O- And H-Isotope Record of Cape Town Rainfall From 1996 to 2008, and Its Application to Recharge Studies of Table Mountain Groundwater, South Africa

Harris, Chris; Burgers, C.; Miller, Jodie; Rawoot, F.

doi:10.2113/gssajg.113.1.33

Cited by 67 publications

(52 citation statements)

References 25 publications

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“…Because the Atlantic Ocean (~266 km) is located closer to Windhoek than the Indian Ocean (~1,904 km; Figures and ), the observed precipitation enrichment in δ 18 O, δ 2 H, and δ 17 O from events originating from the subtropical Atlantic Ocean could be reflecting the continental effect (Dansgaard, ). In addition, the isotope compositions of precipitation originating from the subtropical Atlantic Ocean presented here were generally more enriched in δ 18 O and δ 2 H compared to those reported for Cape Town (Harris et al, ). This could be attributed to the influence of subcloud evaporation at Windhoek as noted by Dansgaard ().…”

Section: Resultssupporting

confidence: 48%

“…Given that the results presented here and the simulations by Nnamchi et al () converge over the same region, they suggest that this SEP could be an extension of the SAOD or a secondary subtropical SAOD defined as SEP and southwest pole and may influence precipitation variability in southern Africa, at least over Windhoek (Figure S4). However, Reason and Jagadheesha () and Harris et al () also suggest the existence of a subtropical SAOD influencing precipitation over the Southwestern Cape region of South Africa, although the location of the dipole is shifted slightly to the left of that proposed in Figure S4. Therefore, the existence of a subtropical SAOD is a distinct possibility, although exact locations may be debatable and outside the scope of this study.…”

Section: Resultsmentioning

confidence: 91%

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Precipitation Origins and Key Drivers of Precipitation Isotope (¹⁸O, ²H, and ¹⁷O) Compositions Over Windhoek

et al. 2018

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Southern African climate is characterized by large precipitation variability, and model precipitation estimates can vary by 70% during summer. This may be partly attributed to underestimation and lack of knowledge of the exact influence of the Atlantic Ocean on precipitation over the region. The current study models trajectories of precipitation events sampled from Windhoek (2012–2016), coupled with isotopes (δ18O, δ2H, δ17O, d, and δ′17O‐δ′18O) to determine key local drivers of isotope compositions as well as infer source evaporative conditions. Multiple linear regression analyses suggest that key drivers of isotope compositions (relative humidity, precipitation amount, and air temperature) account for 47–53% of δ18O, δ2H, and δ17O variability. Surprisingly, precipitation δ18O, δ2H, and δ17O were independent of precipitation type (stratiform versus convective), and this may be attributed to greater modification of stratiform compared to convective raindrops, leading to convergence of isotopes from these precipitation types. Trajectory analyses showed that 78% and 21% of precipitation events during the period originated from the Indian and South Atlantic Oceans, respectively. Although precipitation from the Atlantic Ocean was significantly enriched compared to that from the Indian Ocean (p < 0.05), d was similar, suggesting significant local modification (up to 55% of d variability). Therefore, d may not be a conservative tracer of evaporation conditions at the source, at least for Windhoek. The δ′17O‐δ′18O appeared to be a better alternative to d, consistent with trajectory analyses, and appeared to differentiate El Niño from non‐El Niño droughts. Thus, δ′17O‐δ′18O could be a novel tool to identify drought mechanisms.

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Section: Resultssupporting

confidence: 48%

Section: Resultsmentioning

confidence: 91%

Precipitation Origins and Key Drivers of Precipitation Isotope (¹⁸O, ²H, and ¹⁷O) Compositions Over Windhoek

et al. 2018

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“…It seems probable that the tap water sampled in this region represented recent runoff from rainfall in the mountains, rather than reflecting a longer-term integration of rainfall that the ground water most likely represents. Frontal storm events in the Western Cape can often have a high d (Harris et al, 2010). A large frontal storm may contribute to temporal variability in the tap water in this region, but would be attenuated in the longer-term record of ground water, resulting in the pattern we observed.…”

Section: Accepted Manuscriptmentioning

confidence: 74%

Spatial analysis of hydrogen and oxygen stable isotopes (“isoscapes”) in ground water and tap water across South Africa

West

February

Bowen

2014

Journal of Geochemical Exploration

119

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Stable isotopes in water ( 2 H and  18 O) are important indicators of hydrological and ecological pattern and process.  2 H and  18 O of water are incorporated into geological and biological systems in a predictable manner and have been used extensively as tracers in hydrological, ecological and forensic studies. Physical processes result in spatial variation of  2 H,  18 O in water across the landscape (so-called "isoscapes") and provide the basis for hydrological, ecological, archaeological and forensic studies. Southern Africa is a globally important meeting point for ocean and climate systems, biological diversity and human societies, yet there is little information on the spatial variability in  2 H and  18 O in water across this important region. Here we present the first ground water and tap water isoscapes for southern Africa. We compare and contrast these two water resources, and consider how well global models of precipitation isotopes capture isotopic variation across South Africa.Ground water and tap water samples were collected from across South Africa, analysed for  2 H and  18 O, and used to generate interpolated  2 H,  18 O and deuterium-excess (d =  2 H -8* 18 O) isoscapes. We found coherent spatial structure in  2 H,  18 O and d of ground water and tap water that could be predicted by a geostatistical model based on simple environmental parameters (elevation, mean annual precipitation, precipitation minus potential evaporation, distance to coast and modeled isotope ratio of precipitation). This spatial structure resulted in considerable differences in isotopic composition of water in many of the major wildlife reserves in South Africa, indicating a good potential for wildlife forensics in this region.  2 H and  18 O of ground water, and to a lesser extent tap water, reflected the  2 H and  18 O of long-term weighted annual precipitation at the two GNIP stations in South Africa. A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT2 However, large discrepancies between modelled isotopic composition of precipitation and our ground water and tap water isoscapes, particularly at higher elevations, highlighted uncertainty in the accuracy of modelled precipitation isoscapes for this region. Increased spatial sampling of precipitation, especially for high elevation regions, and temporal sampling of ground and tap water would considerably aid isotopic studies in this region.

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“…In the δ 18 O versus δ 2 H space (Figure a), groundwater samples plot in two groups. Group 1 samples plot with riverwater samples close to the local meteoric water line (LMWL:

δ^{2} normalH =

6.4 δ 18 O + 8.7; Harris et al, ) of Cape Town area (400 km west of the study area). These samples are marked by a δ 18 O and δ 2 H range from −3.5‰ to −4.5‰ and −14‰ to −21‰, respectively.…”

Section: Resultsmentioning

confidence: 99%

Coupling End‐Member Mixing Analysis and Isotope Mass Balancing (222‐Rn) for Differentiation of Fresh and Recirculated Submarine Groundwater Discharge Into Knysna Estuary, South Africa

et al. 2018

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Quantification of submarine groundwater discharge (SGD) is essential for evaluating the vulnerability of coastal water bodies to groundwater pollution and for understanding water body material cycles response due to potential discharge of nutrients, organic compounds, or heavy metals. Here we present an environmental tracer‐based methodology for quantifying SGD into Knysna Estuary, South Africa. Both components of SGD, (1) fresh, terrestrial (FSGD) and (2) saline, recirculated (RSGD), were differentiated. We conducted an end‐member mixing analysis for radon (222Rn) and salinity time series of estuary water over two tidal cycles to determine fractions of seawater, riverwater, FSGD, and RSGD. The mixing analysis was treated as a constrained optimization problem for finding the end‐member mixing ratio that is producing the best fit to observations at every time step. Results revealed highest FSGD and RSGD fractions in the estuary during peak low tide. Over a 24 h time series, the portions of FSGD and RSGD in the estuary water were 0.2% and 0.8% near the estuary mouth and the FSGD/RSGD ratio was 1:3.3. We determined a median FSGD of 41,000 m³ d−1 (1.4 m³ d−1 per m shoreline) and a median RSGD of 135,000 m³ d−1 (4.5 m³ d−1 per m shoreline) which suggests that SGD exceeds river discharge by a factor of 1.0–2.1. By comparison to other sources, this implies that SGD is responsible for 28–73% of total DIN fluxes into Knysna Estuary.

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O- And H-Isotope Record of Cape Town Rainfall From 1996 to 2008, and Its Application to Recharge Studies of Table Mountain Groundwater, South Africa

Cited by 67 publications

References 25 publications

Precipitation Origins and Key Drivers of Precipitation Isotope (¹⁸O, ²H, and ¹⁷O) Compositions Over Windhoek

Precipitation Origins and Key Drivers of Precipitation Isotope (¹⁸O, ²H, and ¹⁷O) Compositions Over Windhoek

Spatial analysis of hydrogen and oxygen stable isotopes (“isoscapes”) in ground water and tap water across South Africa

Coupling End‐Member Mixing Analysis and Isotope Mass Balancing (222‐Rn) for Differentiation of Fresh and Recirculated Submarine Groundwater Discharge Into Knysna Estuary, South Africa

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O- And H-Isotope Record of Cape Town Rainfall From 1996 to 2008, and Its Application to Recharge Studies of Table Mountain Groundwater, South Africa

Cited by 67 publications

References 25 publications

Precipitation Origins and Key Drivers of Precipitation Isotope (18O, 2H, and 17O) Compositions Over Windhoek

Precipitation Origins and Key Drivers of Precipitation Isotope (18O, 2H, and 17O) Compositions Over Windhoek

Spatial analysis of hydrogen and oxygen stable isotopes (“isoscapes”) in ground water and tap water across South Africa

Coupling End‐Member Mixing Analysis and Isotope Mass Balancing (222‐Rn) for Differentiation of Fresh and Recirculated Submarine Groundwater Discharge Into Knysna Estuary, South Africa

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Precipitation Origins and Key Drivers of Precipitation Isotope (¹⁸O, ²H, and ¹⁷O) Compositions Over Windhoek

Precipitation Origins and Key Drivers of Precipitation Isotope (¹⁸O, ²H, and ¹⁷O) Compositions Over Windhoek