70 to 6.10‰ and from −72.7 to 42.1‰ respectively. Groundwater isotopic values for δ18O range from −9.84 to −5.35‰ for Tsumeb; from −10.85 to −8.60‰ for Waterberg and from −8.24 to −1.56‰ for Kuzikus/Ebenhaezer, while that for δ2H range from −65.6 to −46.7‰ for Tsumeb; −69.4 to −61.2‰ for Waterberg and −54.2 to −22.7‰ for Kuzikus/Ebenhaezer. Rainwater scatters along the GMWL. Rainwater collected in January, February and March are more depleted in heavy isotopes than those in November, December, April and May. Waterberg groundwater plots on the GMWL which indicates absence of evaporation. Tsumeb groundwater plots on/close to the GMWL with an exception of groundwater from the karst Lake Otjikoto which is showing evaporation. Groundwater from Kuzikus/Ebenhaezer shows an evaporation effect, probably evaporation occurs during infiltration since it is observed in all sampling seasons. All groundwater from three sites plot in the same area with rainwater depleted in stable isotopic values, which could indicates that recharge only take place during January, February and March. CMB method revealed that Waterberg has the highest recharge rate ranging between 39.1 mm/a and 51.1 mm/a (8.7% – 11.4% of annual precipitation), Tsumeb with rates ranging from 21.1 mm/a to 48.5 mm/a (3.5% – 8.1% of annual precipitation), and lastly Kuzikus/Ebenhaezer from 3.2 mm/a to 17.5 mm/a (1.4% – 7.3% of annual precipitation). High recharge rates in Waterberg could be related to fast infiltration and absence of evaporation as indicated by the isotopic ratios. Differences in recharge rates cannot only be attributed to the precipitation gradient but also to the evaporation rates and the presence of preferential flow paths. Recharge rates estimated for these three sites can be used in managing the savannah aquifers especially at Kuzikus/Ebenhaezer where evaporation effect is observed that one can consider rain harvesting.
Katja Geißler, Jessica Heblack, Shoopala Uugulu, Heike Wanke, and Niels Blaum
Frontiers Media SA
Introduction: Many semiarid regions around the world are presently experiencing significant changes in both climatic conditions and vegetation. This includes a disturbed coexistence between grasses and bushes also known as bush encroachment, and altered precipitation patterns with larger rain events. Fewer, more intense precipitation events might promote groundwater recharge, but depending on the structure of the vegetation also encourage further woody encroachment. Materials and Methods: In this study, we investigated how patterns and sources of water uptake of Acacia mellifera (blackthorn), an important encroaching woody plant in southern African savannas, are associated with the intensity of rain events and the size of individual shrubs. The study was conducted at a commercial cattle farm in the semiarid Kalahari in Namibia (MAP 250 mm/a). We used soil moisture dynamics in different depths and natural stable isotopes as markers of water sources. Xylem water of fifteen differently sized individuals during eight rain events was extracted using a Scholander pressure bomb. Results and Discussion: Results suggest the main rooting activity zone of A. mellifera in 50 and 75 cm soil depth but a reasonable water uptake from 10 and 25 cm. Any apparent uptake pattern seems to be driven by water availability, not time in the season. Bushes prefer the deeper soil layers after heavier rain events, indicating some evidence for the classical Walter’s two-layer hypothesis. However, rain events up to a threshold of 6 mm/day cause shallower depths of use and suggest several phases of intense competition with perennial grasses. The temporal uptake pattern does not depend on shrub size, suggesting a fast upwards water flow inside. δ2H and δ18O values in xylem water indicate that larger shrubs rely less on upper and very deep soil water than smaller shrubs. It supports the hypothesis that in environments where soil moisture is highly variable in the upper soil layers, the early investment in a deep tap-root to exploit deeper, more reliable water sources could reduce the probability of mortality during the establishment phase. Nevertheless, independent of size and time in the season, bushes do not compete with potential groundwater recharge. In a savanna encroached by A. mellifera, groundwater will most likely be affected indirectly.
Zhihong Li, Guangcai Wang, Xusheng Wang, Li Wan, Zheming Shi, Heike Wanke, Shoopala Uugulu, and Collen-Issia Uahengo
Elsevier BV
Abstract Namibia is one of the driest countries in southern Africa. Groundwater has played an important role in the development of Namibia. However, like those at some other places in Namibia, groundwater is unsuitable for drinking in parts of the Northwest of Namibia because of its poor quality. It is significant to assess groundwater quality and understand the hydrogeochemical processes for the management and utilization of groundwater resource in this water-short region. In this paper, we report the investigation and assessment of groundwater quality and associated hydrogeochemical processes in the Cuvelai-Etosha Basin and Kaokoveld region, northwestern Namibia. A total of 24 samples were collected for chemistry and stable hydrogen and oxygen isotopes analysis. The groundwater quality was evaluated by single factor index method. Hydrochemical and isotopic (δD and δ18O) data were used to study the hydrogeochemical processes of groundwater in the areas. The results show that most of the groundwater that originated from precipitation was unacceptable in appearance and tastes, but may be safe for human consumption. The salinity and concentrations of As, U and F− in some of groundwater samples exceeded of the WHO standards. The salinity increase of the groundwater is primarily due to minerals dissolution rather than evaporation. A number of the samples located in the Cuvelai-Etosha Basin and Kaokoveld region are low in TDS. The salinity of them mainly derives from the dissolution of carbonate. Some of the samples located in the west of the Cuvelai-Etosha Basin and Kaokoveld region have medium TDS. The salinity of them originates mainly from the dissolution of carbonate and the oxidation of pyrite. The highest Fe concentration in these samples is up to 13 mg/L. The samples located to the east of Etosha Pan have high TDS. Halite and carbonate dissolution with strong cation exchange are the major source of the high salinity. The low Ca2 + concentrations in groundwater and rich fluorine sediments in Cuvelai-Etosha Basin favor the formation of high F− groundwater. The weak alkaline environment and high HCO3− contents are significant to the higher As and U contents. Insight from this study may be helpful to enhance the understanding of distributions and transfers of major ions and trace elements in groundwater and to improve the management and utilization of groundwater resources in the region and other similar areas.