Preferential Flow: What is it, How we can deal with it, and

1

• Preferential Flow What is it, How we can deal
  with it, and.Do we really need to care about it?
• A Review

Ricardo Oyarzún 2003
2
Introduction

• Typical approach unsat. zone Richards (flow) and
  Conv.Disp. Eq. (solute)
• Assumption a mean pore velocity is what
  governs transport.

3
Introduction

• Typical approach unsat. zone Richards (flow) and
  Conv.Disp. Eq. (solute)
• Assumption a mean pore velocity is what
  governs transport.
• Nevertheless, there are conditions that favours
  preferential movement ?
  assumption no more valid.

4
Introduction

• Currently, widespread concern over pollution
  threat to both SW and GW due to ag. Activities.
• Experimental evidence pf could lead to an
  accelerated leaching
• Therefore, there is a necessity to study this
  kind of process.

5
Concepts and definitions

• Preferential flow process whereby much of the
  water and solute movement through a porous media
  follow favored flow paths, ending in highly
  variable flow rate
• Two main classes
• Fingered
• Macro-pore

6
Concepts and definitions

• Fingered fairly homogeneous medium, flow is
  splitted into isolated pathways
• Macro-pore movement through pathways (cracks
  and wormholes that are larger than would be
  suggested by the particle size distribution of
  the soil.

7
Concepts and definitions

• Fingered fairly homogeneous medium, flow is
  splitted into isolated pathways
• Macro-pore movement through pathways (cracks
  and wormholes) that are larger than would be
  suggested by the particle size distribution of
  the soil.

8
Concepts and definitions

• Fingered fairly homogeneous medium, flow is
  splitted into isolated pathways
• Macro-pore movement through pathways (cracks
  and wormholes that are larger than would be
  suggested by the particle size distribution of
  the soil.
• but. it is occurring in our soil (situation)?

9
How to recognize pf?

• a) Characterization of the soil volumetric water
  capacity curve

10
How to recognize pf?

• b) Tracer experiments

Natural v/s tracer profiles
11
How to recognize pf?

• c) Use of dyes and photography (digitalization,
  GIS)
• ? These methodologies are not free of problems
• ? In fact they are not exclusive but complementary

12
Modeling preferential flow

• Process-based description of preferential flow
  generally invoke dual-porosity (dual-permeability)
  models.
• Two interacting pore regions, one associated with
  macro-pore (fracture) network, the other related
  with micro-pores (inside soil aggregates)

13
Modeling preferential flow

• a) MACRO model (Swedish Univ. Ag. Sci., available
  at www.mv.slu.se/bgf/defeng.htm)
• Two flow domains separated by a boundary wat.
  pot. ?b (? hwe), corresponding ?b, and Kb
• Vert. flow, micropores
• Vert. Flow, macro-pores
• (n account for pore size distrib. in macropore
  region)

14
Modeling preferential flow

• a) MACRO model (Swedish Univ. Ag. Sci., available
  at www.mv.slu.se/bgf/defeng.htm)
• Water exchange can occurs between Macro and
  micro-pores (both ways)
• At surface boundary, infiltrating water is
  partitioned depending on infiltration capacity
  (given by Kb) and rainfall intensity

15
Modeling preferential flow

• a) MACRO model (Swedish Univ. Ag. Sci., available
  at www.mv.slu.se/bgf/defeng.htm)
• For solute transport
• MACRO model should be linked with additional
  algorithm, e.g. , SOILN for N dynamics.
• Use of conv/disp eqn with source/sinks terms ?U
  accounting for mass exchange between flow domains
  and crop uptake

16
Modeling preferential flow

• b) MICMAC model (Bruggeman et al. 1999, Trans
  ASAE 42(6)1743-1752)
• Also two flow and transport domains (systems)
• Pref. Flow is simulate using an
• explicitly cylindrical macro-pore
• located in the center of a soil
• column

17
Modeling preferential flow

• b) MICMAC model (Bruggeman et al. 1999, Trans
  ASAE 42(6)1743-1752)
• Flow in variable sat. Media is described using
  an axisymetric form of Richards Eqn.
• h(?) and K(?) described by Van Genutchen eqn

18
Modeling preferential flow

• b) MICMAC model (Bruggeman et al. 1999, Trans
  ASAE 42(6)1743-1752)
• Flow in macro-pore region is limited by its
  geometry, assume gravity induced and fully
  laminar flow through a cylindrical channel
  Hagen-Poiseuille eqn

19
Modeling preferential flow

• b) MICMAC model (Bruggeman et al. 1999, Trans
  ASAE 42(6)1743-1752)
• Flow between the matrix and the macro-pore
  region is controlled by the ?H at the boundary
• Therefore, no flow occurs from the matrix to the
  macropore (Po) when soil is unsaturated (-h)

20
Modeling preferential flow

• b) MICMAC model (Bruggeman et al. 1999, Trans
  ASAE 42(6)1743-1752)
• For solute transport in matrix, classic
  convect/dispers eqn for transport (without
  adsorption or decay)
• In macro-pore, transport is assumed dominated by
  convection (no diffusion)

21
So far so good, but.

• What is the relative importance / consequence of
  preferential flow processes regarding
  agrochemical transport through soil, and possible
  effect on surface/ground water?

22
So far so good, but.

• What is the relative importance / consequence of
  preferential flow processes regarding
  agrochemical transport through soil, and possible
  effect on surface/ground water?
• Well.. It depends!!!!

23
So far so good, but.

• As water moves through soil, it tends to
  equilibrate with the soil
• ? Water low in solutes tends to remove them from
  soil.
• Water rich in solutes may deposit them

24
So far so good, but.

• As water moves through soil, it tends to
  equilibrate with the soil
• ? Water low in solutes tends to remove them from
  soil.
• Water rich in solutes may deposit them
• and what about flux velocity????

25
So far so good, but.

• Preferential movement may thus results in either
  spikes or thoughs in leaching solute
  concentration curves
• Therefore, we cant expect a single (universal)
  behavior!!!!!

26
So far so good, but.

• a) Nitrate
• Larrson and Jarvis (1999) found better estimates
  of NO3- leaching when 2-domain model was used.
• Main effect of macro-pore flow was a reduction
  in leaching (specially winter) due to
  infiltration of water with low NO3-

27
So far so good, but.

• a) Nitrate
• Larrson and Jarvis (1999) found better estimates
  of NO3- leaching when 2-domain model was used.
• Main effect of macro-pore flow was a reduction
  in leaching (specially winter) due to
  infiltration of water with low NO3-
• But it depends!! (e.g. if most of annual pp
  occurs soon after fertilization, for a crop
  overfertilized, or in short-term studies)

28
So far so good, but.

• b) Pesticides
• Pivetz and Steehnuis (1996), lab test for 2,4-D
  differential degradation, faster in macropores
  (possible due to higher aerobic conditions, and
  therefore, higher bacterial activity)
• Also, different values for sorption coefficient
  between domains!!!

29
So far so good, but.

• b) Pesticides
• Larsson and Jarvis (2000)
• macro-pore flow would be spcially critical for
  compounds which are either strongly sorbed or
  quickly degrading.
• Low relevance for persistent or mobile compounds

30
So far so good, but.

• b) Pesticides
• Larsson and Jarvis (2000)
• macro-pore flow would be speically critical
  forcompounds which are either strongly sorbed or
  quickly degrading.
• Low relevance for persistent or mobile compounds
• c) Soil properties related with flow/transport
• K, differential travel times (Kelly and Pomes,
  1998)

31
Final Remarks

• a) Availability of requiered model parametrs
• b) Uncertaintity caused by heterogeneous nature
  of system
• c) Field studies (quantification of pref. Flow
  under natural climatic boundary conditions,
  larger scales
• d) Better field techniques (for recording water
  movement at the matrix and macropore scale

32
Final Remarks

• a) Availability of requiered model parametrs
• b) Uncertaintity caused by heterogeneous nature
  of system
• c) Field studies (quantification of pref. Flow
  under natural climatic boundary conditions,
  larger scales
• d) Better field techniques (for recording water
  movement at the matrix and macropore scale

33

• Thank you!

34
References

• - Armstrong, A.C., Leeds-Harrison, .B., Harris,
  G.L., Catt, J.A. 1999. Measurements of solute
  fluxes in macroporous soils techniques, problems
  and precision. Soil Use and Management,
  15240-246.
• - Bergstrom, L., Jarvis, N., Larsson, M.,
  Djodjic, F., and Shirmohammadi, A. Factors
  affecting the significance of macropore flow for
  leaching of agrochemicals. In Preferential flow,
  water movement and chemical transport in the
  environment, Proc. 2nd Int. Symp. (3-5 Jan 2001,
  Honolulu, Hawai, USA), eds. D.D. Bosch and K.W.
  King. St. Joseph, Michigan, ASAE Pub 701P0006.
• Available at http//asae.frymulti.com/conference.a
  sp?confidpf2001 . Acceded on January, 27, 2003

35
References

• - Bruggeman, A.C., Mostaghimi, S., and Brannan,
  K.M. 1999. A stochastic model for solute
  transport in macroporous soils. Trans ASAE,
  42(6) 1743-1752.
• - Kelly, B., and Pomes, M. 1998. Preferential
  flow and transport of nitrate and bromide in
  claypan soil. Ground Water. 26(3) 484-494
• - Larsson, M., and Jarvis, N. 1999a. A
  dual-porosity model to quantify macropore flow
  effects on nitrate leaching. J. Environ. Qual.
  281298-1397.
• Larsson, M., and Jarvis, N. 1999b. Evaluation of
  a dual porosity model to predict field-scale
  solute transport in a macroporous soil. Journal
  of Hydrology. 215153-171.
• - Mc conville, C., Kalin, R.M., Johnston, H., and
  McNeill, G.W. 2001. Evaluation of recharge in a
  small catchment using natural and applied ?18O
  profiles in the unsaturated zone. Groundwater,
  39(4) 616-623.

36
References

• - Mohnaty, B.P., Castiglione, P., Shouse, P.J.,
  van Genuthchen, M.Th. Measurements and modeling
  of preferential flow under controlled conditions.
  In Preferential flow, water movement and
  chemical transport in the environment, Proc. 2nd
  Int. Symp. (3-5 Jan 2001, Honolulu, Hawai, USA),
  eds. D.D. Bosch and K.W. King. St. Joseph,
  Michigan, ASAE Pub 701P0006.
• Available at http//asae.frymulti.com/conference.
  asp?confidpf2001 . Acceded on January, 27, 2003
• - Ray, C., Vogel, T., and Gerke, H. Effects of
  chemical reaction variability on preferential
  flow. In Preferential flow, water movement and
  chemical transport in the environment, Proc. 2nd
  Int. Symp. (3-5 Jan 2001, Honolulu, Hawai, USA),
  eds. D.D. Bosch and K.W. King. St. Joseph,
  Michigan, ASAE Pub 701P0006.
• Available at http//asae.frymulti.com/conference.a
  sp?confidpf2001 . Acceded on January, 27, 2003

37
References

• - Regalado, C.M., Munoz-Carpena, R., Alvarez, J.,
  Socorro, A.R., and Hernandez-Moreno. Field and
  laboratory setup to determine preferential flor
  in volvcanic soils. . In Preferential flow,
  water movement and chemical transport in the
  environment, Proc. 2nd Int. Symp. (3-5 Jan 2001,
  Honolulu, Hawai, USA), eds. D.D. Bosch and K.W.
  King. St. Joseph, Michigan, ASAE Pub 701P0006.
• Available at http//asae.frymulti.com/conference.a
  sp?confidpf2001 . Acceded on January, 27, 2003
• - Ritsema, C. 1999. Preface. Journal of Hydrology
  215, 1-3 (Special Issue on Preferential Flow).
• Selker, J., Keller, K., and McCord, J. 1999.
  Vadose zone processes. Lewis Publishers, Boca
  Raton, USA.

38
References

• - Simic, E., and Destouni, G. Significance of
  preferential flow for contaminant transport by
  groundwater in an integrated soil-groundwater
  system. In Preferential flow, water movement and
  chemical transport in the environment, Proc. 2nd
  Int. Symp. (3-5 Jan 2001, Honolulu, Hawai, USA),
  eds. D.D. Bosch and K.W. King. St. Joseph,
  Michigan, ASAE Pub 701P0006.
• Available at http//asae.frymulti.com/conference.a
  sp?confidpf2001 . Acceded on January, 27, 2003