Sensitivity of stream–aquifer seepage to spatial variability of the saturated hydraulic conductivity of the aquifer

In this paper, the sensitivities of stream–aquifer seepage flow and the state of connection or disconnection between stream and aquifer, to spatial variability in the aquifer-saturated hydraulic conductivity, are studied with the variably saturated modelling programme, SWMS_2D, using a Monte Carlo technique. A simple stream–aquifer flow system is used with a rectangular stream, with a clogging layer on its perimeter, partially penetrating an alluvial aquifer. A 2D nearest-neighbour method is used to generate fields of log normally distributed saturated conductivity for the alluvial aquifer for a specific set of degree of heterogeneity and the nearest-neighbour autoregressive parameters. Seven different cases are studied. Five of these investigate the effect of the nearest-neighbour parameters, and three cases investigate the effect of aquifer water table level. The results show that the nearest-neighbour parameters and the aquifer degree of heterogeneity significantly affect the modelling of stream seepage, pressure heads and the state of connection/disconnection between stream and aquifer, particularly when the water table is some distance below the streambed. Moreover, comparisons of these results with a purely deterministic model show good agreement when the stream and the aquifer are connected or disconnected but with a high water table. However, there are significant differences between the deterministic and stochastic models when the water table is well below the streambed.