H. Bendjoudi

Estimating potential evapotranspiration without continuous daily data: possible errors and impact on water balance simulations

Abstract Alternative approaches to estimating monthly and annual potential evapotranspiration (PE) are explored in cases where daily climate data are not routinely recorded. A database consisting of data from 222 weather stations, representing a wide variety of climatic conditions, is used to draw general conclusions. In addition, two PE formulae with different data requirements are used: the standard FAO-56 Penman-Monteith equation, and a simple temperature-based equation. First, we tested the degree of bias introduced by using climate data averaged over long time periods instead of daily data. Second, we explored the sensitivity of PE estimation with respect to variations in sampling frequency of climate variables. The results show that using mean weather data has only a limited effect on monthly and annual PE estimates. Conversely, imperfect sampling of weather data may bias monthly and to a lesser extent annual PE estimates if the sampling period exceeds 5 and 10 days, respectively. Finally, we tested the impact of erroneous weather data on the simulations of annual actual evapotranspiration obtained with the Budyko model. The impact on the Budyko model outputs depends more on the dryness index of a given location than on annual PE; for regions under water stress, the errors in estimation of actual evapotranspiration are very limited, compared to humid regions where available energy is the dominating factor and the propagation of PE errors is important. Citation Oudin, L., Moulin, L., Bendjoudi, H. & Ribstein, P. (2010) Estimating potential evapotranspiration without continuous daily data: possible errors and impact on water balance simulations. Hydrol. Sci. J. 55(2), 209–222.

Hydrogeological modeling constraints provided by geophysical and geochemical mapping of a chlorinated ethenes plume in northern France

A methodological approach is described which combines geophysical and geochemical data to delineate the extent of a chlorinated ethenes plume in northern France; the methodology was used to calibrate a hydrogeological model of the contaminants’ migration and degradation. The existence of strong reducing conditions in some parts of the aquifer is first determined by measuring in situ the redox potential and dissolved oxygen, dissolved ferrous iron and chloride concentrations. Electrical resistivity imaging and electromagnetic mapping, using the Slingram method, are then used to determine the shape of the pollutant plume. A decreasing empirical exponential relation between measured chloride concentrations in the water and aquifer electrical resistivity is observed; the resistivity formation factor calculated at a few points also shows a major contribution of chloride concentration in the resistivity of the saturated porous medium. MODFLOW software and MT3D99 first-order parent–daughter chain reaction and the RT3D aerobic–anaerobic model for tetrachloroethene (PCE)/trichloroethene (TCE) dechlorination are finally used for a first attempt at modeling the degradation of the chlorinated ethenes. After calibration, the distribution of the chlorinated ethenes and their degradation products simulated with the model approximately reflects the mean measured values in the observation wells, confirming the data-derived image of the plume.RésuméUne approche méthodologique combinant des données géophysiques et géochimiques est décrite pour délimiter l’extension d’un panache d’éthènes chlorés dans le Nord de la France; la méthodologie a été utilisée pour caler un modèle hydrogéologique de migration et de dégradation de contaminants. L’existence de conditions fortement réductrices dans certaines parties de l’aquifère est déterminée dans un premier temps en mesurant in-situ le potentiel rédox et l’oxygène dissous et les concentrations en chlorures et fer ferreux dissous. L’imagerie de la résistivité électrique et la cartographie électromagnétique, au moyen de la méthode Slingram, sont ensuite utilisées pour déterminer la forme du panache de polluant. Une relation empirique exponentielle décroissante entre les concentrations en chlorures mesurées dans l’eau et la résistivité électrique de l’aquifère est observée; le facteur de formation de la résistivité calculé en quelques points montre aussi une contribution majeure de la concentration en chlorures à la résistivité du milieu poreux saturé. Le modèle MODFLOW et la réaction en chaîne de premier ordre parent–fille MT3D99 et le modèle RT3D aérobie–anaérobie pour la déchlorination du tétrachloroéthène (PCE)/trichloroéthène (TCE) sont finalement utilisés pour une première tentative de la modélisation de la dégradation des éthènes chlorés. Après calage, la distribution des éthènes chlorés et de leurs produits de dégradation simulée avec le modèle reflète approximativement les valeurs moyennes mesurées dans les puits d’observation, confirmant l’image du panache dérivée des données.ResumenSe describe un enfoque metodológico que combina datos geofísicos y geoquímicos para delinear la extensión de una pluma de etenos clorados en el norte de Francia; la metodología fue usada para calibrar un modelo hidrogeológico de la migración y degradación de contaminantes. La existencia de condiciones fuertemente reductoras en algunas partes del acuífero se determinó primero por medidas in situ del potencial redox y de las concentraciones de oxígeno disuelto, hierro ferroso disuelto y cloruro. Se utilizaron luego imágenes de resistividad eléctrica y mapeos electromagnéticos, usando el método Slingram, para determinar la forma de la pluma contaminante. Se observó una relación empírica exponencial decreciente entre las concentraciones de cloruro medidas en el agua y la resistividad eléctrica del acuífero; el factor de la resistividad de formación calculado en unos pocos puntos también muestra una mayor contribución de la concentración de cloruro en la resistividad del medio poroso saturado. Finalmente se utilizó el software MODFLOW y la reacción en cadena padre–hijo de primer orden del MT3D99 y el modelo aeróbico–anaeróbico RT3D para la declorinación del tetracloroeteno (PCE)/tricloroeteno (TCE) en un primer intento de modelar la degradación de los etenos clorados. Después de la calibración, la distribución de los etenos clorados y sus productos de degradación simulados con el modelo reflejan aproximadamente los valores medios medidos en los pozos de observación, confirmando la imagen de los datos derivados de la pluma.摘要本文探讨了结合地球物理和地球化学资料描述法国北部氯化乙烯污染羽范围的方法;这种方法用于校正污染物迁移和降解的水文地质模型。通过测量原地氧化还原势及溶解氧、溶解亚铁及氯化物含量,首先确定了含水层有些部分存在着很强的还原条件。采用斯陵格兰姆法获得的电阻系数成像和电磁绘图用来确定污染羽的形状。观测到水中测量的氯化物含量和含水层电阻系数之间存在着递减的经验指数关系;几个点上计算的电阻率形成因素也显示了饱和多孔介质电阻率中氯化物含量的主要分布。MODFLOW软件和MT3D99一级父女链式反应式及四氯乙烯/三氯乙烯RT3D好氧/厌氧模型第一次用于模拟氯化乙烯的降解。校准后,用模型模拟的氯化乙烯的分布及其降解物大致反映了观测井中平均测量值,证实了根据资料推断的污染羽图像。ResumoDescreve-se uma abordagem metodológica que combina dados geofísicos e geoquímicos para delinear a extensão de uma pluma de eteno-clorados no norte de França; a metodologia foi utilizada para calibrar um modelo hidrogeológico da migração e degradação dos contaminantes. A existência de condições fortemente redutoras em algumas partes do aquífero é determinada primeiro pela medição in situ do potencial redox e das concentrações de oxigénio dissolvido, ferro ferroso dissolvido e cloreto. Depois é utilizada imagiologia de resistividade elétrica e mapeamento eletromagnético, utilizando o método Slingram, para determinar a forma da pluma poluente. Observa-se uma diminuição da relação exponencial empírica entre a concentração de cloreto medido na água e a resistividade elétrica do aquífero; o fator da resistividade da formação, calculado em alguns pontos, também mostra uma grande contribuição da concentração em cloreto para a resistividade do meio poroso saturado. O software MODFLOW, a cadeia de reações pai–filha de primeira ordem MT3D99 e o modelo aeróbio–anaeróbio RT3D para extração do cloro do tetracloroeteno (PCE)/tricloroetileno (TCE) são finalmente usados para uma primeira tentativa de modelar a degradação dos eteno-clorados. Depois da calibração, a distribuição dos eteno-clorados e dos seus produtos de degradação, simulada com o modelo, reflete aproximadamente os valores médios medidos em poços de observação, o que confirma a imagem da pluma derivada dos dados.