Quantifying Bank Storage of Variably Saturated Aquifers by Hailong Li 1,2 , Michel C. Boufadel 3 , and James W. Weaver 4 Abstract Numerical simulations were conducted to quantify bank storage in a variably saturated, homogenous, and anisotropic aquifer abutting a stream during rising stream stage. Seepage faces and bank slopes ranging from 1/3 to 100/3 were simulated. The initial conditions were assumed steady-state flow with water draining toward the stream. Then, the stream level rose at a constant rate to the specified elevation of the water table given by the landward boundary condition and stayed there until the system reached a new steady state. This represents a highly simplified version of a real world hydrograph. For the specific examples considered, the following conclusions can be made. The volume of surface water entering the bank increased with the rate of stream level rise, became negligible when the rate of rise was slow, and approached a positive constant when the rate was large. Also, the volume decreased with the dimensionless parameter M (the product of the anisotropy ratio and the square of the domain’s aspect ratio). When M was large (.10), bank storage was small because most pore space was initially saturated with ground water due to the presence of a significant seepage face. When M was small, the seepage face became insignificant and capillarity began to play a role. The weaker the capillary effect, the easier for sur- face water to enter the bank. The effect of the capillary forces on the volume of surface water entering the bank was significant and could not be neglected. Introduction Water exchange between the subsurface and the adja- cent water bodies plays important roles in both water quantity and quality. In terms of water quantity, bank storage (the storage of water in stream banks during the rise of stream level due to a flood) has been extensively studied (e.g., Cooper and Rorabaugh 1963; Pinder and Sauer 1971; Hunt 1990, 2005; Barlow and Moench 2000; Zlotnik and Huang 1999; Bolster et al. 2001; Hantush et al. 2002; Chen et al. 2006). Hunt (1990) presented an approximate flood-routing solution for the coupled ground water and open-channel flow equations. Zlotnik and Huang (1999) proposed an analytical model of stream-aquifer interaction that considers the effects from a small degree of aquifer penetration and low-permeabil- ity sediments on the head response to an arbitrary stream- stage hydrograph. Bolster et al. (2001) estimated the spe- cific yield of an unconfined limestone aquifer in southeast Florida using the data from a large-scale canal-drawdown test and the analytical model of Zlotnik and Huang (1999). Boufadel and Peridier (2002) gave an exact ana- lytical expression for water exchange between stream and ground water in a confined aquifer during and after a uni- form rise in stream level. Hantush et al. (2002) developed analytical solutions for routing streamflow and bank stor- age in a linearized unconfined aquifer of semi-infinite extent. Hantush (2005) extended the analysis to the case of ground water recharge and a finite bank width with no- flow or time-dependent prescribed head boundary upgra- dient from the channel, which allows analytical 1 Department of Civil and Environmental Engineering, Temple University, 1947 N. 12th Street, Philadelphia, PA 19122; hailong@ temple.edu 2 School of Environmental Studies & (MOE) Biogeology and Environmental Geology Lab, China University of Geosciences, Wuhan 430074, People’s Republic of China. 3 Corresponding author: Department of Civil and Environmen- tal Engineering, Temple University, 1947 N. 12th Street, Phila- delphia, PA 19122; 01 (215) 204-7871; fax: 01 (215) 204-4696; boufadel@temple.edu 4 National Exposure Research Laboratory, U.S. Environmental Protection Agency, Athens, GA 30605; weaver.jim@epamail.epa. gov Received February 2008, accepted May 2008. Copyright ª 2008 The Author(s) Journal compilation ª 2008 National Ground Water Association. doi: 10.1111/j.1745-6584.2008.00475.x Vol. 46, No. 6—GROUND WATER—November–December 2008 (pages 841–850) 841