The Null Space Monte Carlo Uncertainty Analysis of Heterogeneity for Preferential Flow Simulation

Wednesday, 17 December 2014
Mehdi Ghasemizade1,2, Dirk Radny3 and Mario Schirmer2, (1)EAWAG Swiss Federal Institute of Aquatic Science and Technology, Duebendorf, Switzerland, (2)University of Neuchâtel, Centre for Hydrogeology and Geothermics, Neuchâtel, Switzerland, (3)EAWAG Swiss Federal Institute of Aquatic Science and Technology, Dübendorf, Switzerland
Preferential flow paths can have a huge impact on the amount and time of runoff generation, particularly in areas where subsurface flow dominates this process. In order to simulate preferential flow mechanisms, many different approaches have been suggested. However, the efficiency of such approaches are rarely investigated in a predictive sense. The main reason is that the models which are used to simulate preferential flows require many parameters. This can lead to a dramatic increase of model run times, especially in the context of highly nonlinear models which themselves are demanding. We attempted in this research to simulate the daily recharge values of a weighing lysimeter, including preferential flows, with the 3-D physically based model HydroGeoSphere. To accomplish that, we used the matrix pore concept with varying hydraulic conductivities within the lysimeter to represent heterogeneity. It was assumed that spatially correlated heterogeneity is the main driver of triggering preferential flow paths. In order to capture the spatial distribution of hydraulic conductivity values we used pilot points and geostatistical model structures. Since hydraulic conductivity values at each pilot point are functioning as parameters, the model is a highly parameterized one. Due to this fact, we used the robust and newly developed method of null space Monte Carlo for analyzing the uncertainty of the model outputs. Results of the uncertainty analysis show that the method of pilot points is reliable in order to represent preferential flow paths.