O- and H-isotope record of Cape Town rainfall from 1996 to 2022: the effect of increasing temperature, and the ‘water crisis’ of 2015 to 2018

IF 1.2 4区地球科学 Q2 GEOLOGY

South African Journal of Geology Pub Date : 2023-12-01 DOI:10.25131/sajg.126.0026

C. Harris

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引用次数: 0

Abstract

The O- and H-isotope composition of rainfall collected monthly at the University of Cape Town (UCT) between 2009 and 2022 has been added to a previously published data set from 1996 to 2008 to make a continuous 27 year record. Monthly rainfall over the 27 year period has a range in δD and δ18O values from -57 to +21‰ and -8.1 to +3.5‰, respectively, and shows limited but discernible temperature and amount effects. The 27 year rainfall record defines a Local Meteoric Water Line (LMWL) whose equation is δD = 5.96*δ18O + 7.00 (r = 0.88), a slight change from the LMWL of 1996 to 2009 (6.41*δ18O + 8.89). Annual rainfall at UCT has varied from ~992 mm (2017) to 1996 mm (2013), with no systematic change in annual rainfall amount (r = −0.16). However, from 2015 to 2022, the average annual rainfall of 1 145 mm has been below the 27 year average of 1 313 mm. Mean monthly temperature has increased from 1996 to 2022 (r = 0.53), and the weighted mean annual δD and δ18O values have increased by ~4‰ (r = 0.53) and ~0.5‰ (r = 0.64), respectively, over the 27 years. The UCT data and the data for 1961 to 1974 from Cape Town International Airport plot around the same LMWL, with an average deuterium excess (d-excess) of 13.78 and 12.95, respectively. Natural springs in the area plot close to the LMWL with an average d-excess of 14.15, whereas local well-point and borehole water samples generally plot below the LMWL with an average d-excess of 10.65. These differences can be explained by relatively rapid recharge of springs and slower recharge of groundwater, with the latter containing an additional component, that could either be rainwater that fell during a period of hotter drier climate or, more probably, municipal mains water. Long-term monitoring of groundwater and spring water as well as rainwater would be of great help in assessing the sustainability of groundwater use, among other important questions.

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1996 年至 2022 年开普敦降雨的 O-和 H-同位素记录：气温升高的影响以及 2015 年至 2018 年的 "水危机

开普敦大学（UCT）在 2009 年至 2022 年期间每月收集的降雨的 O-和 H-同位素组成被添加到之前发布的 1996 年至 2008 年的数据集中，从而形成了一个连续 27 年的记录。27 年间的月降雨量的 δD 和 δ18O 值范围分别为 -57 至 +21‰ 和 -8.1 至 +3.5‰，显示出有限但明显的温度和降雨量效应。27 年的降雨记录确定了当地陨水线（LMWL），其方程为 δD = 5.96*δ18O + 7.00（r = 0.88），与 1996 年至 2009 年的当地陨水线（6.41*δ18O + 8.89）相比略有变化。UCT 的年降雨量从约 992 毫米（2017 年）到 1996 毫米（2013 年）不等，年降雨量没有系统性变化（r =-0.16）。然而，从 2015 年到 2022 年，平均年降雨量为 1 145 毫米，低于 27 年的平均值 1 313 毫米。从 1996 年到 2022 年，月平均气温有所上升（r = 0.53），加权年平均 δD 和 δ18O 值在 27 年中分别增加了 ~4‰ （r = 0.53）和 ~0.5‰ （r = 0.64）。UCT 数据和开普敦国际机场 1961 年至 1974 年的数据绘制在同一 LMWL 附近，平均氘过量（d-excess）分别为 13.78 和 12.95。该地区的天然泉水接近 LMWL，平均氘过量为 14.15，而当地的井点和井眼水样本一般低于 LMWL，平均氘过量为 10.65。造成这些差异的原因是泉水的补给相对较快，而地下水的补给较慢，后者含有额外的成分，可能是在气候较炎热干燥期间降下的雨水，也可能是市政自来水。对地下水、泉水和雨水进行长期监测，将大大有助于评估地下水使用的可持续性等重要问题。

本文章由计算机程序翻译，如有差异，请以英文原文为准。

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来源期刊

South African Journal of Geology 地学-地质学

CiteScore

3.60

自引率

5.60%

发文量

审稿时长

>12 weeks

期刊介绍： The South African Journal of Geology publishes scientific papers, notes, stratigraphic descriptions and discussions in the broadly defined fields of geoscience that are related directly or indirectly to the geology of Africa. Contributions relevant to former supercontinental entities such as Gondwana and Rodinia are also welcome as are topical studies on any geoscience-related discipline. Review papers are welcome as long as they represent original, new syntheses. Special issues are also encouraged but terms for these must be negotiated with the Editors.