Impacts of soil conservation on groundwater recharge in the semi-arid Loess Plateau, China

Soil conservation measures undertaken to address land degradation can alter the hydrologic cycle by changing partitioning of water fluxes at the land surface. While effects on runoff are well documented, impacts of soil conservation activities on fluxes to groundwater are poorly understood. The goal of this study was to examine fluxes to groundwater in a semi-arid area of China’s Loess Plateau that has been subject to extensive soil conservation activities. Unsaturated zone pore-water pressures and concentrations of chloride show that impacts on deep drainage differ between ecological and structural soil conservation approaches. High matric potentials and low chloride beneath cultivated terrace and gulley sites are consistent with deep drainage occurring at these sites. Estimated recharge rates for dryland cultivated upland sites were approximately 55–90 mm/year (11–18% of mean annual rainfall) based upon chloride mass balance. In contrast, results suggest that mature tree and shrub plantations prevent deep drainage. Stable isotope signatures of unsaturated-zone moisture and groundwater indicate that focused infiltration through gullies and other topographic lows is likely to be the primary recharge mechanism. The results of this study highlight the potential for inadvertent effects of some soil conservation approaches on regional water resources.RésuméLes mesures de préservation des sols prises pour lutter contre la dégradation des terres peuvent influencer le cycle hydrologique en changeant la répartition des flux à la surface du terrain. Alors que les effets sur le ruissellement sont bien connus, les impacts des mesures conservatoires du sol sur les flux vers la nappe sont peu compris. L’objectif de cette étude était d’examiner les flux vers la nappe dans une région semi-aride du plateau lœssique de Chine qui a fait l’objet de multiples programmes de conservation des sols. Les pressions d’eau interstitielle dans la zone non saturée et les concentrations en chlorures montrent que les impacts sur le drainage profond diffèrent selon les approches écologique et structurale de préservation des sols. Des potentiels matriciels élevés et de faibles concentrations en chlorures sous sites présentant terrasse cultivée et fossé correspondent à un drainage profond. Les taux de recharge estimés sur terre sèche de haute terrasse sont approximativement de 55−90 mm/an (11–18% des précipitations moyennes annuelles) sur la base du bilan de masse des chlorures. Par contraste, des résultats suggèrent que des plantations d’arbres adultes et d’arbustes évitent un fort drainage. La signature des isotopes stables de l’eau de la zone non saturée et de la nappe indique que l’infiltration dirigée par les fossés et autres points bas topographiques est vraisemblablement le mécanisme de recharge primaire. Les résultats de cette étude mettent en évidence l’existence d’effets potentiels inopportuns de certaines mesures de préservation des sols sur la ressource régionale en eau.ResumenLas medidas de conservación del suelo realizadas para hacer frente a la degradación de la tierra pueden alterar el ciclo hidrológico mediante el cambio de flujos de agua en la superficie terrestre. Mientras que los efectos sobre el escurrimiento superficial están bien documentados, los impactos de las actividades de conservación en los flujos de agua subterránea son poco conocidos. El objetivo de este estudio fue examinar los flujos de agua subterránea en un área semiárida del Loess Plateau en China que ha estado sujeta a extensas actividades de conservación de suelos. Las presiones del agua poral en la zona no saturada y las concentraciones de cloruro muestran que los impactos en el drenaje profundo difieren entre los enfoques ecológicos y estructurales de conservación del suelo. Los altos potenciales mátricos y el bajo cloruro debajo de terrazas cultivadas y sitios con cárcavas están en consonancia con el drenaje profundo que ocurre en estos sitios. Las estimaciones de las tasas de la recarga de las tierras secas cultivadas en sitios altos fueron aproximadamente 55−90 mm/año (11−18% de la media anual de la precipitación) basado en el balance de masa de cloruro. En contraste, los resultados sugieren que los árboles maduros y las plantaciones de arbustos impiden el drenaje profundo. Las firmas de isótopos estables de la humedad en la zona no saturada y el agua subterránea indican que la infiltración centrada a través de las cárcavas y otros bajos topográficos es probable que sea el mecanismo primario de la recarga. Los resultados de este estudio ponen de relieve el potencial de efectos inadvertidos de algunos enfoques de la conservación de suelo en los recursos hídricos regionales.摘要因为改变了地表水通量的分配,为应对土地退化采取的水土保持措施能改变水文循环。关于水土保持对地表径流的影响已有很多报导,但对地下水补给影响的研究还不足。本次研究的目的是查明广泛采取水土保持措施的中国黄土高原半干旱区的地下水补给通量。非饱和带的孔隙水压力和氯离子浓度表明,生态和工程上的水土保持措施对土壤水深层入渗的影响是不同的。耕作梯田和沟谷下具有较高的基质势和较低的氯离子浓度,这些地点通常也发生深层入渗。基于氯质量平衡估算的旱地耕作高地的补给量大约是55–90 mm /year (占年平均降雨量的11–18%)。相反,果树和灌木种植阻碍深层入渗。非饱和带水和地下水中的稳定同位素特征表明,经由沟谷和其他地势低洼处的集中入渗方式可能是主要的地下水补给机制。本次研究的结果明确了一些水土保持措施对区域水资源存在负面影响的可能性。ResumoAs medidas de conservação do solo tomadas para lidar com a degradação do terreno podem alterar o ciclo hidrológico, modificando a repartição dos fluxos de água na superfície terrestre. Enquanto os efeitos no escoamento directo estão bem documentados, os impactes das actividades de conservação do solo nos fluxos para as águas subterrâneas são pouco compreendidos. O objectivo deste estudo foi examinar os fluxos para as águas subterrâneas numa zona semi-árida do planalto chinês de Loess que foi sujeita a actividades extensivas de conservação do solo. As pressões intersticiais de água na zona não saturada e as concentrações de cloretos mostram que os impactes na drenagem profunda diferem entre abordagens de conservação do solo ecológicas e estruturais. Os potenciais matriciais altos e os cloretos baixos sob terraços cultivados e locais ravinados são consistentes com a drenagem profunda que ocorre nestes locais. As taxas de recarga estimadas com base em balanços de massa de cloretos para terrenos secos cultivados foram de aproximadamente 55–90 mm/ano (11–18% da precipitação anual média). Em contraste, os resultados sugerem que árvores maturas e plantações arbustivas impedem a drenagem profunda. As assinaturas de isótopos estáveis na água da zona não saturada e nas águas subterrâneas indicam que a infiltração localizada através de ravinas e outras depressões topográficas é provavelmente o mecanismo de recarga primário. Os resultados deste estudo destacam o potencial para efeitos inadvertidos de algumas abordagens de conservação do solo nos recursos hídricos regionais.

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