Importance of Unsaturated Zone Flow forSimulating Recharge in a Humid Climateby Randall J. Hunt1, David E. Prudic2, John F. Walker3, and Mary P. Anderson4

AbstractTransient recharge to the water table is often not well understood or quantified. Two approaches for simulat-

ing transient recharge in a ground water flow model were investigated using the Trout Lake watershed in north-central Wisconsin: (1) a traditional approach of adding recharge directly to the water table and (2) routing thesame volume of water through an unsaturated zone column to the water table. Areas with thin (less than 1 m)unsaturated zones showed little difference in timing of recharge between the two approaches; when water wasrouted through the unsaturated zone, however, less recharge was delivered to the water table and more dischargeoccurred to the surface because recharge direction and magnitude changed when the water table rose to the landsurface. Areas with a thick (15 to 26 m) unsaturated zone were characterized by multimonth lags between infiltra-tion and recharge, and, in some cases, wetting fronts from precipitation events during the fall overtook and mixedwith infiltration from the previous spring snowmelt. Thus, in thicker unsaturated zones, the volume of water infil-trated was properly simulated using the traditional approach, but the timing was different from simulations thatincluded unsaturated zone flow. Routing of rejected recharge and ground water discharge at land surface to sur-face water features also provided a better simulation of the observed flow regime in a stream at the basin outlet.These results demonstrate that consideration of flow through the unsaturated zone may be important when simu-lating transient ground water flow in humid climates with shallow water tables.

IntroductionTransient ground water models are becoming a practi-

cal addition to steady-state modeling owing to theincreased availability of time series data, which arebecoming readily available due to the quality and storagecapacity of new and improved field instrumentation. Inaddition, management decisions often require transientsimulations (e.g., aquifer storage, ground water-surface

water interaction on short timescales). Moreover, in somecases, the steady-state or the successive steady-state ap-proximations simply do not fit observed field data (e.g.,Dripps et al. 2006).

Traditionally, transient recharge in humid climateshas been calculated external to a ground water modelusing a soil water balance approach that subtractsevapotranspiration and runoff from precipitation. Thecalculations range from relatively simple such as aThornthwaite-based approach (e.g., Kim et al. 1999; Pint2002; Dripps and Bradbury 2007) to more sophisticatedapproaches such as linked ground water/surface watermodels (e.g., Hunt and Steuer 2000; Jones et al. 2006)and integrated atmosphere-biosphere models (e.g., Dripps2003). Regardless of the approach used, water that perco-lates beneath the root zone (net infiltration) is commonlyeither averaged over sufficient time so as to approximatesteady-state conditions (Levine and Salvucci 1999; Huntand Steuer 2000) or is assumed to cross the water tableinstantaneously after leaving the root zone. Unsaturatedzone processes are commonly considered to be insignifi-cant to ground water flow and are usually ignored in

1Corresponding author: U.S. Geological Survey, WRD, 8505Research Way, Middleton, WI 53562; (608) 828-9901; fax: (608)821-3817; rjhunt@usgs.gov

2U.S. Geological Survey, WRD, 2730 N. Deer Run Rd., CarsonCity, NV 89701; deprudic@usgs.gov

3U.S. Geological Survey, WRD, 8505 Research Way,Middleton, WI 53562; jfwalker@usgs.gov

4Department of Geology and Geophysics, University ofWisconsin—Madison, 1215 W. Dayton St., Madison, WI 53706;andy@geology.wisc.edu

Received July 2007, accepted November 2007.No Claim to original US government works.Journal compilationª2008National GroundWaterAssociation.doi: 10.1111/j.1745-6584.2007.00427.x

Vol. 46, No. 4—GROUND WATER—July–August 2008 (pages 551–560) 551

humid climates (Romano et al. 1999). The effects of thisassumption are not commonly evaluated, however, andclearly could be problematic for some model applicationssuch as contaminant transport.

Neglecting unsaturated zone processes is not consid-ered appropriate in arid regions, where the thickness ofthe unsaturated zone can reach hundreds of meters andwater recently recharged at the top of the unsaturatedzone may not ever reach the water table given current cli-mate conditions. Furthermore, there are notable exampleswhere the unsaturated zone was explicitly included inwatershed scale transient models in humid climates(Freeze 1972a, 1972b; Prudic 1981; Winter 1983; Smithand Hebbert 1983; Lee 2000; Vanderkwaak and Loague2001; Jones et al. 2006). However, when the unsaturatedzone is included, these efforts commonly used a fullycoupled saturated-unsaturated flow approach based onRichards’ equation (Richards 1931; Freeze 1971; Reevesand Duguid 1975; Frind and Verge 1978; Yeh 1987;Vanderkwaak 1999; Panday and Huyakorn 2004). Suchcodes are notable for the large computational effort re-quired and associated long run times. As a result, manytransient watershed scale ground water flow models inhumid settings do not include unsaturated zone processes,and water leaving the soil/root zone is simulated asrecharge instantaneously transmitted to the water table.

In this work, we evaluate the appropriateness of thisassumption by examining the effect of a thin (less than1 m) and a thick (15 to 26 m) unsaturated zone usinga ground water flow model of the Trout Lake basin innorth-central Wisconsin. This range of unsaturated zonethickness was chosen for our presentation because it canbe considered typical of watersheds in humid climates.Unsaturated zone processes in the Trout Lake basin mightbe considered negligible because this area has relativelyhigh precipitation and permeable sandy soils that rapidlytransmit infiltration to the water table. Thus, the area pro-vides a good endmember for testing the potential utilityof including unsaturated zone flow processes. Both thetiming and the mass balance (volume) of water deliveredto the water table are assessed. Recognizing that fullycoupled modeling using the Richards’ equation is beyondthe computational resources commonly available for suchwatershed scale transient models, we made the evalua-tions using the recently developed Unsaturated-ZoneFlow (UZF) Package (Niswonger et al. 2006) available forthe widely used ground water flow model MODFLOW(Harbaugh 2005). A brief description of the UZF Packageis given subsequently; the reader is referred to Niswongeret al. (2006) and Niswonger and Prudic (2008) for adetailed description of the theory and a comparison to afully coupled variably saturated flow model.

The UZF Package routes water through the unsatu-rated zone using a one-dimensional (1D) kinematic waveapproximation to Richards’ equation (Colbeck 1972;Smith 1983; Charbeneau 1984) that ignores capillaryforces (Niswonger et al. 2006). UZF can partition infil-trating water into evapotranspiration, unsaturated zone stor-age, and recharge. The Method of Characteristics (Abbott1966) is used to simulate unsaturated flow through homo-geneous sediment between the land surface and the water

table; as a result, the package cannot currently simulateperched water tables. Both the 1D vertical assumption andassumption of a homogeneous unsaturated zone are con-sidered suitable for regional scale soil water flow (Harterand Hopmans 2004). A Brooks-Corey function is used torelate unsaturated hydraulic conductivity to water content.The resulting kinematic wave formulation is markedlymore efficient than solving the full Richards’ equationand thus is suited for watershed scale transient modeling.

Site DescriptionThe Trout Lake watershed (Figure 1) is located

within the North Temperate Lakes Long Term EcologicalResearch (NTL-LTER) site (Magnuson et al. 2006) andthe USGS Trout Lake Water, Energy, and BiogeochemicalBudgets (WEBB) site (Walker and Bullen 2000). Groundwater–derived base flow accounts for more than 90% oftotal streamflow (Gebert et al. 2008). The aquifer consistsof 40 to 60 m of unconsolidated Pleistocene glacial depo-sits, mostly glacial outwash sands and gravel (Attig 1985).Given land surface elevations and heads, the unsaturatedzone thickness ranges from 0 to about 50 m. Saturatedhorizontal hydraulic conductivities are estimated to aver-age about 10 m/d (Okwueze 1983; Hunt et al. 1998). Ver-tical anisotropy in hydraulic conductivity is relativelysmall, with the ratio of horizontal-to-vertical conductiv-ity ranging from 4:1 to 8:1 at a scale of a few meters(Kenoyer 1988). The lakes occupy depressions in the gla-cial deposits that may penetrate more than 80% of theaquifer thickness. Annual precipitation averages about81.5 cm/year (National Climatic Data Center 2004); aver-age ground water recharge is estimated to be 27 cm/year(Hunt et al. 1998) and has been estimated to range fromabout 15 to 50 cm/year (Dripps et al. 2006). Annual evap-oration off the lakes is about 54 cm/year (Krabbenhoft etal. 1990; Wentz and Rose 1991). Lakes are well con-nected to the ground water system, and many lakes areflow-through lakes with respect to ground water.

The Trout Lake basin has been the focus of sev-eral modeling studies (Cheng 1994; Hunt et al. 1998;Champion and Anderson 2000; Pint 2002; Pint et al.2003; John 2005) that represent stages in the developmentand refinement of a regional ground water model, whichwill be used in future studies to address a variety of re-search problems, including the effects of climate change.We modified the model of Pint (2002) and Pint et al.(2003) for the work described here.

Model DesignMODFLOW-2005 (Harbaugh 2005) was used for the

three-dimensional model with a uniform horizontal nodalspacing of 75 m and four layers (Figure 2). The bottomthree layers ranged in thickness from 5 to 15 m, whereasthe upper layer was relatively thick, with a saturatedthickness between 8 and 35 m, to minimize the possibilityof nodes drying. All hydraulic conductivity zones werespecified with calibrated values reported by Pint (2002)and Pint et al. (2003). A two-dimensional analytic element(AE) model using GFLOW (Haitjema 1995) was modified

552 R.J. Hunt et al. GROUND WATER 46, no. 4: 551–560

from an existing regional model of the Trout Lake areaand was used to derive perimeter boundary conditions forthe finite-difference model using the methodology of Huntet al. (1998). In this approach, ground water fluxes calcu-lated at the boundaries of the MODFLOW grid by the AEmodel were distributed to the upper three layers of thefinite-difference model on the basis of layer transmissivityand were input to the Well Package of MODFLOW. Thecrystalline bedrock, assumed to be impervious, formed thebottom boundary of the model.

Recharge flux was entered into the model in twoways. First, a traditional approach was used wherebywater leaving the soil zone was instantaneously added tothe water table via the Recharge (RCH) Package forMODFLOW (Harbaugh 2005). The second approach wasto add the water leaving the soil zone as ‘‘infiltration’’ tothe top of the unsaturated zone simulated by the UZFPackage (Niswonger et al. 2006). MODFLOW variablesare included in Table 1 and discussed subsequently. Re-charge to the saturated zone in UZF was calculated usingunsaturated zone properties, including the vertical satu-rated hydraulic conductivity (VKS) of the soil and a para-meter that describes the relation of hydraulic conductivityto soil moisture (a Brooks-Corey epsilon or EPS vari-able), as well as the transient loading of water specifiedby the time-varying infiltration (FINF) rate. Unsaturatedzone thickness is also used by UZF to simulate rechargeand is calculated by subtracting the simulated water table

elevation from the land surface elevation that is specifiedby the top elevation (TOP) in the DIS file of MODFLOW.It is important to note that TOP is a critical parameter inthe calculation of recharge within UZF—this differs fromthe RCH Package approach whereby the variable TOP foran unconfined top layer is commonly set to some arbi-trary value because it is not used in most calculations ofground water flow.

In both approaches, the water leaving the soil zonewas calculated externally using results from IBIS (Inte-grated Biosphere Simulator; Foley et al. 1996), an atmo-sphere-biosphere model that has been shown to provideestimates similar to other field and model estimates forthe site (Dripps et al. 2006). The IBIS model uses landcover and soil data along with hourly/daily inputs of pre-cipitation, solar radiation, air temperature, relative humid-ity, and wind speed to estimate the daily net watertransmitted through the soil zone. Because evapotranspi-ration losses are incorporated into IBIS, the evapotranspi-ration capabilities of UZF were not invoked. The dailyflows calculated by IBIS were combined into monthlyrates, which were in turn specified as either monthlyrecharge directly to the water table (RCH Package) orinfiltration to the top of the unsaturated zone (UZF Pack-age). Thus, the infiltration rates are identical in both RCHand UZF approaches, and only the method for moving thewater to the water table differs. A 10-year transient runextending from November 1988 through December 1998

92°

90°

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4 5 kmBase from U.S. Geological Survey1:24,000; Boulder Junction, 1981, Sayner, 1982, White Sand Lake, 1981,and Woodruff, 1982

K7

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U N I T E D S T A T E SU N I T E D S T A T E S

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TroutLake

Trout River

outlet

(fig. 7)thin unsat

zone (fig. 4)

thick unsat

zone (fig. 6)

D3

K30

Figure 1. Site map showing location of the Trout Lake area, Wisconsin, and locations of interest. D3, K7, and K30 are loca-tions of Long Term Ecological Research wells used to assess parameter sensitivity; these wells are in areas with unsaturatedzone thickness of 3 to 5 m. The location of the Trout River outlet and thin and thick unsaturated zone locations referred to inthe text are also shown.

R.J. Hunt et al. GROUND WATER 46, no. 4: 551–560 553

consisted of monthly stress periods divided into 15 timesteps, starting from an initial steady-state stress periodcalculated using long-term average recharge rates. Thesteady-state stress period was used to provide initial con-ditions for head and initial unsaturated zone saturation(THTI variable in the UZF Package).

Although lateral unsaturated zone flow is not simu-lated, one valuable capability of UZF that was used inthis work is the lateral routing of surface runoff tostreams and lakes simulated in MODFLOW. Runoff inUZF can be generated by the following: (1) rejected infil-tration (infiltration rate that exceeds the transmittingcapacity of the soil); (2) rejected recharge (unsaturatedzone storage insufficient to accommodate the volume ofinfiltrating water: Theis 1940); and (3) surface seepageresulting from ground water levels above land surface.The Lake Package (Merritt and Konikow 2000) was usedto simulate lake stages and water budgets in 30 lakeswithin the Trout Lake basin or near its boundary (Pint2002); streams located within the basin were simulated

using the SFR Package (Prudic et al. 2004; Niswongerand Prudic 2005). Runoff from low-lying areas identifiedin a digital elevation model was routed to the adjacentstreams and lakes via the IRUNBND array specified inUZF. Lakes and streams distant from the area of interestwere represented as specified heads using the River Pack-age (Harbaugh 2005); these features were not included inthe routing of runoff.

It should be noted that the objective of the modelingwork considered here was not to obtain a fully calibratedmodel of the basin, which is the basis of ongoing workusing the fully coupled ground water/surface watermodel, GSFLOW (Markstrom et al. 2008), which alsosimulates unsaturated zone flow using UZF. Input varia-bles for aquifer properties were taken from a previoustransient calibration of the Trout Lake model without theunsaturated zone (Pint 2002). Unsaturated zone input var-iables were assigned reasonable values based on the liter-ature for the type of sediment at Trout Lake (Table 2).Transient model parameters considered in the sensitivity

R2

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Layer 2

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X

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515m

480m475m

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Figure 2. (A) Overview of MODFLOW model design (taken from Pint et al. 2003) showing location of layer 1 high hydraulicconductivity (Kb, Ks), recharge (R1, R2), and lakebed leakance (L-1 through L-7) zones. (B) Cross section shows the verticallayering of hydraulic conductivity zones. Vertical exaggeration is 90.

554 R.J. Hunt et al. GROUND WATER 46, no. 4: 551–560

analysis were specific yield (Sy) and specific storage (Ss)for the RCH model and three additional parameters forthe UZF model, including saturated soil water content(THTS), vertical saturated hydraulic conductivity (VKS),and Brooks-Corey epsilon (EPS). Parameter sensitivitieswere calculated by the parameter estimation code PEST(Doherty 2005) using 1% perturbations.

Time series values of measured ground water headwere used as observation data to calculate parameter sensi-tivity for the RCH and UZF models. Heads measured atthree locations in the basin (labeled D3, K7, and K30 inFigure 1) were used to evaluate sensitivity for one thinner(around 3 m at D3) and two intermediately thick (around 5m at K7 and K30) unsaturated zones. The measured datawere obtained from the NTL-LTER database (http://lter-query.limnology.wisc.edu [registration required]). The ob-served and simulated heads were processed into equivalentdrawdown time series using the PEST utility TSPROC(Doherty 2003). Each drawdown observation in each wellwas given equal weight in the sensitivity calculation.

Results: RCH vs. UZF RunsThe additional data required to substitute UZF for

the RCH Package into a MODFLOW transient run can be

minimal (as few as three additional parameters in UZFinput file—VKS, THTS, and EPS; Table 1). However,the memory requirements of runs using UZF (controlledprimarily by the parameters NTRAIL2 and NSETS2;Table 1) can be substantially larger than the same runusing the RCH Package. In addition, the use of UZFappreciably increased the run time of the 10-year tran-sient run—in some cases by as much as 100%. Even withthe potential for longer run times, however, runs usingUZF are still considerably shorter than run times fora fully coupled saturated/unsaturated flow model usinga Richards’ equation approach. Moreover, includingunsaturated zone processes can result in distinct simula-tion advantages, as described subsequently.

Parameter Sensitivity ResultsThe additional input variables for UZF provide more

flexibility in calibration, but the importance of new pa-rameters relative to the traditional parameters for calibrat-ing transient models using the RCH Package (storage,specific yield) is not immediately obvious. Composite-scaled sensitivities to the measured water table fluc-tuations at three locations in the basin (Figure 1) showdifferences among the parameters (Figure 3). The com-posite-scaled sensitivity reflects the total amount of infor-mation provided by the observations for the estimation ofa parameter (Hill and Tiedeman 2007). The sensitivity ofparameters can change as their values change; thus, thefollowing sensitivities are strictly quantitative only for thevalues shown in Table 2. Specific yield (Sy) is the mostsensitive parameter for both the UZF and the RCH ap-proaches, though the UZF sensitivity is larger (Figure 3).In both the UZF and the RCH models, Sy is used for thetraditional purpose of calculating changes in saturatedzone storage during fluctuation of the water table. UnlikeRCH, however, UZF additionally uses Sy for calculationsinvolving both the timing and the volume of recharge aswetting front velocities and the amount of water stored inthe unsaturated zone (and available to be captured by a ris-ing water table) are affected by values of Sy. All threeadditional UZF variables (VKS, EPS, and THTS) are less

Table 1MODFLOW Variables Used in Text

Name MODFLOW Package Short Description

NTRAIL2 UZF Number of trailing waves used to define the water content profileNSETS2 UZF Number of wave sets used to simulate multiple infiltration periodsIRUNBND UZF Array of integer values used to define the routing of rejected recharge

to surface water featuresVKS UZF Soil-saturated vertical hydraulic conductivityEPS UZF Brooks-Corey epsilonTHTS UZF Saturated water content (thetasat)THTI UZF Initial water content (thetainit)FINF UZF Infiltration rateNSTP DIS Number of time stepsTOP DIS Elevation of the land surfaceSy BCF or LPF Specific yieldSs BCF or LPF Confined specific storage term

Table 2MODFLOW Parameter Values Used in the Study

ParameterValue Usedin Model

Specific yield (Sy)1 0.23Specific storage (Ss)1 53 1024/mSaturated vertical hydraulicconductivity (VKS)1

2.4 m/d

Brooks-Corey epsilon (EPS)2 3.3Saturated water content ofunsaturated zone (THTS)2

0.24

1From Pint (2002).2From Brooks and Corey (1964).

R.J. Hunt et al. GROUND WATER 46, no. 4: 551–560 555

sensitive than Sy, but two variables, VKS and EPS, aremore sensitive than specific storage (Ss). The saturatedwater content (THTS) had no sensitivity for this transientmodel, given the 1% perturbation. Thus, it appears thatSy will continue to be an important variable for simulat-ing water tables in humid settings. Given the sensitivity ofsome of the UZF parameters, however, additional flexibil-ity provided by these additional variables may furtherenhance simulations of system dynamics.

Results for Thin Unsaturated ZonesThere was little difference in timing between infiltra-

tion (water added to the top of unsaturated zone) andrecharge at the water table in areas with thin (less than1 m; Figure 1) unsaturated zones (Figure 4a), a phenome-non facilitated by the soils having relatively high verticalhydraulic conductivity (2.4 m/d). However, due to differ-ences in how recharge is handled in the RCH and UZFPackages, there can be discrepancies in the actualrecharge rates applied in some areas of the model.

Discrepancies in the recharge rate simulated by theRCH and UZF Packages can occur via two mechanisms.First, the rate of water added to the unsaturated zone canexceed the rate that the fully saturated soil can transmitvertically, which, in turn, diverts some of the ‘‘rejectedinfiltration’’ to Hortonian overland flow (Horton 1933).This is not a dominant mechanism in the results shownhere due to averaging of extreme events into monthlystress periods and the use of relatively high verticalhydraulic conductivity (2.4 m/d) for the soil zone sedi-ments. Second, a rising water table reduces the unsatu-rated zone storage available to accommodate infiltratingwater so that a portion of infiltrating water (‘‘rejectedrecharge,’’ Theis 1940; ‘‘saturation excess,’’ Dunne andBlack 1970a, 1970b) is diverted into Dunnian overlandflow. A third effect can occur owing to the presence of theunsaturated zone: because a rising water table captureswater stored in the unsaturated zone and adds it asrecharge to the saturated zone, the recharge rate can occa-sionally exceed the infiltration rate during periods of

rising water tables (asterisks in Figure 4a). Because thevolume of water stored in thin unsaturated zones is com-monly less than the volume of infiltrating water, however,the effect is usually modest.

As a result of the first two mechanisms, a portion ofthe monthly infiltration input to a cell in the UZF modelbecame part of the discharge from that node, and theactual amount of recharge added to the aquifer is lessthan that specified as infiltration (Figure 4b). This differ-ence, along with ground water discharge at the land sur-face (seepage from the ground water system resultingfrom the head being above land surface), become runoff,which can be removed from the model via evapotranspi-ration or routed to streams and lakes. Given the effects ofmonthly averaging of recharge events and sandy soils inthe site area, areas where runoff occurred were not wide-spread but were generally concentrated in wetland areasnear streams and lakes (red areas in Figure 5 the reader is

0.000.0

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(VKS)

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(THTS)

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Com

posi

te s

cale

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nsiti

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Figure 3. Composite-scaled parameter sensitivities calcu-lated by PEST using head data from the three LTER wellsshown in Figure 1.

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Simulated actual infiltration

Figure 4. (a) Simulation results for a thin unsaturated zone(less than 1 m). The pink line shows the IBIS-derived watervolumes applied to the top of the unsaturated zone (UZFPackage) or directly to the water table (RCH Package). Theblue line shows the recharge at the water table after routingthrough the unsaturated zone using the UZF Package. Aster-isks show where recharge volume at the water table ex-ceeded the infiltration volume due to additional water beingcontributed from unsaturated zone storage during times ofrising water tables. (b) Simulation for a thin unsaturatedzone in a node nearby to that shown in Figure 4a. In thisnode, the water table location near land surface results inUZF simulating less water entering the aquifer (blue) thanwas specified in the model input (blue 1 pink). The RCHPackage does not account for this inability of the aquifer toaccept the entire infiltration volume but instead adds theentire specified infiltration amount to the aquifer.

556 R.J. Hunt et al. GROUND WATER 46, no. 4: 551–560

referred to videos S1 and S2 in the online supplementarymaterial for animations of the 10-year recharge and dis-charge distribution).

When using the RCH Package in the Trout Lakebasin at a node like that shown in Figure 4b, all the in-filtrated water is transmitted to the water table as rechargeand ground water discharges are limited to model cellsdirectly connected to streams, lakes, and rivers (about 75m away for the node shown in Figure 4b). This conceptu-alization results in locally flooded cells in which thewater table in the unconfined aquifer exceeded the landsurface elevation. Thus, the standard RCH Packageapproach of adding infiltrating water directly to the watertable can locally add more water to the water table than ispossible given the soil conditions and topography presentin a watershed. Or put another way, although a traditionalRCH Package approach can approximate the correct tim-ing of recharge in thin unsaturated zones, it may lead towater budget errors as a result of overestimating transientrecharge because it neglects the effects of areas whererecharge is rejected and thin unsaturated zones reduce thenet infiltration rate.

Incorrect representation of the distribution of groundwater recharge in areas where the water table fluctuatesbetween land surface and a few meters below land sur-face is expected when using shorter stress periods (lessaveraging of extreme events), in low-permeability soils(less ability to transmit water in the soil column), and inareas with high water tables (less unsaturated zone stor-age available). Moreover, externally computed infiltrationrates (such as the IBIS rates used here) may exceed theability of the soil to accept water (infiltration rate isgreater than the saturated vertical hydraulic conductiv-ity)—a phenomenon that cannot be considered whenapplying the rates directly in the RCH Package but that isconsidered when applying infiltration rates in UZF. Thus,UZF simulates the difference between potential andactual recharge, whereas RCH assumes the actualrecharge is equal to the potential recharge.

Results for Thick Unsaturated ZonesIn an area with a thick unsaturated zone (between 15

and 26 m; Figure 1), runoff is not expected owing to rela-tively large unsaturated zone storage, monthly averagingof infiltration volumes, and the relatively high saturatedvertical hydraulic conductivity of soils in the basin. How-ever, effects of discharging ground water to land surfaceand rejecting recharge in other areas of the basin propa-gated laterally through the aquifer in the model andaffected the distribution of ground water head throughoutthe basin. For example, simulated heads were about 1 mlower in the thicker unsaturated zone areas (Figure 1) forUZF compared to RCH. Lower heads are attributed to thefollowing: (1) shorter flow paths from recharge to dis-charge areas (Figure 5) and (2) reduction in total groundwater recharge because of areas with rejected recharge.This difference suggests that the steady-state modeldeveloped by Pint (2002), which used the RCH Package,may have had somewhat different calibrated parametervalues if UZF had been used instead. This also demon-strates that the utility of using a UZF approach extends tosteady-state models as well as transient models because itcan help determine areas where shallow water tables maygenerate runoff as a result of saturation excess, groundwater discharge at land surface, and rejected recharge.

In addition to the effects of the spatial variability inrecharge and the effects on ground water head, the timingof recharge can be appreciably different between simu-lations that use UZF compared with RCH (Figure 6).When infiltrating water was routed through the unsatu-rated zone using UZF, recharge was temporally diffuseand characterized by appreciable lag time between infil-tration from the root zone and recharge at the water table.Additionally, mixing of wetting fronts occurred wherebyinfiltration from one season overtook and combined withinfiltration from a previous season, which resulted insignificant differences in the recharge rates for a givenmonth (Figure 6). The greater the unsaturated zone thick-ness, the larger the effect (Figure 6a vs. 6b). Thus, inthicker unsaturated zones, the local volume of waterinfiltrated was properly simulated using the standardapproach using the RCH Package (because infiltrationwas not being diverted to runoff), but the timing was lessrealistic. Or put another way, although a traditional simu-lation using the RCH Package can provide the correctvolume of recharge when averaged over a sufficientlylong time period, it may produce poor fits on the shorterterm timing of recharge because lag times and coalescingwetting fronts are not simulated. These effects are ex-pected to be most acute when using shorter stress periods(less averaging of extreme events) and in less-permeablesoils (slower transmission through the unsaturated zonecauses larger discrepancy between simulations with UZFcompared with RCH).

Implications for Transient Models ofGround Water-Surface Water Interaction

In addition to the effects on simulation of theground water system, the ability of UZF to route rejected

Figure 5. Net flux (recharge minus discharge) across watertable for simulation using the UZF Package at end of secondyear (day 665). Values are in cubic meters per day; positive val-ues (blue) are recharge and negative values (red) are discharge.

R.J. Hunt et al. GROUND WATER 46, no. 4: 551–560 557

recharge to streams and lakes can be important for simu-lating ground water-surface water interaction. Whenrejected recharge simulated with UZF was not routedto streams and lakes, the major outlet of the system(Trout River; Figure 1) went dry during part of the 10-year transient run (Figure 7)—a result that was not con-sistent with measured streamflow during this simulationperiod (USGS streamflow data [http://nwis.waterdata.usgs.gov/wi/nwis/measurements]). Routing rejected re-charge from low-lying areas near the streams and lakesusing the IRUNBND array in UZF allowed for continu-ous streamflow over the 10-year simulation, in agree-ment with field measurements. The simulation usingthe RCH Package also resulted in continuous stream-flow but routed the water through a locally over-pressurized ground water system rather than routing thewater via overland flow, which may result in longertravel times to discharge points. This lag, in turn, maycause inaccurate representation of fluctuations of lakestage and streamflow.

ConclusionsWe used a ground water flow model of the Trout

Lake basin in northern Wisconsin, which is character-ized by a relatively humid climate and transmissivesoils, to compare simulations routing infiltrationthrough the unsaturated zone using the UZF Package forMODFLOW with simulations where infiltrated waterwas input directly to the water table using the RCHPackage. Four primary conclusions can be drawn fromthis work:

1. Areas with thin (less than 1 m) unsaturated zones were

characterized by little difference in timing of recharge

between methods. However, recharge was reduced when

runoff occurred in some areas with the UZF Package—

a process that cannot be explicitly simulated with the

RCH Package. Thus, in areas with thin unsaturated zones,

a traditional simulation using the RCH Package may have

similar timing as a simulation using UZF, but the water

budget, as reflected in volume of water recharged to the

water table, can be appreciably different.

2. In an area with a relatively thick unsaturated zone (15 to

26 m), the timing of recharge was more diffuse when

routed through the unsaturated zone, often characterized

by lags and mixing of wetting fronts. For a given month,

mixing of wetting fronts can result in recharge rates that

are appreciably different from infiltration rates. Thus, in

thicker unsaturated zones, the long-term average volume

of water infiltrated in these areas was properly simulated

when using the RCH Package, but the timing and short-

term rate of recharge were incorrect.

3. Even though the same volume of infiltration was speci-

fied in both models, heads calculated using the UZF

Package were, in places, more than 1 m lower than

heads calculated using the RCH Package due to addi-

tional discharge locations and rejection of recharge dur-

ing months with high infiltration. As a result, calibration

of either a steady-state or a transient model to heads

would likely have resulted in somewhat different

0.00E+00

5.00E+04

1.00E+05

1.50E+05

2.00E+05

10/1/96 4/1/97 10/1/97 4/1/98 10/1/98

Str

eam

flow

(m

3 /d)

RCH PackageUZF with runoff routingUZF without runoff routingMeasured

dry

Figure 7. Simulation results and field-measured flows forthe Trout River downstream of the Trout Lake outlet. Thesimulation where runoff simulated by UZF was not routed tostreams and lakes resulted in lower flows and a period of noflow (marked ‘‘dry’’)—a condition not observed in the fieldduring this time (http://nwis.waterdata.usgs.gov/wi/nwis/measurements).

0

5

10

15

20

25

30(a)

(b)

10/1/88 4/1/89 10/1/89 4/1/90 10/1/90 4/1/91 10/1/91

Infiltration rate out of root zoneRecharge rate at water table

Nod

al v

olum

etric

rat

e (m

3 /d)

Nod

al v

olum

etric

rat

e (m

3 /d)

10/1/88 4/1/89 10/1/89 4/1/90 10/1/90 4/1/91 10/1/910

5

10

15

20

25

30Infiltration rate out of root zoneRecharge rate at water table

Mixing ofinfiltration

fronts Lag

LagLag

Lag

Mixing of infiltration

fronts

Figure 6. (a) Simulation results for a thick unsaturated zone(15 m). The pink line shows the Integrated Biosphere Simula-tor (IBIS)-derived water volumes applied to the top of theunsaturated zone (UZF Package) or directly to the watertable (RCH Package). The blue line shows the recharge at thewater table after routing through the unsaturated zone (UZFPackage). Note that recharge at the water table during Octo-ber 1990 does not return to baseline observed in the summer,indicating the mixing of fall 1990 infiltration front with theprevious spring’s infiltration. (b) Simulation results fora thick unsaturated zone (26 m). The pink line shows theIBIS-derived water volumes applied to the top of the unsatu-rated zone (UZF Package) or directly to the water table (RCHPackage). The blue line shows the recharge at the water tableafter routing through the unsaturated zone (UZF Package).

558 R.J. Hunt et al. GROUND WATER 46, no. 4: 551–560

parameter values. This underscores the potential utility

of a UZF approach for calibrating steady-state as well as

transient models.

4. In addition to providing a more representative simulation

of the transient watershed scale ground water system

response, routing runoff to surface water by the UZF

Package resulted in a more realistic simulation of stream-

flow.

These results demonstrate that unsaturated zone pro-cesses may be important in representing ground waterflow in humid areas. Moreover, for the relatively smallcost in additional parameterization and increased runtimes, it is likely that the UZF Package will be a valuabletool for obtaining more representative simulations inmany transient MODFLOW models.

AcknowledgmentsSupport was provided by the USGS Ground-Water

Resources Program; the USGS Trout Lake Water, Energy,and Biogeochemical Budgets (WEBB) project; and theNorth Highland LTER Project funded by the NationalScience Foundation (DEB-9632853). Richard G. Nis-wonger (USGS) provided many helpful discussions andsuggestions and reviewed many facets of this work. PaulBarlow (USGS), Walter A. Illman, and two anonymousreviewers are also thanked for their review of the manu-script. Greg Pohll (Desert Research Institute) is thankedfor help creating the supplemental material. We thankJohn McCray, who was the guest Editor-in-Chief for thispaper.

Supplementary MaterialThe following supplementary material is available

for this article:

Video S1. Ten-year animation of spatial distributionof recharge over the basin calculated by the UZF Pack-age. Recharge values are shown in cubic meters per day(legend in upper left) and time is in years (timer in upperright).

Video S2. Ten-year animation of spatial distributionof discharge over the basin calculated by the UZF Pack-age. Discharge shows less spatial variability than the re-charge distribution, and discharge locations are generallylocated in low-lying wetland areas adjacent to lakes andstreams. Discharge values are show as cubic meters perday (legend in upper left) and time is in years (timer inupper right).

This material is available as part of the online articlefrom: http://www.blackwell-synergy.com/doi/abs/10.1111/j.1745-6584.2007.00427.x

(This link will take you to the article abstract).Please note: Blackwell Publishing is not responsible

for the content or functionality of any supplementary ma-terials supplied by the authors. Any queries (other thanmissing material) should be directed to the correspondingauthor for the article.

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