Estimation of groundwater recharge along a precipitation gradient for Savannah aquifers in Namibia with special emphasis on the impact of vegetation
| dc.contributor.author | Uugulu, Shoopala | |
| dc.date.accessioned | 2023-02-14T08:52:35Z | |
| dc.date.available | 2023-02-14T08:52:35Z | |
| dc.date.issued | 2022 | |
| dc.description | A dissertation submitted in fulfilment of the requirements for the Degree of Doctor of Philosophy in Science (Geology) | en_US |
| dc.description.abstract | The quantification of groundwater resources is essential especially in water scarce countries like Namibia as well as the assessment of the influence of vegetation on groundwater recharge for a better management and sustainability of savannah aquifers. This study has two specific objectives which are firstly to identify groundwater recharge processes and quantify such along a precipitation gradient in Namibian savannah aquifers. Secondly, to determine the influence of savannah vegetation on groundwater recharge by determining the active root depth and source water sources for S. mellifera and B. albitrunca. The study was carried out along a precipitation gradient at three sites namely: Tsumeb; Waterberg and Kuzikus/Ebenhaezer. The chloride mass balance (CMB) method and water stable isotope methods were used in determining groundwater recharge rates at the three sites. Precipitation samples from Tsumeb and Waterberg were collected during the rainy season from 2017 to 2018, while Kuzikus/Ebenhaezer samples were collected between 2014 and 2015. Groundwater samples were collected before, during and after rainy seasons from 2016 to 2017. A deuterium tracer was inserted at different plots with varying depths in December 2016 in order to assess the active root depths for both S. mellifera and B. albitrunca at Ebenhaezer farm. Both woody plants were sampled for the xylem and transpired water. A scattered distribution of rain sample isotopic ratios along the global meteoric water line in the areas was attributed to a seasonal effect. Local meteoric water line equations for Tsumeb, Waterberg and Kuzikus/Ebenhaezer were obtained as: δ2H = 7.78 δ18O + 6.74, (R 2 = 0.95); δ2H = 7.37 δ18O + 5.77, R2 = (0.97); δ2H = 7.16 δ18O + ii 9.88, (R 2 = 0.96) respectively. All the slopes obtained from three study sites are all lower than that of a global meteoric water line equation. A lower slope indicates that the local precipitation has experienced some sub cloud evaporation, leading to enrichment of heavy isotopes. Waterberg groundwater plots on the GMWL which indicates little or 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. All groundwater isotopic values from three sites match those of precipitation depleted in heavy stable isotopes, which indicates that recharge only take place during January, February and March where the precipitation is highest and depleted in heavy stable isotopes. CMB method revealed that savannah aquifers are recharged at low rates, mostly below 10% of the annual precipitation The analysis of woody plant water isotopes at Ebenhaezer farm revealed their source water and the active root depth for S. mellifera. Of 49 transpired water samples, only one S. mellifera sample showed a high deuterium content of 515.9 ‰ where the tracer was inserted at 2.5 m soil depth. Elevated deuterium contents were observed in two S. mellifera xylem samples where the tracer was applied at 2.5 m and 3 m, a possible sign of the active root depth for S. mellifera. However, the active root depth of B. albitrunca could not be determined due to the absence of the tracer in the sampling depths used. S. mellifera δ 18O values indicate that it is using both groundwater and soil water while B. albitrunca δ 18O values show that it is mainly using groundwater. Groundwater recharge rates from this study can be used to guide policy makers on decisions regarding safe yields for the sustainability of the aquifers. | en_US |
| dc.identifier.uri | http://hdl.handle.net/11070/3580 | |
| dc.language.iso | en | en_US |
| dc.publisher | University of Namibia | en_US |
| dc.subject | Ground water | en_US |
| dc.subject | Savanah vegitation | en_US |
| dc.subject | Water recharge | en_US |
| dc.title | Estimation of groundwater recharge along a precipitation gradient for Savannah aquifers in Namibia with special emphasis on the impact of vegetation | en_US |
| dc.type | Thesis | en_US |