Effect of Pilot-Points Location on Model Calibration: Application to the Northern Karst Aquifer of Qatar

Husam Baalousha,Marwan Fahs,Fanilo Ramasomanana,Anis Younes

doi:10.3390/w11040679

Abstract

In hydrogeological modelling, two approaches are commonly used for model calibration: zonation and the pilot-points method. Zonation assumes an abrupt spatial change in parameter values, which could be unrealistic in field applications. The pilot-points method produces smoothly distributed parameters compared to the zonation approach; however, the number and placement of pilot-points can be challenging. The main goal of this paper is to explore the effect of pilot-points number and locations on the calibrated parameters. A 3D groundwater flow model was built for the northern karst aquifer of Qatar. A conceptual model of this aquifer was developed based on MODFLOW software (United States Geological Survey). The model was calibrated using the parameter estimation and uncertainty analysis (PEST) package employing historical data of groundwater levels. The effect of the number and locations of pilot-points was examined by running the model using a variable numbers of points and several perturbations of locations. The calibration errors for all the runs (corresponding to different configurations of pilot-points) were maintained under a certain threshold. A statistical analysis of the calibrated parameters was then performed to evaluate how far these parameters are impacted by the pilot-point locations. Finally, an optimization method was proposed for pilot-points placement using recharge and observed piezometric maps. The results revealed that the pilot-points number, locations, and configurations have a significant effect on the calibrated parameter, especially in the high permeable regions corresponding to the karstic zones. The outcome of this study may help focus on areas of high uncertainty where more field data should be collected to improve model calibration. It also helps the placement of pilot-points for a robust calibration.

Highlights

In many hydrogeological modelling studies, models are calibrated using the zonation approach, in which a model domain is discretized into a finite number of zones representing the variability of a calibrated parameter, reducing the number of variables [1,2,3]
The pilot-points were defined using a grid of 17 rows and 11 columns created over the model domain
The northern aquifer of Qatar has a high variability in hydraulic conductivity as the aquifer is karst, but has a lower hydraulic conductivity in general

Summary

Introduction

In many hydrogeological modelling studies, models are calibrated using the zonation approach, in which a model domain is discretized into a finite number of zones representing the variability of a calibrated parameter, reducing the number of variables [1,2,3]. The full parameterization of a parameter at each node is an alternative to the zonation approach, but the computation cost is high [4]. De Marsily [6] introduced the pilot-points method as a means to reduce the computational burden of the full parameterization approach. In the pilot-points approach, the hydraulic parameter domain is represented by a finite number of points distributed over the model domain.

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