Uncertainties in the ‘ monitoring - conceptualisation - modelling ’ sequence of catchment research

Holocene valley fill material in old stream beds are generally composed of weathering material from the hills. The grain size can vary from gravel to clay. The spatial distribution of the valley fill material can be simulated by using the Markov chain approach. The transition probabilities of this approach indicate the chance that one type of sediment is changing into another sediment type. The concept, which normally is used as time dependent and one-dimensional, is adapted to enable the simulation of spatial variability in more dimensions. In the Noor catchment, located in the chalk region in the south of the Netherlands, the lowermost part of the valley is filled up during Holocene with such valley fill sediments. Field experiments showed a significant small scale variation in hydraulic heads and chemical composition in this valley fill. By using the Markov chain technique 100 equally probable realizations are constructed, using well logs from drillings in the valley fill as known profiles. With these realizations 100 steady-state MODFLOW models are constructed. The influence of gravel and sand beds on the groundwater flow is determined with a statistical analysis of the modelling results. It could be concluded that the realizations of the valley fill have the same statistical properties, but the differences in the occurrence of the sediment layers are large. So the technique used here can not be used to predict the occurrence of a particular sediment type in a particular cell, but it could be used to predict the occurrence of continuous coarse grained layers in a predominantly fine grained valley fill and also the possible effect of these layers on the groundwater flow.