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Hydrogeology Journal

, Volume 17, Issue 6, pp 1427–1441 | Cite as

Comparison of groundwater recharge estimation methods for the semi-arid Nyamandhlovu area, Zimbabwe

  • Tenant Sibanda
  • Johannes C. Nonner
  • Stefan Uhlenbrook
Report

Abstract

The Nyamandhlovu aquifer is the main water resource in the semi-arid Umguza district in Matebeleland North Province in Zimbabwe. The rapid increase in water demand in the city of Bulawayo has prompted the need to quantify the available groundwater resources for sustainable utilization. Groundwater recharge estimation methods and results were compared: chloride mass balance method (19–62 mm/year); water-table fluctuation method (2–50 mm/year); Darcian flownet computations (16–28 mm/year); 14C age dating (22–25 mm/year); and groundwater modeling (11–26 mm/year). The flownet computational and modeling methods provided better estimates for aerial recharge than the other methods. Based on groundwater modeling, a final estimate for recharge (from precipitation) on the order of 15–20 mm/year is believed to be realistic, assuming that part of the recharge water transpires from the water table by deep-rooted vegetation. This recharge estimate (2.7–3.6% of the annual precipitation of 555 mm/year) compares well with the results of other researchers. The advantages/disadvantages of each recharge method in terms of ease of application, accuracy, and costs are discussed. The groundwater model was also used to quantify the total recharge of the Nyamandhlovu aquifer system (20 × 106–25 × 106 m3/year). Groundwater abstractions exceeding 17 × 106 m3/year could cause ecological damage, affecting, for instance, the deep-rooted vegetation in the area.

Keywords

Groundwater recharge/water budget Groundwater age Groundwater flow Nyamandhlovu area Zimbabwe 

Comparaison de méthodes d’estimation de la recharge des aquifères en contexte semi-aride, zone de Nywamandhlovu, Zimbabwe

Resumé

L’aquifère du Nyamandhlovu est la ressource principale en eau du district d’Umguza dans la province nord de Matebeleland du Zimbabwe. La forte augmentation de la demande en eau de la ville de Bulawayo a nécessité l’évaluation des ressources en eau souterraine dans une optique de gestion durable. Les résultats de la mise en œuvre de différentes méthodes d’estimation: la méthode du bilan des chlorures (19–62 mm/an); la méthode d’analyse des variations piézométriques (2–50 mm/an); les calculs de réseaux d’écoulement appliquant la loi de Darcy (16–28 mm/an); la datation à l’aide du 14C (22–25 mm/an) et la modélisation des eaux souterraines (11–26 mm/an). Les méthodes de calculs des écoulements et de modélisation hydrogéologique donnent de meilleures estimations pour une recharge spatialisée que les autres méthodes. L’estimation de la recharge à partir des précipitations et de la modélisation est ainsi de l’ordre de 15–20 mm/an. Cette estimation est considérée comme réaliste, considérant qu’une partie de l’eau participant à la recharge est associée à la transpiration du milieu racinaire au niveau de la nappe. La recharge estimée (2.7–3.6% des précipitations annuelles de 555 mm/an) est cohérente avec les résultats de travaux de recherche antérieurs. Les avantages et inconvénients de chaque méthode d’évaluation de la recharge sont discutés considérant les questions d’application, de précision et de coût. La modélisation hydrogéologique a été utilisée également pour quantifier la recharge totale du système aquifère de Nyamandhlovu (20 × 106 to 25 × 106 m3/an). L’exploitation des eaux souterraines à des volumes supérieurs à 17 × 106 m3/an pourrait ainsi être la cause de dommage écologique affectant la végétation possédant des systèmes racinaires profonds dans la région concernée par l’étude.

Comparación de métodos para estimar la recarga de aguas subterráneas para el área semiárida de Nyamandhlovu, Zimbabwe

Resumen

El acuífero Nyamandhlovu es la principal fuente de agua en el distrito semiárido de Umguza en la Provincia de Matebeleland Norte en Zimbabwe. El rápido incremento en la demanda de agua en la ciudad de Bulawayo ha motivado la necesidad de cuantificar los recursos de aguas subterráneas disponibles para su utilización sustentable. Se compararon los métodos y resultados para estimar la recarga subterránea: método del balance de masa de cloruro (19–62 mm/año); método de fluctuación del nivel freático (2–50 mm/año); cómputos de red de flujo Darciana (16–28 mm/año); datación por 14C (22–25 mm/año); y modelación de aguas subterráneas (11–26 mm/year). El método de computación de la red de flujo y el de modelación de aguas subterráneas proporcionan las mejores estimaciones para la recarga areal que los otros métodos. Una estimación final de la recarga (a partir de la precipitación) en el orden de 15–20 mm/año basada en la modelación de aguas subterráneas se considera realista, suponiendo que parte de la recarga de agua transpira desde el nivel freático debido a la vegetación de raíces profundas. Esta estimación de la recarga (2.7–3.6% de la precipitación anual de 555 mm/año) se compara bien con los resultados de otros investigadores. Se discuten las ventajas/desventajas de los métodos de recarga en términos de facilidad de aplicación, precisión y costos. El modelo de aguas subterráneas también fue usado para cuantificar la recarga total del sistema acuífero de Nyamandhlovu (20 × 106 a 25 × 106 m3/año). Las extracciones de aguas subterráneas que exceden 17 × 106 m3/año podrían causar daños ecológicos, afectando, por ejemplo, la vegetación de raíces profundas en el área.

津巴布韦Nyamandhlovu半干旱区地下水补给估算方法的比较

摘要:

Nyamandhlovu含水层是津巴布韦北马塔贝莱兰省翁古扎半干旱区主要的水资源。由于布拉瓦约市需水的快速增长, 为可持续利用计, 需对可获地下水资源进行量化。比较了地下水补给估算方法及其结果 : 氯质量平衡法 (19–62 mm/年) ; 水位变动法 (2–50 mm/年) ; 达西流网计算法 (16–28 mm/年) ; 14C年龄法 (22–25 mm/年) 和地下水模拟 (11–26 mm/年) 。流网计算法和模拟方法给出了较其它方法好的大气补给估算量。基于地下水模拟, 最终认为补给15–20 mm/年是合理的, 假定一部分补给水消耗于深根植被的蒸腾。这一补给估计 (约占年降水量555 mm/年的2.7–3.6%) 与其它研究人员的结果接近。据应用难易程度、精度和成本, 讨论了每种补给估算方法的优缺点。地下水模型也用来量化确定Nyamandhlovu含水层系统的总补给量 (20 × 106 到25 × 106 m3/年) 。超过17 × 106 m3/年的地下水抽取量可能导致生态退化, 例如, 将影响该地区的深根植被。

Comparação de métodos de estimação da recarga subterrânea para a área semi-árida de Nyamandhlovu, Zimbabwe

Resumo

O aquífero de Nyamandhlovu constitui a principal origem de água no distrito semi-árido de Umguza, Província de Matebeleland Norte, no Zimbabwe. O aumento rápido da procura de água na cidade de Bulawayo levou à necessidade de quantificar os recursos hídricos subterrâneos disponíveis para um uso sustentável. Foram comparados métodos de estimação de recarga e os resultados obtidos: método de balanço de cloretos (19–62 mm/ano); método da flutuação do nível freático (2–50 mm/ano); Computação da rede de fluxo Darciano (16–28 mm/ano); Datação por 14C (22–25 mm/ano); e modelação de água subterrânea (11–26 mm/ano). O método de computação da rede de fluxo e o método de modelação dão melhores estimativas para a recarga aérea do que os outros métodos. Com base na modelação da água subterrânea, obteve-se uma estimativa final para a recarga (da precipitação) da ordem dos 15–20 mm/ano, o que se crê ser realista, assumindo que parte da água de recarga transpira da superfície freática através da vegetação de raízes profundas. Este valor de recarga estimado (2.7–3.6% da precipitação anual de 555 mm/ano) é comparável aos resultados apresentados por outros investigadores. As vantagens/desvantagens de cada um destes métodos de recarga em termos da facilidade de aplicação, adequação dos resultados à realidade e custos, são discutidas. O modelo de água subterrânea foi também usado para quantificar a recarga total do sistema aquífero de Nyamandhlovu (20 × 106 to 25 × 106 m3/ano). Extracções de água subterrânea superiores a 17 × 106 m3/ano podem provocar danos ecológicos, afectando, por exemplo, a vegetação de raízes profundas da área.

Notes

Acknowledgements

The authors would like to thank the Dutch government for providing sponsorship for the studies of the first author at UNESCO-IHE, Delft, The Netherlands. We would also like to thank members of staff of the Zimbabwe National Water Authority for their support and assistance during data collection and fieldwork. Many thanks go to the University of Zimbabwe, Geology Department, in particular to Mr. Pride Mangeya for providing additional data and advice during the research.

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Copyright information

© Springer-Verlag 2009

Authors and Affiliations

  • Tenant Sibanda
    • 1
  • Johannes C. Nonner
    • 2
  • Stefan Uhlenbrook
    • 2
    • 3
  1. 1.World VisionHarareZimbabwe
  2. 2.Department of Water EngineeringUNESCO-IHEDelftThe Netherlands
  3. 3.Delft University of TechnologyDelftThe Netherlands

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