Modeling Groundwater-surface water interaction and development of an inverse groundwater modeling methodology

Abstract

In the last two decades, modeling groundwater-surface water interaction has been an important research topic in hydrology due to its role in conjunctive use, riparian zone management, contaminant transport, etc. and also due to the emergence of the discipline - ecohydrology. Despite the research focus on this subject, quantifying the interaction between the two systems remains difficult. This thesis focuses on quantifying the exchange flux of a river-aquifer interaction using distributed recharge as a main constraint for a highly detailed groundwater model, and developing a methodology for determining hydraulic conductivities by assimilating distributed recharge and other fluxes.

Estimation of distributed recharge based on a GIS based rainfall-runoff model, WetSpa, and its use in groundwater-surface water interaction is presented using Kleine Nete catchment and Aa River, Belgium, as a case study area. Water balance simulation of the Kleine Nete catchment for the period 2005-2006 indicates that over half of the precipitation is lost to the atmosphere and about 43% of the precipitation recharges the groundwater. During this period, the main input to the river flow comes from the groundwater. The distributed recharge simulated by WetSpa is used to build a highly detailed groundwater model around Aa River based on the well calibrated MODFLOW model of the Kleine Nete catchment. The detailed model is able to simulate the spatial and temporal variation of the exchange flux between the river and the underlying aquifer. During normal river flow condition exfiltration is found to be the dominant exchange flux although some infiltration occurred. In contrast, during flood events infiltration was the dominant flux exchange. The exchange fluxes are found to be in the order of 5% compared to the total groundwater inflow-outflow balance of the area. In addition, the exchange fluxes are around 1% compared to the river discharge.

This thesis also investigates two discrete analog formulations to the continuum equations: block centered heads and 3D stream functions. The discrete analog block centered head method is mathematically and algorithmically equivalent to the standard block centered finite difference method, whereas the 3D stream function method is a new approach in reservoir engineering and groundwater flow modeling. The mathematical derivation and the numerical applications show that the two formulations are mathematically equivalent, although the 3D stream function method is quite different from an algorithmic point of view. After proofing the equivalence of the 3D stream function method with the finite difference formulation, it is used to 'translate' the methodology of Electrical Impedance Tomography (EIT) to Hydraulic Impedance Tomography (HIT). HIT is a direct inversion method to calculate spatially distributed conductivities using hydraulic and geohydrologic data. In direct inversion, hydraulic conductivities are calculated directly without the need of calibration. The main advantage of HIT is that the back projection system is linear in the impedivities. In addition, HIT satisfies the continuity equation exactly. The method is demonstrated on several theoretical examples using both actual and noisy data. The test examples indicate that HIT can provide stable and reliable impedivity estimates. The applicability of HIT is also investigated for downscaling problems using various theoretical impedivity fields simulating different types of aquifers. HIT provides reasonable impedivity estimates for consistent boundary conditions irrespective of the starting impedivity field.

Date of Award	2009
Original language	English
Supervisor	Okke Batelaan (Promotor)