Numerical analysis of flow and transport from a multiple-source system in a partially saturated heterogeneous soil under cropped conditions

Field-scale solute transport in a three-dimensional, heterogeneous, variably saturated soil, originating from multiple planar sources (MS) is analyzed and compared with its counterpart originating from a single planar source (SS). The case under consideration is a citrus grove planted on a Hamra Red Mediterranean soil (Rhodoxeralf) in the central part of the coastal region of Israel, with a distinct rainy period during the winter and irrigations during the rest of the year. Results of the analyses show that for both the MS and the SS cases, solute transport during the irrigation season is characterized by a restricted downward movement and spread and by a considerable increase in concentration, while the opposite situation occurs during the rain season. In addition, results of the analyses suggest that as compared with the SS case, the MS case is characterized by a larger uncertainty in the concentration point values, by a slower solute convection and a smaller longitudinal solute spread, by a larger transverse solute spread, and by larger skewness and uncertainty in the solute breakthrough curve (BTC). Regarding the solute sampling design problem, our findings suggest that for both cases a pair of sampling points located in the horizontal plane of the field may be sufficient in order to provide relatively accurate estimates of characteristics of the transport (i.e., displacement and spread of the solute plume in the direction of the mean flow and mean solute BTC) for the entire spatially heterogeneous domain. To achieve these desirable results, in the MS case the first sampling point must be located within the wetted area in the vicinity of one of the planar sources, while in the SS case these results are independent of the location of the first sampling point. For both cases, however, an effort to quantify the uncertainty in the mean solute BTC requires few additional sampling points.