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

Preferential Flow: What is it, Preferential Flow: What is it,

How we can deal with it, How we can deal with it,

and….Do we really need to and….Do we really need to

care about it?care about it?

A ReviewA Review

Ricardo OyarzúnRicardo Oyarzún

20032003

Ricardo OyarzúnRicardo Oyarzún

20032003

Introduction

• Typical approach unsat. zone: Richards (flow) and Conv.Disp. Eq. (solute)

• Assumption: “ a mean pore velocity is what governs transport”.

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.

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.

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


• 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.



• Macro-pore: movement through pathways (cracks and wormholes) that are larger than would be suggested by the particle size distribution of the soil.



• 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)?

How to recognize pf?

a) Characterization of the soil volumetric water capacity curve

1 2

Log

-d/d

Bimodality behavior


b) Tracer experimentsEC

Depth

Natural v/s tracer

profiles


c) Use of dyes and photography (digitalization, GIS)

These methodologies are not “free of problems”

In fact they are not exclusive but complementary

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)


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)

SwSrz

Kzt

1

*

)(

n

bs

mabs KKq




• 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




For solute transport:•MACRO model should be linked with additional algorithm, e.g. , SOILN for N dynamics.

•Use of conv/disp eq’n with source/sinks terms U accounting for mass exchange between flow domains and crop uptake

Uqcz

cD

zt

SC


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



ASAE 42(6):1743-1752)

• Flow in variable sat. Media is described using an axisymetric form of Richards Eq’n.

• h() and K() described by Van Genutchen eq’n

1)()(1

)(z

hhK

zr

hhrK

rrt

h

ht

hhC zr



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 eq’n

8

max4mm gRK

Q



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)



ASAE 42(6):1743-1752)

• For solute transport in matrix, “classic” convect/dispers eq’n for transport (without adsorption or decay)

• In macro-pore, transport is assumed dominated by convection (no diffusion)

z

cV

r

cV

z

cD

zr

crD

rrt

c zrzr

1

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?


• 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!!!!


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


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????


Preferential movement may thus results in either spikes or thoughs in leaching solute concentration curves

Therefore, we can’t expect a single (“universal”) behavior!!!!!

FlowNitrate

Time


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-


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)


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!!!


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


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)

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

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

Thank you!

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, 15:240-246.

- 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.asp?confid=pf2001 . Acceded on January, 27, 2003

http://asae.frymulti.com/conference.asp?confid=pf2001



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. 28:1298-1397.

- 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. 215:153-171.

- 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.

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.


- 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.








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.


- 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.




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.





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