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OUTLINE
Challenges in Developing Models
Describing Complex Soil Systems

 How Complex Should Models Be?
 The Use of Complex Models
 Selected HYDRUS-BasedHYDRUS Based Models(e.g., HP1, colloid transport)

Jirka Šimůnek1 and Diederik Jacques2

 Models Verification and Validation(model inter-comparisons; benchmarking)

1Department of Environmental Sciences,

University of California, Riverside, CA, USA

 Documentation of Complex Models

2Belgian Nuclear Research Centre (SCK•CEN), Mol, Belgium

Presentation at the 2014 AGU
San Francisco, California, December 19, 2014

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(technical and user manuals, online help, source code)

 DangerD((misuse)i) off C
Complexl M
Modelsd l

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How Complex Should Models Be?
“Everything should be made as
simple as possible
possible, but no simpler.”
simpler ”or“Make things as simple as possible,
but not simpler.”

The Use of Complex Models
Is there demand for complex models?
 My experience is that there is not.
 There are frequent calls for more complex models
 However, once new processes are included in existing

Adapted by modelers as:

Alb t Ei
AlbertEinsteint i“Models
Models should be
as simple as possible,
but no simpler.”
simpler.

models they are often ignored by model users
 While we have hundreds (or even thousands) of users forthe basic standard HYDRUS (MODFLOW, PHREEQC)
models, we have very few users for specialized modules(e g UnsatChem,(e.g.,
UnsatChem HP1/2/3,
HP1/2/3 CRide,
CRide DualPerm).
D alPerm)To help protect y our priv acy , PowerPoint prev ented this external picture from being automatically downloaded. To download and display this picture, click Options in the Message Bar, and then click Enable external content.

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The Use of Simpler Models
MODFLOWThe Use of Simpler Models

ISI, Web of Knowledge

Web of Knowledge
HYDRUS

PHREEQC

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The Use of Complex Models
UnsatChem (Major Ion Chemistry Model)

SOILCO2 (Carbon Dioxide Transport and Production Model)

The Use of Complex Models
CW2D (Wetland Module)

HP1 (Coupled HYDRUS + PHREEQC)

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OUTLINEThe Use of Complex Models
 Quantitative mechanistic models that consider basic physical,

mechanical, chemical, and biological processes have the potential to be
powerful tools to integrate our understanding of complex soil systems
systems.
 The soil science community has often called for models that would
include a large number of these diverse processes.
 However,Honce attempts
tt
t havehbbeen maded tto ddevelopl suchh models,
d l ththeresponse from the community has not always been overwhelming:- these models are consequently highly complex- requiring a large number of parameters (not all of which can be easily (or atall) measured and/or identified, and which are often associated with large uncertainties),- requiring from their users deep knowledge of all/most of these
implementedppphysical,y, mechanical,, chemical and biologicalgpprocesses.
 Real, or perceived, complexity of these models then discourages users
from using them (even for relatively simple applications, for which they would be
perfectlypy adequate)q).
 It is virtually impossible to verify these types of models analytically (orvalidate them), raising doubts about their applicability.

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 How Complex
p Should Models Be?
 The Use of Complex Models
 Selected
S l t d HYDRUS
HYDRUS-Based
B dM
Modelsd l(e.g., HP1, colloid transport)

 Models Verification and Validity(model inter-comparisons; benchmarking) Documentation of Complex Models(technical and user manuals, online help, source code)

 Danger (misuse) of Complex Models

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Why to Develop Complex Models?

How to Develop Complex Models?

Environmental Problems:

HP1/2/3 models for complex environmental problems:

Water Flow

Solute Transportp

BiogeochemicalgReactions

Advanced (complex) mathematical/numerical models areneeded to analyse complex environmental problems involving
water flow and nutrient/contaminant transport, as well as
many biogeochemical reactions in soils

Water Flow

Solute Transport

HYDRUS

HP1/2/3

Biogeochemical
Reactions

PHREEQCTo help protect y our priv acy , PowerPoint prev ented this external picture from being automatically downloaded. To download and display this picture, click Options in the Message Bar, and then click Enable external content.

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HP1/2/3 (HYDRUS+PHREEQC)
Simulating water flow, transport and biogeochemical reactions in environmental
soil quality problems

HYDRUS (1D/2D/3D)
Software for Simulating Water Flow and
Solute Transport in One/Two/Three Di
Dimensionalil Variably-Saturated
V i bl S t
t dS
Soils
il
Using Numerical Solutions

Flow and transport model
HYDRUS-1D 4.0
HYDRUS (2D/3D) 2.x- thousandshd off users aroundd the
h world
ld- thousands of applications published- used by scientists, students, and/or practicing professionals

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HP1/2/3 (HYDRUS+PHREEQC)

A Coupledp
Numerical Code for
Variably Saturated Water Flow,
Solute Transport and
BioGeoChemistryin Soil Systems

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Uniform Flow and Transport Model in HP1HYDRUS--1D or HYDRUS (2D/3D):HYDRUSyVariably-Saturated
Water Flow Solute Transport
 Heat Transport
 Gas Transport
 Root Water Uptake

Three phase systemqsolid and gas
g phasep
Aqueous,d phaseSolid

PHREEQC [Parkhurst and Appelo,
Appelo, 1999]:

Transport
Water flow
Heat transport
Advection-dispersion in water
Diffusion in gas phase

s
a w

Available Chemical Reactions:Aqueous Complexation
 Redox Reactions
 Ion Exchange (Gains-Thomas)
 Surface Complexation (diffuse double-layer model and nonelectrostatic surface complexation model)
 Precipitation/Dissolution
Precipitation/Dissol tion
 Chemical Kinetics
 Biological Reactions

Biogeochemical model
PHREEQC 2 4
PHREEQC-2.4

ws

Poro
osity



wa

Gas phase

Sr,w
Sr,s,iSc,iAqueous phase

Homogeneous sink/source terms S
Root water uptake Sr,w
SoluteSou e root
oo w
water
e up
uptakee Sr,s,i
rs i
Degradation/decay/transform. Sc,i
Heterogeneous mass exchange 
Aqueous – solid phase ws
Aqueous – air phase wa

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Uniform Flow and MIM Transport in HP1

Dual-Porosity Water Flow and Solute Transport in HP1Three phase system
MobileImmobile

Three phase system

Aqueous, solid and gas phase

MobileImmobile

Transport

s

Poro
osity

a w
wa

ws,m

ws,imi
i

Sr,w
Sr,s,iSc,iGas phase

Water flo
flow
Heat transport
Advection-dispersion in water
Diffusion in gas phase
Solute exchange iMobile / Immobile

Transport

s
a w

Homogeneous sink/source terms S
Root water uptake Sr,w
Solute root water uptake Sr,s,i
Degradation/decay/transformation Sc,i

wa

ws,m

ws,imi
i
w
Sr,w
Sr,s,iSc,iHeterogeneous mass exchange 

Aqueousq– solid pphase ws(in both mobile/immobile domains)
Aqueous – air phase wa

Gas phase

Aqueous phase

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Aqueous, solid and gas phase

Two domains
d phase
Solid

Mobile / Immobile

Poro
osity

d phaseSolid

Two liquid
liquid-phase
phase domains

Water flow
Heat transport
Advection-dispersion in water
Diffusion in gas phase
Water exchange w
Solute exchange i

Homogeneous sink/source terms S
Root water uptake Sr,w
S l root water uptake
Solutek Sr,s,i
Degradation/decay/transformation Sc,i

Heterogeneous mass exchange 

Aqueousq– solid p
phase ws
(in both mobile/immobile domains)
Aqueous – air phase wa

Aqueous phase

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HYDRUS GUI for HP1/2/3

Jacques, D.,
Jacques
D and JJ. Šimůnek
Šimůnek, Notes on the HP1 software – a coupled code for variably-saturated
variably saturated water
flow, heat transport, solute transport and biogeochemistry in porous media, HP1 Version 2.2,
SCK•CEN-BLG-1068, Waste and Disposal, SCK•CEN, Mol, Belgium, 114 pp., 2010.

HYDRUS (2D/3D) GUI for HP2/3

Four text editors to define the geochemical model, required output, andsolution compositions are fully incorporated into the GUI.

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Transport and Cation Exchangey Metals ((HP1 Application)
pp
)
Heavy

((HP1 Application)pp)

Podzol Soil

4.2

Parameters:
q=2 cm/d, l=0.2 cm, CEC=11 mmol/cell.
Initial concentrations: Al=0.5,lBr=11.9, K=2, Na=6, Mg=0.75, Cd
Cd=0.09, Pb
Pb=0.1, Zn
Zn=0.25 mmol/L.
l/Boundary concentration: Al= 0.1, Br=3.7, Cl=10, Ca
Ca=5, Mg=1 mmol/L.
Species and Complexes: Al3+, Al(OH)2+, Al(OH)2+, Al(OH)3, Al(OH)4-, Br-, Cl-, Ca2+, Ca(OH)+ , Cd2+, Cd(OH)+, Cd(OH)2, Cd(OH)3-,
Cd(OH)42-, CdCl+, CdCl2, CdCl3-, K+, KOH, Na+, NaOH, Mg2+, Mg(OH)+, Pb2+, Pb(OH)+, Pb(OH)2, Pb(OH)3-,
Pb(OH)42-, PbCl+, PbCl2, PbCl3-, PbCl42-, Zn2+, Zn(OH)+, Zn(OH)2, Zn(OH)3-, Zn(OH)42-, ZnCl+, ZnCl2, ZnCl3, ZnCl42
AlX3, AlOHX2, CaX2, CdX2, KX, NaX, MgX2, PbX2, ZnX2

Exchange Species:

Concentrattion (m ol/l)Concentrat ion (m ol/l)

Cl

0.008
Na

0.006

Ca

0.004
0.002

C C
CEC

40

3.8

Atmospheric

3.6

5 cm depth

20
3.4

150

2

3

4

5

151

152

6

pH
U-species replacedby other cations Increased deprotonation
Increased U-sorption
p153

154 155 156
Time (year)

157

158

159

160

 Water content variations induce pH variations(dry soil => low pH)
 pH variations => variations in sorption potential(low pH => low sorption – higher mobility)

C, Ca, Cl, F, H, K, Mg, N(5), Na, O(0), O(-2), P, S(6), U(6)4E 0044E-004Pb

 Multi-site cation exchange reactions- Related to amount of organic matter

Cd Surface complexation reactions- Specific binding to charged surfaces (FeOH)- Increases with increasing pH- UO22+ adsorbs

2E-004- Related to amount of Fe-oxides

Al

0E+000

0
0

4

SC

Total

60

 Aqueous speciation reactions

Zn

6E-00480

08E-0040.01

Steady-state100% U (V I) adso rbed

An 8-cm soil column initially contains a solution (with heavy metals) in equilibrium with the cationexchanger. The column is then flushed with three pore volumes of solution w/o heavy metals.

U-Transport in Agricultural Field Soils

pH

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3

6

9

Time (days)

1215

 200-year time series of synthetic meteorological data

0

3

6

9

12

15

Time (days)

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 P-fertilizer (Ca(H2PO4)2): ~3000 Bq 238U/kg, applied eachyear on May 1 (1 g P/m2)

Jacques et al. (VZJ, 2008)

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Chemical Degradation of Concrete

Feedback of Geochemical Changes onpPropertiesp
Transport
 HP1 allows dynamic updates of- porosity- soil hydraulic properties- tortuosity
y in the aqueousq
and ggaseous pphase,,- dispersion,- heat conductivity,- heat dispersivity
 Users have great flexibility in implementing

any relationships via BASIC-functions in an
input file((HP1 Application)
pp
)




Chemical degradation of concrete in contact with soil water
Disposal site with a low-level radioactive waste
Concrete is multi
multi-scale
scale porous medium
 Cement Hydrates:
portlandite (Port), monocarboaluminate (Mc), strätlingite(Strat), calcite (Cal), OH-hydrotalcite (Ht), CO3hydrotalcite (Htc),(Htc) the ideal solid solution between jennite(Jen) and tobermorite (Tob), and the ideal solid solution
between ettringite (Ett) and tricarboaluminate (Tca) Model for Cement Phase (p):General effective medium homogenization
scheme (Oh and Yang, 2004)(consist of capillary pores (cp), gel pores (gp)
incorporated in calcium-silicate hydrates (CSH), solidparts of CSH (csh), and other cement hydrates (ch)(mainly portlandite) Updated Transport Properties:- Porosity- Tortuosity- Permeability (Wissmeier and Barry, 2009)To help protect y our priv acy , PowerPoint prev ented this external picture from being automatically downloaded. To download and display this picture, click Options in the Message Bar, and then click Enable external content.

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Mathematical Model

Coupled Reactive Transport Model

Link Between Different Processes
Water flow equation (variably-saturated flow)

MixingDisequilibrium

Transport properties
Permeability
Diffusitivity    K h  h  1t z  zTransport

microcracks cement pastes  r1  h   K S e   K s S 1  1  S en / n 1LeState II
Portlandite

C j
C j  qC j


 Rj
  Dhd
z
t
z 
z 

State III
State IVC-S-H, AFm, AFt Calcite

1000

m 2

Cement model
Thermodynamic calculation

100

0.5

900

0.4

solid phase

800

0.3
700
0.2

600

Porosity

Solid phase volume (cm³//1000 cm3)

Microstructural/ multi-scale
modelState INa- & K-oxideReactive solute transportpequationq((# number of components)p)

G
Geochemistryhi taggregate

 r

n m






Physicalporosity

500

0.1
10

-2

10

-1

0

1

10
10
10
Cumulative leached water (kg)

2

10

3

4

10

ChemicalSolution
Reactive phases

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 : tortuosity
D0 : free water diffusion coefficient
Factors:
S() effect of water saturation
G(T) effect of temperature
H(t) effect of degree of hydration
M() effect of microstructural changes

Dhd  

q



10-1

 Dp

10-2


ITZ

 h  

D p  τD0 S θ G T H t M φ 

()

10

-3

10

-4

GEM
Empiricalprelation I
Empirical relation II
GEM + ITZ

10-5
0

0.1

cp (cm3 cm-3)

0.2

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Chemical Degradation of Concrete((HP1 Application)pp)

Uranium Transport from a Mill Tailing Pile

Porosity changes during leaching of the concrete core at different depths:

Evolution of tortuosity and water flux:

Jacques et al. (2010)

105*25 m
1560 FE
Aqueous Complexation for Uranium Species
Calcite and Gypsum Precipitation/Dissolution
Cation Exchange

0.3

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HP1/2 Examples

Colloid, Virus, and Bacteria TransportAir Transport of Heavy Metals (Zn2+, Pb2+, and Cd2+) subject to a

Air-Water Interface Colloids, caca

multiple pH
pH-dependent
dependent Cation Exchange Transport and mineral dissolution of Amorphous SiO2 andGibbsite Infiltration of a Hyperalkaline Solution in a clay sample

kdca

kaca

Mobile Colloids, Cc(kinetic precipitation-dissolution of kaolinite, illite, quartz, calcite,
dolomite, gypsum, hydrotalcite, and sepiolite)

Water
kstr

 Kinetic biodegradation of NTA (biomass, cobalt)

kac

kdc

 Long-term Uranium transport following mineral phosphorus

ffertilizationtili ti (pH-dependent surface complexation and cation exchange) Transport of Explosives, such as TNT and RDX
 Property Changes (porosity/conductivity) due toprecipitation/ dissolution reactions

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sstrStrained Colloids, Scstrs
Attached Colloids, Scatt

S lid
Solid

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Why is colloid release different under transientyconditions?
from steady-stateParticle Transport in Hydrus
 w CcS A C  q C   c  aw c    w Dc c   c c  Rc
t
t
t
x 
x 
x
Colloid mass
mass--transfer between the aqueous and solid phases:ScS str
S att  c   c   w sstr k str Cc  ( w s kac Cc   kdc Sc )  Rsc
t
t
t

Colloid mass
mass--transfer between the aqueous phase and the air
air––water interface:Aaw  c  w aca kaca Cc  Aaw kdca  c  Rac
t
Cc
Sc
Scstr
Scatt

c

w
Dc
qc



colloid concentration in the aqueous phase [nL-3]
colloid concentrations adsorbed to the solid phase [nM-1]solid-phase
solid
phase concentrations of strained colloids [nM-1]solid-phase concentrations of attached colloids [nM-1]
colloid concentrations adsorbed to the air–water interface[nL-2]
volumetric water content accessible to colloids [L3L-3] (dueto ion or size exclusion, w may be smaller than the total
volumetric water content )dispersion coefficient for colloids [L2T-1]
volumetric water flux density for colloids [LT-1]
bulk density [ML-3]

Aaw
Ri
kstr
kac
kdc

s
aca
kaca
kdca

air-water interfacial area per unit volume [L2L-3]
various chemical and biological reactions [nL-3T-1]
fi
first-orderd straining
i i coefficientffi i[T-11]
first-order colloid attachment coefficient [T-1]
first-order colloid detachment coefficient [T-1]
dimensionless colloid retention function [-]dimensionless colloid retention function for the air–water interface (-)
first-order colloid attachment coefficient to the air–water interface [T-1]
first-order colloid detachment coefficient from the air–water interface [T-1](Lazouskaya et al. 2013; JCIS)
Colloid retention and release (at/from SWI) are controlled by forces andttorques Release is a diffusion controlled process under steady-state conditions
 Transients conditions alter the adhesive and/or hydrodynamic forces Imbibition: AWI is destroyed (colloids are released to the water phase) Draining: Colloids are removed from SWI to AWI and the aqueous phaseTo help protect y our priv acy , PowerPoint prev ented this external picture from being automatically downloaded. To download and display this picture, click Options in the Message Bar, and then click Enable external content.

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Effects of Imbibition and Draining on Colloids

Imbibition: Release of Microbes from AWI

Imbibition: AWI is destroyed (colloids are released to the water phase)
Draining: Colloids are removed from SWI to AWI and the aqueous phase




Eaw =

Aaw 
=  kaaw a Aaw c - Aaw kdaw Γ - Era  f aw Eswa
tEra  Aaw
dAaw     
Ho 
Ho 


t
d t
 t 
 t 

Eswa  b feq

Si dAaw   
H o    H o  d c w f 
As d t t 

Era - destruction of air-water interface duringg wettingg
Eswa - colloids removed from the solid phase due to drainage
faw - a fraction of Eswa that is transferred to the air-water interface

Bradford et al. (2014)

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Cells at the AWI or triple point are released during imbibition due toexpansion of water films and destruction of AWI.
 The amount of D21g release is highly dependent on the initial amountand distribution of cells.
Bradford et al. (2014)

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Colloid Release under Transient
Chemical Conditions

Cycles of Drainage and Imbibition

IS=10 mM

Multiple cycles of drainage and imbibition are needed to release D21go thee SWI.
SW .
from
 Release depends on the initial amount of retention on the SWI and thesaturation history.


IS=50 mM

IS=100 mM

Af

b

Seq
t

 EIS   b Feq

Si dAf CIS C 
H o   IS 
Aif dCIS t t 

 Values of Af or Smax (denoted in red) change with the physicochemical conditions

Bradford et al. (2014)

 The amount of release is related to changes in Af or Smax

Bradford et al. (2014)

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Colloid Release under Transient
Chemical Conditions

Colloid-Facilitated Solute Transport
Air Release of D21g with

Contaminant sorbed to colloids at air-water interface, Sac

ga reduction in IS

kdca

kaca
Contaminant sorbed tomobile colloids
colloids, Smc

kaac

m

kdac
kamc

Water
 Release of D21g withan increase in pH

kstr

Kd

S lid
Solid

kdmc

Dissolved
Di
l dContaminant, C

Instantaneously Sorbed
Contaminant,, Se

kac
kdic



kaic

i
Kinetically Sorbed
Contaminant,, Sk

Contaminant sorbed to immobile colloids, Sic

Bradford et al. (2014)

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Particle-Facilitated Solute Transport
Pang et al. [2005]: Bacteria act as carriers for heavy
metals in gravel aquifers

kdc

OUTLINE How Complex
p Should Models Be?
 The Use of Complex Models
 Selected
S l t d HYDRUS
HYDRUS-Based
B dM
Modelsd l(e.g., HP1, colloid transport)

 Models
M d l Verification
V ifi i andd Validation
V lid i(model inter-comparisons; benchmarking)Since bacteria may be excluded from small
pores, they move through interconnectedlarger pores and cracks where water moves
quicker.

 Documentation of Complex Models

Provide a vehicle for rapid transport of lessmobile contaminants.

 Danger (misuse) of Complex Models(technical and user manuals, online help, source code)To help protect y our priv acy , PowerPoint prev ented this external picture from being automatically downloaded. To download and display this picture, click Options in the Message Bar, and then click Enable external content.

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Models Verification

Models Verification

 Model Verification: is the process of confirming that the numerical Available analytical solutions are often limited to idealized transportdomains, homogeneous and isotropic media, and uniform initial andconstant boundary conditions.
 The very reason for developing numerical models is to go beyond therange of available analytical solutions, i.e., to allow irregularttransportt domains,di non-homogeneoushandd anisotropici t i media,divariable boundary conditions, and nonlinear processes, i.e. to usethem for situations or conditions for which they can not be directly
verified.
verified Verification in such conditions is often accomplished using
approximate tests of having internal consistency and accuracy, suchas:- mass conservation- global mass-balance errors- insensitivityy to changes
g in mesh sizes and time stepsp- insensitivity to changes in units

model is correctly implemented with respect to the conceptual (ormathematical)hi l) model.
d l During
D i verification
ifi i theh modeld l isi tested
d to findfi dand fix errors in the implementation of the model. The objective ofmodel verification is to ensures that the implementation of the model(i e governing equations) is correct.
(i.e.,
correct
 Verification of a numerical code consists of showing that theresults generated by the model for simpler problems are consistent withavailable analytical solutions
solutions.

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Models Verification (Benchmarking)

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Benchmarking Studies((Scanlon et al.,, 2002))

 Scanlon et al. (WRR, 2002) compared water balance Alternatively, one can show that the results generated
by the model are the same, or similar, as results
generated with other numerical codes. The latterprocedure is often also called Benchmarking.

simulation results from seven different codes (HELP,(HELP HYDRUS
HYDRUS-1D
1D,
SHAW, SoilCover, SWIM, UNSAT-H, and VS2DTI) using three-year
water balance monitoring data from non-vegetated engineered
covers ((3 m deep)p) in warm ((Texas)) and cold ((Idaho)) desert regions.
g Code Inter-Comparison
Inter Comparison is then likely the most suitable
method to assess code capabilities and model
performance. However, this requires existence ofmultiple models of similar/overlapping capabilities,
which may not always exist.
Time series of cumulative evaporation for the Texas site.
The main outliers are VS2DTI with daily precipitation
input (V(d)), HELP (H), and SoilCover with hourly
precipitation input (SC(h)).

Time series of dailyypprecipitationpand measured and simulated
drainage at the Idaho Site. Drainage curves were restarted on 1
October each year. The main outliers are HELP (H) for the firstfew months, SHAW (SH) for 1999, and VS2DTI (V) for 1999 and2000.

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Benchmarking Studies

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((Vanderborght
g et al.,, 2005))

 Vanderborght et al. (VZJ, 2005) developed and used a set ofBenchmarking Studies((Vanderborght
g et al.,, 2005))

Vanderborght et al. (VZJ, 2005)

analyticalybenchmarks ((of different complexity)py) to test numericalmodels (HYDRUS-1D, MACRO, MARTHE, SWAP, and WAVE) of
flow and transport in soils.
 Analytical Solutions:- Kirchhoff Transform (water flow; Darcy e.)- Laplace Transform (solute transport; CDE)
- Boltzmann Transform (water flow; Richards e.)- Traveling Wave Solution (water flow and solute transport)

 Scenarios:- Steady-state flux in layered profile- Steady-state evaporation from a water table- Infiltration in an initially dry soil
- Transient evaporation from a soil profile- Steady-state linear solute transport in homogeneous soil profile
Steady-state
state nonlinear solute transport in homogeneous soil profile- Steady- Steady-state nonequilibrium linear solute transport with flow interruption- Steady-state linear solute transport in a dual-porosity medium

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Benchmarking Studies((Vanderborght
g et al.,, 2005))

Evaporation rate, Eact , from initially wet (a) sandy, (b, c) loamy, and (d) clayey soil
profiles Dashed lines are simulated Eact using a spatial discretization of 1 cm
profiles.
cm, fulllines using a discretization of 0.25 cm. (Black line is analytical benchmark; R2 iscalculated for simulations with a discretization of 0.25 cm.)

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Benchmarking Studies((Vanderborght
g et al.,, 2005))

Vanderborght et al. (VZJ, 2005)

Vanderborght et al. (VZJ, 2005)

Simulated cumulative drainage at the 22-m
m depth (full lines) and cumulative net
infiltration (precipitation - actual evaporation) (dashed lines) in (a) sandy, (b)loamy, and (c) clayey soil profiles. (Full black line is the cumulative precipitation)

Simulated Solute Fluxes at the 22-m
m depth in (a) sandy,
sandy (b) loamy
loamy, and (c) clayey soil
profiles for climatic boundary conditions.

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Benchmarking Studies

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((Reactive TransportpModels)) A set of well-described benchmark problems for complex reactive
transport numerical models was developed in the special issue of
Computational Geosciences (CrunchFlow, HP1, MIN3P, PFlotran,
and TOUGHREACT) (e.g., Steefel et al., 2015; Xie et al., 2015).
- kinetic dissolution- clogging in a simple (1D) geochemical system- clogging in a complex (2D heterogeneous) geochemical system

Steefel, C., S. Yabusaki, and U. Mayer, Special Volume on Subsurface Environmental Simulation
Benchmarks, Computational Geosciences, in press, 2015.
Steefel,, C. I.,, C. A. J. Appelo,
pp , B. Arora,, D. Jacques
q 14, T. Kalbacher,, O. Kolditz,, V. Lagneau,g, P. C.
Lichtner, K. U. Mayer, J. C. L. Meeussen, S. Molins, D. Moulton, H. Shao, J. Šimůnek, N.
Spycher, S. B. Yabusaki, and G. T. Yeh, Reactive transport codes for subsurface environmental
simulation, Computational Geosciences, doi:10.1007/s10596-014-9443-x, in press, 2015.
Xie, M., K. U. Mayer, F. Claret, P. Alt-Epping, D. Jacques, C. Steefel, C. Chiaberge, and J. Šimůnek,Implementation and evaluation of permeability
permeability-porosity
porosity and tortuosity
tortuosity-porosity
porosity relationships
linked to mineral dissolution-precipitation, Computational Geosciences, in press, 2015.

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Benchmarking Studies((Reactive TransportpModels))Xie et al. (2015)

Benchmarking Studies((Reactive TransportpModels))Xie et al. (2015)

Profiles (B2) of porosity, hydraulic conductivity, and volume fraction of gypsum andcalcite, h (d) at 10, 100, and 120 years by CrunchFlow, HP1, MIN3P, PFlotran, andTOUGHREACT.

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Benchmarking Studies((Focus Scenarios)) FOCUS Scenarios: standardized scenario's have been developed for 9

locations and approximately 14 crops per location.
 Scenario's are a combination of crop, location, the long-term application

schedule (i.e. annual, biennial or triennial applications) and agronomic
parameters (particularly irrigation data).
 Scenarios collectively represent agriculture across Europe for the purpose

potential.
of Tier 1 EU-level assessment of leachingg p
 In the initial assessments of pesticide registration, models are used to get afirst indication of the leaching potential of a pesticide.
 Models: PEARL, PELMO, PRZM (and Macro for one location)

Hydraulic conductivity initial distribution, and distribution of porosity simulated by
MIN3P, PFlotran, CrunchFlow, and TOUGHREACT at 300 years.

 FOCUS:FOCUS ForumF
forf International
I ttil Co-ordination
Cdi ti off pesticide
ti id fate
f t models
d land their Use (FOCUS).

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Benchmarking Studies

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((Focus Scenarios))(FocusScenarios))(

Substance A

Location: Chateaudun, Fr.
Domain: 450 cm, 7 layers
Time:
26 years, atmospheric daily meteo
Solute: multiple substances with different Kd and half-life
Plants: Root growth
Models: Pearl and HYDRUS
C
Carriedi d outt b
by: StStathisthi Di
Diamantopoulostl

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Benchmarking Studies((Focus Scenarios))

Substance B

Substance D

Substance C

Substance C’s daughter productBenchmarking Studies

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Additional Benchmarking Studies
 Oster et al. (AWM, 2012) compared the simulated crop yields grownunder production practices and (transient) conditions (involvingpressure head and osmotic stresses) in the western San Joaquin Valley of
California using the ENVIRO-GRO, HYDRUS-1D, SALTMED, SWAP,
and UNSATCHEM models.
y g in their spatial,p, Hanson et al. ((2004)) evaluated 13 models varying
mechanistic, and temporal complexity for their ability to capture intraand inter-annual components of the water and carbon cycle for an
upland, oak-dominated forest of eastern Tennessee.
 Rosenzweig et al. (2013) described the Agricultural Model
Intercomparison and Improvement Project (AgMIP), which is a majorinternational effort linking the climate, crop, and economic modelingcommunities
iti with
ith cutting-edge
tti
d iinformationfti technologyt h l
tot producedimproved crop and economic models and the next generation of climate
impact projections for the agricultural sector.
 WCRP (World Climate Research Programme) Working Group on
Coupled Modeling catalogues a large number of Model Intercomparison
Projects (MIPs) related to various climate related models.

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Model Validation - Definitions

International Validation Projects

Narasimhan (1987):
 Model Verification is related to the accuracy of the invoked numericalsolution schemes and the coding of a model
model, and Model Validation to the inherent capability (or the degree of validity) ofa model in describing a set of processes (in our case subsurface flow andtransport processes).

 INTRACOIN ((International Nuclide Transportp

Code Intercomparison Study, SNPI, 1986)
 INTRAVAL ((International Projectj to Studyy

 International Atomic Energy Agency: a validated model gives 'a

Validation of Geosphere/Transport Models, SNPI,
1987)

good representation of the actual processes occurring in a real systemsystem'(IAEA, 1982)

 HYDROCOIN (Hydrologic Code Intercomparison

Study, OECD/NEA, 1990)

 U.S. Departmentpof Energy:
gy a validated model 'reflects the behavior

 Advances in Water Resources - two special issuesof the real world' (US DOE, 1986)

to the topic of 'Validation of geo-hydrologicalmodels' (Hassanizadeh & Carrera, 1992)
models

OECD/NEA: 'validation is a process of obtaining assurance that a modeli a correctt representationist ti off theth process or systemtforf whichhi h it isi
intended’ (OECD/NEA, 1990)

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Validation of Geo-Hydrological Models


Konikow & Bredehoeft (1992): Ground-water models cannot bevalidated. Since groundwater models are embodiment of scientifichhypothesisth i ththey cannott beb proven or validated,
lid t d similarly
i il l as any
scientific hypothesis or theory, but only tested and invalidated. The
terms validation and verification are misleading and their use in
groundwater science should be abandoned in favor of more meaningfulmodel-assessment descriptors.

de Marsily et al. (1992): We do not validate our models, but we try toshow that they are not invalidated by the data!



Anderson & Woessner (1992): the issue of model validation is mainly aregulatory one, not a scientific one. A model can never be proven valid
from a scientific standpoint because our understanding of a system will
always be incomplete.



Oreskes et al. (1994): Verification and validation of numerical models
of natural systems is impossible since such systems are never closed
and model results are always non-unique.

OUTLINE How Complex
p Should Models Be?
 The Use of Complex Models
 Selected
S l t d HYDRUS
HYDRUS-Based
B dM
Modelsd l(e.g., HP1, colloid transport)

 Models
M d l Verification
V ifi i andd Validation
V lid i(model inter-comparisons; benchmarking) Documentation of Complex Models(technical and user manuals, online help, source code)

 Danger (misuse) of Complex Models

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OUTLINE

Documentation of Complex Models
Important Factors for Acceptance of Complex Models: Detailed description of all processes in Technical and User Manuals- HYDRUS-1D (342 pages)- HYDRUS (2D/3D) (260 pp. Technical manual, 301 pp. User manual,
over 1000 pages of online help, + documentation of modules)- SWAP (284 pages)
 Availability of the source code
 Availability of examples/tutorials
 Verification/validation
 Training (short courses) (Hydrus short courses, currently annually in Europe(Prague), US (Golden(Prague)(Golden, Colorado)
Colorado), China (Beijing)(Beijing), and Brazil,
Brazil and semiannually in Israel(Sede Boqer) and Australia (Adelaide)) How Complex
p Should Models Be?
 The Use of Complex Models
 Selected
S l t d HYDRUS
HYDRUS-Based
B dM
Modelsd l(e.g., HP1, colloid transport)

 Models
M d l Verification
V ifi i andd Validation
V lid i(model inter-comparisons; benchmarking) Documentation of Complex Models

 Numerical robustness
 …

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Danger (misuse) of Complex Models(technical and user manuals, online help, source code)

 Danger (misuse) of Complex Models

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Conclusions
 It is a challenge,
challenge not only to develop
complex models describing complex soil
systems,y, but also to p
persuade the soil
science community in using them.
 As a result, complex quantitative
mechanistic models are still anunderutilized tool in soil science researchresearch.

By developing and making available powerful numerical models, we are providing
people with tools, which they are sometimes applying without fully understanding
the theories underlying these tools and in conditions, for which they are not always
appropriate.