Evaluation of the soil water balance in an alluvial flood plain with a shallow groundwater table

Abstract

Many of the hydrological and ecological functions of alluvial flood plains within watersheds depend on the water flow exchanges between the vadoze soil zone and the shallow groundwater. The water balance of the soil in the flood plain is investigated, in order to evaluate the main hydrological processes that underlie the temporal dynamics of soil moisture and groundwater levels. The soil moisture and the groundwater level in the flood plain were monitored continuously for a three-year period. These data were integrated with the results derived from applying a physically-based numerical model which simulated the variably-saturated vertical water flow in the soil. The analysis indicated that the simultaneous processes of lateral groundwater flow and the vertical recharge from the unsaturated zone caused the observed water table fluctuations. The importance of these flows in determining the rises in the water table varied, depending on soil moisture and groundwater depth before precipitation. The monitoring period included two hydrological years (September 2009-September 2011. About 13% of the precipitation vertically recharged the groundwater in the first year and about 50% in the second. The difference in the two recharge coefficients was in part due to the lower groundwater levels in the recharge season of the first hydrological year, compared to those observed in the second. In the latter year, the shallow groundwater increased the soil moisture in the unsaturated zone due to capillary rise, and so the mean hydraulic conductivity of the unsaturated soil was high. This moisture state of soil favoured a more efficient conversion of infiltrated precipitation into vertical groundwater recharge. The results show that groundwater dynamics in the flood plain are an important source of temporal variability in soil moisture and vertical recharge processes, and this variability must be properly taken into account when the water balance is investigated in shallow groundwater environments.Key words soil water balance; groundwater recharge; lateral groundwater flow; 1D Richards model Evaluation du bilan hydrique du sol dans une plaine d'inondation alluviale avec une nappe phréatique peu profonde Résumé La plupart des fonctions hydrologiques et écologiques des plaines inondables alluviales des bassins versants dépendent des échanges entre la zone vadose et la nappe phréatique peu profonde. Nous avons étudié le bilan hydrique du sol dans la plaine d'inondation, afin d'évaluer les principaux processus hydrologiques qui sous-tendent la dynamique temporelle de l'humidité du sol et des eaux souterraines. L'humidité du sol et le niveau de la nappe phréatique de la plaine d'inondation ont été suivis en continu pendant une période de trois ans. Ces données ont été intégrées avec les résultats issus de l'application d'un modèle numérique à base physique simulant l'écoulement vertical de l'eau dans la zone du sol à saturation variable. L'analyse a montré que les processus simultanés de l'écoulement souterrain latéral et de la...

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