Abstract
Physical processes are at the root of determining hydrologic response at all scales. Here, the physical mechanisms linking (1) subsurface heterogeneities to soil moisture and (2) resulting land-surface energy feedbacks to the atmosphere, are examined at the hillslope scale using a fully coupled surface-subsurface-land-surface model, ParFlow. A hillslope with a heterogeneous subsurface and uniform topography was modeled numerically using summer atmospheric conditions and a single precipitation event under controlled boundary conditions in order to isolate the contribution of hydraulic conductivity to land-surface hydrological processes and energy interactions. Patterns of subsurface hydraulic conductivity are shown to govern soil-moisture distribution at the hillslope scale following precipitation. This variability in soil moisture is closely linked to the variability in land-surface energy feedbacks. The role that vegetation plays in subsurface soil moisture and land energy communications is also examined. Results show that hillslope soil moisture variation is first established by patterns in vertical hydraulic conductivity, while later on in the dry-down period, vegetation exerts greater control on the land-surface energy fluxes and controls the rate of hillslope dry down. Furthermore, as compared to bare-soil simulations, grass-cover simulations show an increase in near-surface soil moisture despite water up-take along the rooting depth.
Résumé
Les processus physiques sont à la base de la réponse hydrologique à toutes les échelles. Ici, les mécanismes physiques reliant (1) les hétérogénéités de subsurface à l’humidité du sol et (2) les pertes par évapotranspiration sont examinés à l’échelle du versant avec un modèle numérique couplant état de surface-surface-subsurface du sol, Parflow. Un versant présentant une subsurface hétérogène et une topographie uniforme a été modélisé en entrant des conditions atmosphériques estivales et un seul épisode pluvieux avec des conditions aux limites fixées, afin de séparer des interactions énergétiques la contribution de la conductivité capillaire aux processus d’évapotranspiration. On montre que la perméabilité capillaire de subsurface contrôle la distribution de l’humidité à l’échelle du versant à la suite d‘une précipitation. Cette distribution de l’humidité du sol est étroitement reliée à la variabilité des pertes par évapotranspiration. Le rôle que la végétation joue sur l’humidité de subsurface et sur l’évapotranspiration est aussi examiné. Les résultats montrent que la variation d’humidité du versant est d’abord conditionnée par la capilarité verticale, alors qu‘en période de sécheresse, la végétation exerce un plus grand contrôle sur l’évapotranspiration et contrôle le taux de dessication du versant. De plus, comparativement à un sol dénudé, les simulations montrent qu’une couverture herbeuse accroît l’humidité de subsurface malgré le prélèvement d’eau radiculaire.
Resumen
Los procesos físicos están en la raíz de las determinaciones de las respuestas hidrológicas a toda escala. Aquí se examinan los mecanismos físicos que vinculan (1) la heterogeneidad subsuperficial a la humedad del suelo y (2) la retroalimentación de energía de la superficie de la tierra a la atmósfera, a una escala de ladera usando un modelo completamente acoplado superficie – subsuperficie –superficie del terreno, ParFlow. Se modeló numéricamente una ladera con una subsuperficie heterogénea y una topografía uniforme usando condiciones atmosféricas de verano y un solo evento de precipitación bajo condiciones de contorno controladas con el fin de aislar la contribución de la conductividad hidráulica a los procesos hidrológicos de la superficie terrestre y las interacciones energéticas. Se demuestra que los esquemas de conductividad hidráulica subsuperficial gobiernan la distribución de la humedad del suelo a la escala de ladera y posteriormente a la precipitación. Esta variabilidad en la humedad del suelo está estrechamente ligada con la variabilidad en la retroalimentación de energía en la superficie terrestre. También se examinaron el rol que juega la vegetación en la humedad subsuperficial del suelo y en las comunicaciones energéticas en el terreno. Los resultados muestran que la variación de la humedad del suelo en la ladera está establecida en primer lugar por el esquema en la conductividad hidráulica vertical, mientras que posteriormente por el período de secado, en el cual la vegetación ejerce el mayor control sobre los flujos de energía superficial terrestre y controla la ritmo del secado de la ladera. Además, en comparación con las simulaciones de suelo descubierto, las simulaciones de la cubierta vegetal muestra un incremento en la humedad del suelo próxima a la superficie a pesar que el agua es tomada a través de la profundidad del enraizado.
摘要
在任一尺度上, 物理过程都是研究水文响应的基础。本文在山坡尺度上, 采用完全耦合的地表-地下-陆地-地表模型 (ParFlow) 将物理机制与 (1) 地下非均质性对土壤水分的影响; (2) 陆-地对大气的能量反馈等相联系起来进行研究。对一个地下为非均质介质而地形一致的山坡进行数值模拟, 其控制边界条件限定为夏季大气条件下的一次降水过程, 以区分开渗透系数和陆地水文过程与能量相互作用的不同影响。作出了地下渗透系数特征分布图, 以明确降水后山坡尺度上土壤水分的分布。土壤水分的变化与陆-地能量反馈的变化密切相关。另外, 本文分析了植被在土壤水汽与陆地能量交换中所发挥的作用。结果表明, 山坡尺度上, 土壤水汽的变化首先受到垂向渗透系数的控制, 然后在逐渐变干的过程中, 植被逐渐控制陆地表面能量通量与山坡变干的速率。进一步分析表明, 与裸土模拟相比, 有植被覆盖的模拟表明土壤水汽含量在近地表增加, 尽管根部在向上吸水。
Resumo
Os processos físicos são a base para determinar a resposta hidrológica a todas as escalas. Aqui, os mecanismos físicos que ligam (1) as heterogeneidades subsuperficiais à humidade do solo e (2) as respostas energéticas resultantes da ligação superfície do terreno à atmosfera, são examinados à escala da vertente de encostas, utilizando o modelo conjunto de superfície-subsuperfície-superfície do terreno, ParFlow. Uma vertente de encosta com uma subsuperfície heterogénea e topografia uniforme foi modelada numericamente, utilizando condições atmosféricas de verão e um evento de precipitação único, sob condições de fronteira controladas, de forma a isolar a contribuição da condutividade hidráulica para os processos hidrológicos da superfície do terreno e para as interacções energéticas. Demonstra-se que os padrões da condutividade hidráulica subsuperficial condicionam a distribuição da humidade do solo à escala da vertente de encosta após a precipitação. Esta variabilidade na humidade do solo está proximamente ligada à variabilidade da resposta energética da superfície do terreno. Examina-se também o papel que a vegetação desempenha na humidade do solo subsuperficial e nas comunicações de energia do terreno. Os resultados demonstram que a variação de humidade das vertentes de encosta se estabelece em primeiro lugar por padrões na condutividade hidráulica vertical, enquanto mais tarde, durante o período de secagem, a vegetação exerce um maior controlo nos fluxos energéticos da superfície do terreno e controla a taxa de secagem da vertente de encosta. Para além disso, quando comparadas com simulações de solo descoberto, as simulações do coberto de relva mostram um aumento na humidade do solo próximo da superfície, apesar de haver absorção de água ao longo da profundidade das raízes.
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Acknowledgements
This research was supported in part by the Golden Energy Computing Organization at the Colorado School of Mines using resources acquired with financial assistance from the National Science Foundation and the National Renewable Energy Laboratory. We wish to thank Salvatore Manfreda and the anonymous reviewer for comments that improved the quality and clarity of our paper.
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Atchley, A.L., Maxwell, R.M. Influences of subsurface heterogeneity and vegetation cover on soil moisture, surface temperature and evapotranspiration at hillslope scales. Hydrogeol J 19, 289–305 (2011). https://doi.org/10.1007/s10040-010-0690-1
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DOI: https://doi.org/10.1007/s10040-010-0690-1