HYDROLOGICAL PROCESSES Hydrol. Process. 28, 3824–3832 (2014) Published online 14 March 2014 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/hyp.10186
Comparing MODFLOW simulation options for predicting intra-meander flux Bangshuai Han† and Theodore A. Endreny* Department of Environmental Resources Engineering, State University of New York, College of Environmental Science and Forestry, 1 Forestry Drive, 402 Baker Labs, Syracuse, NY, 13210, USA
Abstract: During the evolution of meander bends, the intra-meander groundwater head gradients steepen and generate zones of accelerated water and nutrient intra-meander fluxes important for ecosystem processes. This paper compares and contrasts three MODFLOW groundwater model packages based on their simulation of intra-meander flux for two stages of meander evolution observed in a sandbox river table and one level of river bed clogging, where the hydraulic conductivity in the river bed is lower than in the adjacent aquifer. These packages are the Time-Variant Specified Head package [constant head (CHD)], River package (RIV), and Streamflow-Routing package (SFR2), each controlling the groundwater or river head bounding the intra-meander region. The RIV and SFR2 packages fix river stage and allow for variation in groundwater head below the river, which is suggested for simulating intra-meander flux for all sinuosities with and without river bed clogging whenever river bed parameters are available. The CHD package fixes below river groundwater head and fails to simulate intra-meander head loss and flux in meanders with high sinuosity or river bed clogging. In low sinuosity meanders and in cases without river bed clogging, there were no significant differences between MODFLOW packages for simulating river intra-meander head loss and flux. This research demonstrates why MODFLOW users need to consider the limitations of each package when simulating intra-meander flux in reaches with river bed clogging, high sinuosity, or similarly steep hydraulic gradients. Copyright © 2014 John Wiley & Sons, Ltd. KEY WORDS
intra-meander head and flux; MODFLOW; River package; constant head package; Streamflow-Routing package; groundwater head
Received 17 September 2012; Accepted 24 February 2014
INTRODUCTION Intra-meander flux is important to river ecosystem functions because of its role in mixing surface water and groundwater and associated oxygen, nutrients, and solutes (Krause et al., 2011). The intra-meander flux passes through the intra-meander porous medium area known as the hyporheic zone. Intra-meander flux is governed by the meander bend planimetry and substrate conductivity, which affects hydraulic gradients and the rate and spatial extent of biogeochemical changes across the intra-meander hyporheic zone. Intra-meander flux has been investigated in lab studies (Han and Endreny, 2012), field studies (Kasahara and Hill, 2007; Nowinski, 2010; Nowinski et al., 2011), and analytical models using homogeneous and isotropic aquifers with river head
*Correspondence to: Theodore A. Endreny, Department of Environmental Resources Engineering, State University of New York, College of Environmental Science and Forestry, 1 Forestry Drive, 402 Baker Labs, Syracuse, NY, 13210, USA. E-mail:
[email protected] † Current Address: Department of Geosciences, Boise State University, Boise, ID, 83725, USA
Copyright © 2014 John Wiley & Sons, Ltd.
boundary conditions (Boano et al., 2006; Cardenas, 2009a, b; Boano et al., 2010). Operational models that can parameterize real world geologic heterogeneity and represent valley and river head boundary conditions are needed to simulate intra-meander flux and the accompanying chemical fate and transport process. These operational models will fill the gaps between costly direct observation and simplified analytical models. Since the early 1990s, MODFLOW (Harbaugh, 2005) has been the most widely used operational model to simulate river–groundwater interactions (McDonald and Harbaugh, 2003; Brunner et al., 2010). Earlier research has demonstrated the limitations and proper use of MODFLOW river head packages, including the standards for vertical and horizontal discretization (de Lange, 1998; Haitjema et al., 2001; Brunner et al., 2010) and interactions between saturated and unsaturated zones (Osman and Bruen, 2002; Fox, 2003). Research has not examined the sensitivity of MODFLOW’s river head simulation packages to river meander planimetry, where increasing curvature can cause steep intra-meander groundwater head gradients that interact with the bounding river and valley groundwater head. This paper advances
MODFLOW SIMULATION OPTIONS FOR INTRA-MEANDER FLUX
the use of MODFLOW as a tool for intra-meander flux analysis by testing how changes in meander curvature and river bed conductivity affect the accuracy of three widely used MODFLOW river head packages: constant head (CHD) package, River (RIV) package, and SFR2 package. Spatial and temporal intensification of intra-meander flux was postulated by the theoretical work of Boano et al. (2006) and Revelli et al. (2008). Han and Endreny (2012) used a physical model 1 : 500 scaled from the river evolution model planimetry of Boano et al. (2006) and tested the spatial and temporal intensification of intrameander flux. Spatial intensification of flux rate is characterized by intra-meander flux rate increasing from its lowest value at the apex (e.g. tip of the meander) to its highest value at the meander neck (Figure 1 planimetry A or B with flux graphed in C). Temporal intensification of flux rate is characterized by intra-meander flux rate increasing with evolutionary age because of meander elongation and narrowing (Figure 1 planimetry A or B with flux graphed in D). The phenomena of spatial and temporal intensification are governed by meander
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planimetry and river head. In an operational model like MODFLOW, river head boundary conditions are needed to predict the intra-meander flux associated with varying meander planimetry and assist in river and ecosystem management. In this research paper, the MODFLOW groundwater model was parameterized using three standard packages to simulate the steady state lateral flux for the physical meandering models of Han and Endreny (2012), and the impact of MODFLOW parameterization on predictive accuracy was explored. METHODS We compare and contrast MODFLOW CHD, RIV, and SFR2 package performance in predicting intra-meander flux for a well-studied set of ideal meanders, the younger M1 and the older M3 (Boano et al., 2006; Han and Endreny, 2012). MODFLOW is used to simulate the M1 and M3 physical river and valley domain described in detail by Han and Endreny (2012) and summarized in Table I. Meander M1 has a sinuosity of 2.3, and meander
Figure 1. Planimetry of meander age M1 (A) and M3 (B), showing transects NN′ at the neck and MM′ near the apex. Spatial intensification predicts that intra-meander flux is higher at transect NN′ than at transect MM′ for M1 and M3 planimetry (C), and temporal intensification predicts that intra-meander flux is higher in planimetry M3 than in planimetry M1 along transect NN′ and MM′ (D) Copyright © 2014 John Wiley & Sons, Ltd.
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Table I. MODFLOW model parameters Parameters Kaquifer/Kriver bed η Horizontal grid size Kaquifer Porosity Valley gradient Mean channel width Mean river depth
Values 1 and10 1 * 1 cm 0.01 cm/s 0.3 1.4% 4 cm 4 cm
M3 has a sinuosity of 5.2. The horizontal domain of the models is 90 cm crossvalley and 190 cm downvalley (Figure 2), represented in MODFLOW as 90 columns by 190 rows, each horizontal cell given a 1- by 1-cm surface area. The sandbox river table valley and unconfined aquifer were homogenous and saturated with 0.2-mm diameter sand of 8-cm depth and at a 1.4% valley slope, represented in MODFLOW as five layers, divided into 1 cm in the upper two layers and 2 cm in the bottom three layers. The laboratory river was 4 cm wide and 0.4 cm deep and was represented in MODFLOW with four cells, each 1 cm wide. The MODFLOW grid size and vertical discretization were considered adequate based on quality
Figure 2. Oblique view of the laboratory sandbox river table system with a dashed oval showing the meander location used for intra-meander flux analysis (panel A). Plan view of meander planimetry M1 and M3 with the normalized curvilinear distance S* along the meander centerline denoted in bold (panel B)
Copyright © 2014 John Wiley & Sons, Ltd.
control tests (de Lange, 1998; Haitjema et al., 2001). The MODFLOW valley and aquifer were simulated as a homogeneous and isotropic sand system with a hydraulic conductivity of 0.01 cm/s. MODFLOW represented the sandbox river table valley surface as a linearly interpolated plane between measured upvalley and downvalley elevations, with the river cells set 0.4 cm below the adjacent valley cell elevation. The MODFLOW simulations used a CHD condition to represent the sandbox river table upvalley and downvalley reservoir boundary conditions. River stage for each MODFLOW river cell was set to initial river stage based on the observed constant river slope along the streamwise profile of the M1 and M3 rivers. We used the upvalley and downvalley reservoir boundary conditions and river length to find the river slope and then used slope to assign river stage to each river cell. The M1 river slope was 0.6%. The M3 river slope was 0.5%. The valley walls were parameterized in MODFLOW as no flow boundaries. All simulations were run in steady state condition and did not simulate direct precipitation, overland run-off, or evapotranspiration. The previously mentioned MODFLOW conditions were used as a base for each of the three river head packages. The MODFLOW CHD package, also referred to as the Time-Variant Specified Head package, assumes that the river head does not vary over a specified time period. The CHD package assigns a constant river stage value over a specified stress period, and this remains constant for the stress period. During the stress period, the groundwater head below the river cells is equal to the river stage and will not change despite river flux. The equation for calculating flux through a CHD river cell is identical to MODFLOW flux calculations for surface layer cells. The summation of flux into or out of the six faces of a CHD Δh A , where cell uses Darcy’s Law: QCHDn ¼ ∑K CHDn Δr K CHDn is the hydraulic conductivity, Δh is the groundwater head difference, Δr is the distance between the nodes of the constant cell and its neighbouring cell for a certain face, and A is the area of the cell face normal to the neighbour cell. This approach was used by many MODFLOW studies of river aquifer exchange (Soriano and Samper, 2000; Kasahara and Wondzell, 2003; Di Matteo and Dragoni, 2005; Lautz and Siegel, 2006; Chen, 2007; Wondzell, 2011; Han and Endreny, 2012). The RIV package is specifically designed to simulate river flow effects on the head-dependent interactions of surface water and groundwater. The RIV package assigns a constant river stage value over a specified stress period, similar to the CHD package, but does not fix the groundwater head below the river cell. During the stress period, the groundwater head below the river cells can change because of river flux, and this RIV package Hydrol. Process. 28, 3824–3832 (2014)
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feature differs with the CHD package. The flux into or out of the six faces of an RIV cell depends on the relative elevation of the bounding groundwater head. If the groundwater head bounding the river is higher than the river bottom elevation, the flow between the river and the groundwater system for the reach is given by QRIV n ¼ C RIV n H RIV n hi;j;k , where QRIV n is the flow between the river and the aquifer, taken as positive if it is directed into the aquifer, H RIV n is the water level (stage) in the river, hi,j,k is the head at the node in the cell underlying the river reach, and C RIV n is the hydraulic conductance of the river–aquifer interface, calculated as Ln W n , where Ln, Wn, and Mn denote the river C RIV n ¼ K nM n cell length, width, and river bed thickness respectively. If the groundwater head bounding the river is lower than the river bottom elevation, the flow between the river and the groundwater system for the reach is given by QRIV n ¼ C RIV n ðH RIV n RBOT n Þ, where RBOT n is the river bottom elevation at the cell. This approach was used by several MODFLOW studies of river aquifer exchange (Krause et al., 2007; Kim et al., 2008; Chen et al., 2010; Endreny et al., 2011b; Fabian et al., 2011; Munz et al., 2011). The Streamflow-Routing package (SFR2) is a replacement to previous stream flow packages Stream (STR) and SFR1. The SFR2 package assigns a constant river stage value over a specified stress period, similar to the CHD and RIV packages. During the stress period, the groundwater head bounding the river cells can change because of river flux, similar to the RIV package but in contrast with the CHD package. The SFR2 uses the RIV package flux calculation equations for hydraulically connected or disconnected rivers between stress periods but also calculates flow routing between river cells in the specified stream network and interaction of river stage with the unsaturated zone. These functions allow for more detailed representation of channel geometry controls on the river water surface profile. The governing equation in the unsaturated zone is a simplified form of the Richards’ equation (Richards, 1931). The SFR2 package has five options for computing stream depth at the midpoint of a reach: Manning’s equation in a rectangular four-vertex cross section, Manning’s equation in an eight-vertex cross section, a stage to discharge rating curve, a stage to discharge table, and direct assignment of river stage. We used the direct assignment of river stage option in this research. The STR, SFR, and SFR2 packages have each been used in MODFLOW studies simulating streamflow routing (Hunt et al., 2008; Mallakpour, 2011; Zhang et al., 2012), but we found no reported use of the SFR2 package in MODFLOW studies of hyporheic exchange. River bed sediment is not typically the same as the aquifer sediment surrounding the river bed in field conditions; it is often with lower conductivity and referred to as river bed clogging. To examine the Copyright © 2014 John Wiley & Sons, Ltd.
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sensitivity of the river head packages to river bed clogging, we ran additional simulations. Field research has shown that even a thin clogging layer can dramatically affect the groundwater aquifer head and hyporheic exchange rate (Rosenberry et al., 2010; Rosenberry and Healy, 2012). Because the CHD package does not parameterize any river bed hydraulic conductivity, the aquifer to river bed hydraulic conductivity ratio, called the river bed clogging ratio (η), is 1 in all CHD simulations (Table I). The RIV and SFR2 packages were used in the additional simulations to intentionally create a lower conductivity river bed clogging layer relative to the high conductivity aquifer. Our variation of the aquifer to river bed clogging ratio, η, represented observed ratios ranging from 1 to 1000 (Schälchli, 1992; Fox, 2007). In this research, we use the river bed clogging ratios of 1 and 10 to demonstrate how predictive differences between MODFLOW packages change without and with river bed clogging.
RESULTS The choice of river package had a pronounced effect on the intra-meander groundwater head and intra-meander flux. Along a transect following the river planimetry, there were larger differences between CHD, RIV, and SFR2 predictions of groundwater head in the high curvature meander, M3, and smaller differences in the lower curvature meander, M1 (Figure 3). In the river planimetry transects, the CHD package kept groundwater head equal to its initial condition, parallel to the river stage, indicated by the negative sloping straight line. The RIV and SFR2 packages predicted departures from the initial groundwater head, which generated undulations in the head. These undulations created a groundwater to surface water gradient favouring a losing river at the start of the meander and a gaining river at the end of the meander. The magnitude of the difference between groundwater head and river stage was affected by the river bed clogging ratio. When the clogging ratio is 10, the aquifer conductivity is an order of magnitude larger than the river conductivity. In these cases, the magnitude of the difference between groundwater head and river stage is greater than when the clogging ratio was 1. In the M3 scenario, the departure of groundwater head from river stage was initiated further upstream of the meander than in the M1 scenario, and the departure extended further downstream for the M3 than for the M1 scenario. Along a straight upvalley-to-downvalley transect cutting through the meander neck, there were also larger differences between CHD, RIV, and SFR2 predictions of groundwater head in the M3 scenario than in the M1 scenario (Figure 4). The upvalley-to-downvalley transect Hydrol. Process. 28, 3824–3832 (2014)
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Figure 3. Groundwater head (cm) below river cells proceeding from upstream to downstream, with the meander bend between river curvilinear distance 0 and 1, for meander ages M1 and M3. The CHD predicted head declines at a constant slope, the RIV and SFR2 predicted head undulates along the meander, and undulation is greatest when the river bed clogging ratio was 10
Figure 4. Groundwater head (cm) along a straight valley transect proceeding from upstream to downstream, passing under the river at two locations shown by the arrows, for meander ages M1 and M3. The CHD and RIV and SFR2 with a river bed clogging ratio of 1 predicted an isolated steep decline in head within the intra-meander zone, while the RIV and SFR2 with a river bed clogging ratio of 10 predicted a steeper decline in head along the entire transect
runs parallel with the river in the upvalley region, crosses perpendicular to the river at the neck, and then runs parallel with the river in the downvalley region. Differences between the three packages along this transect are because of the differences in valley groundwater head extending from the meander neck. Unlike the RIV and SFR2 packages, the CHD package kept initial values of groundwater head below the river cells, which caused it to have relatively high head at the upstream neck and low head at the downstream neck. This caused the CHD package to predict a sharp increase in the groundwater slope crossing the meander bend (most pronounced in Figure 4 M3). The RIV and SFR2 packages predicted departures (e.g. losing and gaining) from the initial groundwater head gradient, which lowered groundwater head at the upstream neck and increased groundwater head at the downstream neck. The magnitude of the difference between groundwater head gradient along the upvalley-to-downvalley transect at the meander neck was affected by the river bed clogging ratio. When the river bed clogging ratio was 10, the meander neck groundwater head gradient was smaller than when the clogging ratio was 1. Copyright © 2014 John Wiley & Sons, Ltd.
The intra-meander flux estimates (where positive flux indicates transport from the river into the intra-meander region) along the river planimetry transect were different for the CHD, RIV, and SFR2 simulations, and these differences were larger in the high curvature meander, M3, than in the lower curvature meander, M1 (Figure 5, note scale difference along the y-axis). Along the river planimetry transect, the CHD, RIV, and SFR2 packages generated comparable intra-meander flux spatial patterns compared with the intra-meander flux observed in the sandbox river table and predicted by the theoretical model for M1 and M3 (Boano et al., 2006). The intra-meander flux magnitude of the theoretical model flux was larger than sandbox river table observations and MODFLOW predictions because it was simulating a river of greater depth. However, spatial patterns of intra-meander flux were similar between the 2D theoretical model and the 3D MODFLOW model simulations, indicating that vertical flow components were small compared with the predominant horizontal flow components. The CHD package generated a larger flux rate than the RIV and SFR2 packages, and this overestimation was larger in M3 than in M1. This overestimation could be explained by the Hydrol. Process. 28, 3824–3832 (2014)
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Figure 5. Intra-meander flux along the meander curvilinear distance s* as predicted by MODFLOW (primary y-axis) and predicted by Boano et al. (2006; secondary y-axis) and observed in our sandbox river table for meander ages M1 and M3 with a river bed clogging ratio of 10 simulated by the RIV and SFR2 packages
larger head gradient across the meander neck using the CHD package (Figure 5). The RIV and SFR2 packages predicted smoother intra-meander flux curves than the CHD package, and SFR2 prediction is even smoother than the RIV curves, indicating the MODFLOW groundwater–surface water interactions within RIV and SFR2 packages regulated against artificially abrupt transitions in groundwater head. In the upstream meander section, the intra-meander flux curve predicted river flow into the intra-meander zone, creating a losing river section, balanced by a gaining river in the downstream meander section (Figure 5). The differences in intrameander flux estimates between the CHD and the RIV and SFR2 packages were affected by the river clogging ratio. When the clogging ratio increased above 1, the RIV and SFR2 flux estimates became smaller and had a greater difference with the CHD estimates. The SFR2 estimates of intra-meander flux were smaller than the RIV and CHD estimates, which was attributed to the SFR2 simulating the unsaturated zone.
DISCUSSION Our research has shown that the MODFLOW CHD, RIV, and SFR2 packages used to generate intra-meander head and flux have different sensitivities to river sinuosity. Even though the CHD, RIV, and SFR2 packages use the same river stage values, they do not generate similar groundwater head values below or bounding the river. As such, MODFLOW users must assess whether the groundwater head below their river cells should be equal to the river stage or allow for departures and the associated river losing and gaining flux. It has been widely accepted that the simplified CHD package can be used to simulate the river boundary in the segment to watershed scale (Soriano and Samper, 2000; Kasahara and Wondzell, 2003; Di Matteo and Dragoni, 2005; Lautz and Siegel, 2006; Chen, 2007; Wondzell, 2011), and no discussion was given to how different packages impact Copyright © 2014 John Wiley & Sons, Ltd.
flux predictions. However, we show that the RIV and SFR2 packages, compared with CHD predictions, can generate relatively large differences in intra-meander flux in high sinuosity meanders with clogging. The SFR2 package predicted an intra-meander flux that was 37% smaller than the CHD package prediction at the meander neck when sinuosity was 5.2 and the river bed clogging ratio was 10 (Figure 5). With no river bed clogging, our research determined that sinuosities less than 2.3 generated few differences between the CHD, RIV, and SFR2 estimates of intra-meander head and flux. For sinuosities greater than 5.2, regardless of river bed clogging, there were pronounced differences between the estimates of the CHD package and the RIV and SFR2 packages, indicating that the threshold sinuosity is somewhere between these test cases. In general, this experiment shows that SFR2 and RIV packages are the best in detecting intra-meander head and flux differences for reaches with river bed clogging or high sinuosity reaches. Because of the limited accuracy of the physical sandbox river table observations, it is premature to conclude that any package is universally best in matching laboratory or field intra-meander flux observations. In MODFLOW modelling exercises where the goal is to simulate abrupt transitions in groundwater head and flux at the meander scale, the SFR2 and RIV packages provide more physical parameters than the CHD package and can enhance representation of the flux across such gradients. River scientists can use MODFLOW to consider other river and geologic influences on intra-meander head and flux. In our study, we used a river stage with constant slope; however, many meander bends have river sections with varying slopes, such as at riffle-pool sequences (Montgomery and Buffington, 1997) and around large woody debris and restoration structures (Zhou and Endreny, 2011) where relatively flat water surfaces in a backwater pool abruptly steepen. The river stage slope will influence the longitudinal pattern of intra-meander head and flux. Valley substrate was represented as homogeneous in our study; however, heterogeneity in Hydrol. Process. 28, 3824–3832 (2014)
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hydraulic conductivity is common in intra-meander regions (Rosenberry and Healy, 2012) and would influence the intra-meander head and flux rates and patterns. Hillslope influences on intra-meander head were not considered in our study; however, researchers have documented sensitivity in surface water–groundwater flux with changes in hillslope proximity and flow rates (Storey et al., 2003; Cardenas, 2009a, b; Tonina and Buffington, 2009). Variations in river bed morphology, such as dune-riffle patterns or small vertical drops, are known to induce local and regional flow path upwelling and downwelling patterns (Harvey and Bencala, 1993; Elliott and Brooks, 1997; Kasahara and Hill, 2007; Endreny et al., 2011a,2011b) that might interrupt the linear trends in intrameander head and flux predicted along the idealized meanders in our study. However, these bedform influences are likely small relative to the larger influence of the planimetry on intra-meander flux (Boano et al., 2006; Revelli et al., 2008), which was observed in our sandbox river table experiment where pool-riffle features did not drive the overall pattern of intra-meander flux. The potential for the three MODFLOW packages in simulation of intra-meander flux in laboratory sandbox river table is applicable for field conditions with similar sinuosity and river bed clogging. The parameters to consider in site selection are those used by MODFLOW in its representation of the groundwater flux, including hydraulic gradient, hydraulic conductivity, and the crosssectional area of flow. Dimensional analysis has shown that geometric and dynamic similitude is not always maintained between natural rivers and physical models, and instead, physical modellers may use distorted models or Froude models to maintain the proportionality for governing parameters while distorting others (Peakall et al., 1996). Even in the natural world, geometric and dynamic similitude between sites is challenging as meanders may range across metres to kilometres, introducing significant variation in river width-to-depth ratios and soil physical (e.g. local permeability and hydraulic conductivity) and hydraulic properties (e.g. Reynolds, Froude, and Peclet numbers), even when the meanders maintain the same planimetric shape, bulk hydraulic conductivity, and water surface slope (Leopold and Wolman, 1957; Stølum, 1998). Given the variation in geometric and dynamic conditions across scales, direct translation of our experimental results to predict model performance for natural rivers may encounter distortions in scaling and should be tested case by case. As larger meanders are often in high order rivers, where flow conditions are often complex, groundwater modellers should use caution in selecting river head packages when simulating these systems. To summarize our findings, each of the MODFLOW river head packages had advantages and disadvantages for Copyright © 2014 John Wiley & Sons, Ltd.
intra-meander flux simulation. The CHD package provided advantages of simplicity and stability, but it could not simulate unsaturated zones, it lacked terms representing seepage, and it fixed groundwater head below the river cells, which caused overestimation of head loss and intra-meander flux. The RIV and SFR2 packages allowed a more realistic simulation of the influence of river stage on groundwater below the river cells. Simulation results with the RIV and SFR2 packages illustrated how groundwater head can depart from river water surface elevation, especially in high sinuosity or high river bed clogging scenarios (Figures 3 and 4). Models of meandering river systems that use the CHD package assume equality between the groundwater head and the river stage, and this may introduce MODFLOW insensitivities to field conditions not considered in earlier sensitivity studies (Wondzell, 2011). A limitation of the RIV and SFR2 packages is the assumption that groundwater bounding the river cell only interacted with the river through the river bed rather than riverbanks. This bed dominance is generally the case for wide shallow rivers where the width to depth ratio is large. However, for single thread meanders with low width to depth ratios or in valleys with pumping wells alongside the river, significant errors might be introduced in river–aquifer flux calculations by assuming the impermeable river side walls. In sum, although the RIV and SFR2 packages are more robust than the CHD package for intra-meander head loss and flux simulation, they should still be examined critically before being employed in meandering rivers, and the simulation results should be validated. The CHD, RIV, and SFR2 MODFLOW river head packages used in simulations of intra-meander flux do not represent the same physical system. The RIV and SFR2 packages represent the river stage, while the CHD package represents the groundwater head bounding the river. In cases of surface water–groundwater mixing, the river stage is typically different than the below-river groundwater head. We recommend use of a field investigation to determine whether the CHD package is appropriate for MODFLOW simulation of intra-meander flux. In these investigations, below-river groundwater head should be compared with the river stage – if they are different, then the CHD package parameterized by river stage is erroneous, and either the RIV or SFR2 packages are likely to provide more accurate simulation results.
CONCLUSION MODFLOW simulated that groundwater head patterns and intra-meander flux in meandering river systems with homogenous valley substrate are directly affected by meander sinuosity, river bed conductance, and the Hydrol. Process. 28, 3824–3832 (2014)
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selection of the river head package. In summary, this study determined the following: 1. The MODFLOW CHD, RIV, and SFR2 packages were all able to simulate the river boundary, generating comparable intra-meander head loss and flux patterns for low sinuosity (
