Water Flow and Solute Transport in the Subsurface
Precipitation

Vadose zone

A HYDRUS approach to couple vadose zone andground water flow

Modeling
subsurfacehydrology

Water table
Recharge fluxmodels

MODFLOWHYDRUS short-course and workshop

Navin K. C. Twarakavi & Jirka Šimůnek
Department of Environmental Sciences, University ofCalifornia Riverside, Riverside, CA, USA

Colorado School of Mines, Golden, CO

RunoffRoot water uptake

HYDRUS

Based onwater table

Ground water

Sophia SeomodelsTranspiration
Evaporation

Czech University of Life Sciences, Faculty of Agrobiology, Food andNatural Resources, Prague, Czech Republic&PC-Progress, Ltd (Visit us at: http://www.pc-progress.cz)
March 26-28, 2008

MODFLOW• MODFLOW strictly models ground water flow processes.
• MODFLOW can incorporate various sinks/ sources suchas rivers, streams, wells and drains• MODFLOW follows the concepts of packages. Anyprocesses can be added to the MODFLOW environmentby writing an add-on package.

Subsurface flow models that adequately couple vadose zone flow to ground water would
be very beneficial.

Incorporating vadose zone flow in to ground watermodels

Modeling vadose zone-groundwater interactions in MODFLOW

Ground water
recharge using thewater-balance
budgetKinematic wave
approximation offlow in the vadose
zone

Rigorously solving theground water andvadose flow using the3D Richards equation

Incorporating vadose zone flow in MODFLOW: Comparison of
approaches

Modeling vadose zone-groundwater interactions

MODFLOW package

Recharge using thewater-balance
budgetKinematic wave
approximation offlow in the vadosezone

Solving the groundwater and vadose flowusing the 3D Richardsequation

REC & ET packages

UZF1 package

VSF package

Accuracy

HYDRUS package for MODFLOW• Objective:– Develop a vadose zone package for MODFLOW that canreasonably incorporate vadose zone processes.
– Allow application of the package to modeling problems of varying
spatial scales.
– Extend the package to model solute transport in the vadose zone
and ground water.
• The HYDRUS package for MODFLOW was developed to meet theseobjectives (Seo et al., 2007; Twarakavi et al., 2008).
• The HYDRUS package considers the following vadose zone processes• Precipitation• Infiltration• Evaporation• Redistribution

Modeling speedVadose Zone processes
Large-scale modeling• Capillary rise
• Root water uptake• Water accumulation
at the groundsurface• Surface runoff• Soil moisture storage

HYDRUS package: Approach

HYDRUS Package: Zoning

The HYDRUS package for MODFLOW uses two sub-models interacting with eachother in time and space: (a) HYDRUS and (b) MODFLOW.

MODFLOW model domain is grouped in to zones based on similarities in soil
hydraulic characteristics, hydrogeology and meteorology.
A HYDRUS vertical profile is assigned to each of the zones on which the 1D
Richards equation is used.

The HYDRUS sub-model uses the 1D Richards Equation to simulate vadose zone
processes.

Reservoir

Precipitation/ snowfall
Evapo-transpiration (ET)Surface runoff

farm

Land surface

HYDRUS package(1D Richards Equation)

Infiltration
Root water uptake
ET extinction depth
Vadose zone storage

MODFLOW

Water table

Water table

Recharge
Ground water flow

Zone 1Zone 2

HYDRUS-MODFLOW Interaction

Interaction in Time

MODFLOW and HYDRUS exchange information at the end of each of time foreach of the zones.

MODFLOW Sub-model

Average watertable depths

Bottom fluxes asrecharge at the watertable for the next
MODFLOW time stepHYDRUS Sub-modelSolve for bottom fluxes in eachprofile using the atmospheric
data and 1D Richards Equation

Time steps

t=1

start

t=2

end

Meteorologicalconditions(daily)(seasonal)
MODFLOW(ground water)

Water table

Zone 1

Zone 2HYDRUS Package: Ease of Use
Like other packages, HYDRUS is coded within MODFLOW

Average watertable depths
for each zone.

Total flux atthe water tablefor each zone

Averagewater table
depths foreach zone.

Total flux atthe water tablefor each zone

Averagewater table
depths foreach zone.

Total flux atthe water tablefor each zoneHYDRUS(vadose zone)

Case Study 1
One-Dimensional Las Cruces infiltration experiment (Wierenga et al., 1991)

Case Study 2

1

Case Study 3

234ROWS
657891015

Two-dimensional Vauclin water table recharge experiment (Vauclin et al., 1979)

14

Hypothetical regionalscale ground waterflow problem:

131211COLUMNS

10Model domain, spatial
distribution of hydraulicconductivities and specificyields, wells (red circles) andgeneral head boundaries.

98765432
1
Alluvial aquifer (Hydraulic conductivity=53 m/d, specific yield=0.2)
Uplands (Hydraulic conductivity=11 m/d, specific yield=0.1)
Inactive cells
Pumping well

General head boundary(a) Land surface elevation (m)3314050501106070

3 1815080330

3 26328

32 2312
314

320

324

30316

40310

W

NW

53528763N

6050(c) Initial water table depth (m)4Zonation showing thespatial distribution ofprecipitation

70

290

130

120

4432

Hypothetical regional-scale ground water flow problem:

5050333 3 4 336
330
33 2Case Study 350

7060330 332

3 20202S46810 Kilometers(a)

Legend (in m)
(b)
303.8

24.3

0.3

1586345 2
14 131392703 Kilometers(c)4134710 11 9 5 652105ESE379 8
11 12 1311a) Land surface elevationb) depth to bedrockc) water table depth at thebeginning of the simulation

50

3 36

Hypothetical regionalscale ground waterflow problem:(b) Aquifer thickness (m)100Case Study 3

12 11334.5

144.4

18.4

Precipitation rate factor
1258No DataCase Study 3

Case Study 3

Hypothetical regional-scale ground water flow problem:

Hypothetical regionalscale ground waterflow problem:

MODFLOW zones
used to define
HYDRUS soilprofiles

NWES3(b)(a)

03 Kilometers

Ground water table fluxes(recharge vs discharge) as
predicted by the HYDRUS
package at the end of stressperiods (a) 3, (b) 6 and (c) 12.

(c)
NZones
12
3
4
5
6
7
8
910

11
12
13
14
15
16
17
18
1920

Conclusions & Future Directions• The HYDRUS package for MODFLOW has proven to be a viable
alternative for ground water models of different spatial scales.
• The HYDRUS package for MODFLOW has been recently expanded tofacilitate solute transport modeling.
• The HYDRUS Package for MODFLOW produces concentrations as afunction of time that can be incorporated into the source function for
MT3DMS.

WES

404Recharge
Discharge

8 Kilometers