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Long-term increase in diffuse groundwater recharge following expansion of rainfed cultivation in the Sahel, West Africa

Augmentation sur le long terme de la recharge diffuse des aquifères suite à l’expension des cultures pluviales dans le Sahel, Afrique de l’Ouest

Incremento a largo plazo en la recarga difusa de agua subterránea siguiendo la expansión de cultivos de secano en el Sahel, África Occidental

西非Sahel地区靠雨水灌溉的耕种面积扩大之后地下水弥散补给长期增加

Aumento da recarga difusa de águas subterrâneas a longo prazo após a expansão das culturas regadas a partir da precipitação no Sahel, África Ocidental

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Abstract

Rapid population growth in sub-Saharan West Africa and related cropland expansion were shown in some places to have increased focused recharge through ponds, raising the water table. To estimate changes in diffuse recharge, the water content and matric potential were monitored during 2009 and 2010, and modeling was performed using the Hydrus-1D code for two field sites in southwest Niger: (1) fallow land and (2) rainfed millet cropland. Monitoring results of the upper 10 m showed increased water content and matric potential to greater depth under rainfed cropland (>2.5 m) than under fallow land (≤1.0 m). Model simulations indicate that conversion from fallow land to rainfed cropland (1) increases vadose-zone water storage and (2) should increase drainage flux (∼25 mm year−1) at 10-m depth after a 30–60 year lag. Therefore, observed regional increases in groundwater storage may increasingly result from diffuse recharge, which could compensate, at least in part, groundwater withdrawal due to observed expansion in irrigated surfaces; and hence, contribute to mitigate food crises in the Sahel.

Résumé

La forte croissance démographique en Afrique de l’Ouest sub-saharienne et l’expansion associée des zones cultivées ont induit dans certains secteurs une augmentation de la recharge via le réseau de drainage, conduisant à une hausse piézométrique. Afin d’estimer les modifications de la recharge diffuse, la teneur en eau et le potentiel hydrique matriciel ont été enregistrés sur la période 2009-2010, et une modélisation utilisant le code Hydrus-1D a été réalisée sur deux sites dans le Sud-Ouest du Niger : (1) terres en jachère et (2) cultures pluviales de millet. Les résultats du suivi dans les premiers 10 m de la zone non saturée ont induit une augmentation de la teneur en eau et du potentiel hydrique matriciel à plus grande profondeur sous cultures pluviales (>2.5 m) que sous terres en jachère (≤1.0 m). Les simulations indiquent que la conversion de terres en jachère en terres cultivées non irriguées (1) augmente le stockage de l’eau dans la zone vadose et (2) devrait augmenter le flux de drainage (∼25 mm/an) à 10 m de profondeur avec un temps de transit de 30 à 60 ans. Par conséquent, les augmentations régionales observées du stockage de l’eau dans l’aquifère peuvent de plus en plus résulter de la recharge diffuse, qui pourrait compenser, au moins en partie, les diminutions de niveau piézométrique dues à l’expansion des surfaces irriguées; et ainsi, contribuer à atténuer les crises alimentaires au Sahel.

Resumen

Se demostró que en algunos lugares el rápido crecimiento rápido de la población en el sur del África Occidental subsahariana y la subsecuente expansión de tierras agrícolas han incrementado la recarga focalizada a través de bañaderos, elevando el nivel freático. Para estimar los cambios en la recarga difusa se monitorearon el contenido de agua y el potencial matricial durante 2009 y 2010, y se realizó un modelizacion usando el código Hydrus-1D para dos sitios de campo en el sudoeste de Niger: (1) barbucheras y (2) cultivos de mijo de secano. Los resultados del monitoreo de los 10 m superiores mostraron un incremento del contenido de agua y del potencial matricial a mayor profundidad bajo cultivos de secano (>2.5 m) que bajo barbecho (≤1.0 m). Las simulaciones de los modelos indican que la conversión de barbecho a cultivos de secano (1) incrementa el almacenamiento de agua en la zona vadosa y (2) debería incrementar el flujo del drenaje (∼25 mm year−1) a una profundidad de 10-m después 30–60 años de retardo. Por lo tanto, incrementos regionales observados en el almacenamiento de agua subterránea pueden incrementarse progresivamente como resultado de la recarga difusa, lo cual podría compensar, al menos en parte, la depresión del agua subterránea debido a la expansión observada en las superficies irrigadas; y por lo tanto, contribuir a mitigar la crisis de alimentos en el Sahel.

摘要

西非撒哈拉以南地区一些地方的人口快速增长及相应的耕地面积扩大增加了通过池塘对地下水的补给,抬高了地下水位。为估算弥散补给变化,监测了2009年 到2010年间的含水量和基质势,采用Hydrus-1D编码对尼日尔西南部两个野外点进行了模拟:(1)休耕地和(2)靠雨水灌溉的小米耕地。上部10米的监测结果显示,靠雨水灌溉的农田含水量和基质势呈增加趋势的深度(>2.5米)比休耕地的(≤1.0)要大。模型模拟表明,休耕地转换成雨水灌溉的农田:(1)渗流带储水量增加,(2)30–60年延迟之后在深度10米处排水通量应当增加 (大约25 mm yr−1)。因此,观测的地下水储量区域性增加可能越来越多地归因于弥散补给,弥散补给至少在某种程度上可以对由于观测到的耕地面积扩大而抽取地下水进行补偿;从而有助于缓解Sahel地区的粮食危机。

Resumo

O crescimento populacional rápido na África Ocidental subsariana e a correspondente expansão de culturas agrícolas fez com que, nalguns locais, houvesse um aumento da recarga concentrada através de charcos, elevando o nível freático. Para estimar as alterações da recarga difusa, monitorizou-se o teor de humidade e o potencial matricial em 2009 e 2010, e fez-se a modelação utilizando o código Hydrus-1D em dois locais de estudo no sudoeste do Níger: (1) terreno em pousio e (2) cultura de painço usando a precipitação. Os resultados da monitorização dos 10 m superiores mostraram um teor de humidade e do potencial matricial mais elevado até uma maior profundidade para a cultura regada por precipitação (>2.5 m) em relação à terra de pousio (≤1.0 m). As simulações do modelo indicam que a conversão de terrenos de pousio para as culturas regadas por águas de precipitação (1) aumenta o armazenamento de água da zona vadosa e (2) deverá aumentar o fluxo de drenagem (∼25 mm ano−1) à profundidade de 10 m após um período de 30–60 anos. Portanto, os aumentos observados do armazenamento de água subterrânea podem cada vez mais resultar da recarga difusa, que poderia compensar, pelo menos em parte, a extração de águas subterrâneas devido à expansão observada dos regadios; e assim, contribuir para atenuar crises alimentares no Sahel.

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Acknowledgements

The first author was funded by a PhD grant from the French Ministry of Foreign Affairs. The research projects AMMA-Catch (African Monsoon Multi-disciplinary Analysis; http://www.amma-catch.org) and GHYRAF (Gravity and HYdrology in Africa, 2007–2010) partly supported field work and laboratory analysis. We thank Robert Reedy (University of Texas at Austin) for calibration of heat dissipation sensors and support in data interpretation. Ibrahim Maïnassara, Jean-Philippe Chazarin and Monique Oï (IRD, HydroSciences Montpellier) are acknowledged for support in installation of heat dissipation sensors and data acquisition. We thank Reed Maxwell, associate editor, and two anonymous reviewers for their helpful comments on the manuscript.

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Correspondence to Mathieu Le Coz.

Appendix: ET computation

Appendix: ET computation

The daily reference evapotranspiration ETo was computed using the Penman-Monteith equation (Allen et al. 1998) based on climatic records from two Eddy covariance stations installed in Wankama catchment (Ramier et al. 2009). The crop coefficients, Kc, which differentiate the vegetation covers from the reference surface of grass, were estimated for the successive growing stages of fallow and millet based on tables delivered by Allen et al. (1998). The values of Kc were reduced by a factor depending on the actual vegetation development (Allen et al. 1998):

$$ {\mathrm{Kc}}_{\mathrm{adj}}=\mathrm{Kc}-{A}_{\mathrm{cm}} $$
(6)

with

$$ {A}_{\mathrm{cm}}=1-{\left[\frac{\mathrm{LAI}}{{\mathrm{LAI}}_{\mathrm{dense}}}\right]}^{0.5} $$
(7)

where Kcadj is the reduced Kc value; A cm is the adjustment coefficient; LAI is the actual leaf area index measured for fallow or millet (J. Demarty, IRD, HydroSciences Montpellier, personal communication, 2011); and LAIdense is the leaf area index expected for fallow or millet under standard management practices. The potential evapotranspiration ET was computed at a daily time-step for both fallow and millet by multiplying ETo and the corresponding Kcadj values.

Bare soil evaporation can be estimated by multiplying ETo by an evaporation coefficient Ke, which equals 1.05 (Allen et al. 1998). The value of Ke was reduced to account for vegetation development:

$$ {\mathrm{Ke}}_{\mathrm{adj}}=\mathrm{Ke}-\left(1-{A}_{\mathrm{cm}}\right) $$
(8)

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Ibrahim, M., Favreau, G., Scanlon, B.R. et al. Long-term increase in diffuse groundwater recharge following expansion of rainfed cultivation in the Sahel, West Africa. Hydrogeol J 22, 1293–1305 (2014). https://doi.org/10.1007/s10040-014-1143-z

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