Drainage networks in soils. A concept to describe bypass-flow pathways

By using linear scaling factors of water characteristic functions we have been able to reconstruct a probable long-term drainage network through a sandy soil under coniferous forest in the north of Germany. The topology of the drainage network closely resembles one of mountainous streams. The fractional area of the entire profile occupied by the network was found to decrease exponentially with depth. For solutes preferentially travelling through such a network, the transport volume will therefore decrease exponentially with depth, and so the effective velocity should increase correspondingly. Assuming one-dimensional (1D), piston-flow through this effective transport volume of the network, we have been able to predict how much cumulative infiltration is, on average, needed for inert solutes to reach any given depth. Comparing our predictions with the results of a tracer experiment, we were able to estimate well the arrival time of the peak concentration of the tracer. We consider that this network analysis can be improved further by using more dynamic transport properties to define the pathways of the network.