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Earth's Systems / Biological processes / Terrestrial ecosystem functioning / Living soil system / Water infiltration, retention and movement / Preferential flow and macropore bypass

Preferential flow and macropore bypass

How continuous macropores let water and solutes bypass the soil matrix during intense rain, carrying nitrate and pesticides rapidly to depth.

Boundary: where this process starts

Starts with: Cracks, worm burrows and root channels form a continuous macropore network

Boundary: where it ends

Ends after: Fast transport of Nitrate, pesticides and pathogens to drains and groundwater

PARTICIPANTSRESOURCESMacroporesEarthwormburrowsRoot channelsSoil waterSoil matrixOtherparticipantsWaterPore spaceNitrateClay mineralsImpact +3 of 5. Click for details and evidence.Impact +3 of 5. Click for details and evidence.Impact +4 of 5. Click for details and evidence.40% intense rain, macroporesImpact +3 of 5. Click for details and evidence.45% gentle rain, matrix flowImpact +3 of 5. Click for details and evidence.15% repellent or layeredImpact +3 of 5. Click for details and evidence.Impact -2 of 5. Click for details and evidence.↻ balances · moderate · 0.3Impact +4 of 5. Click for details and evidence.Impact +3 of 5. Click for details and evidence.Impact +2 of 5. Click for details and evidence.Impact -3 of 5. Click for details and evidence.↻ balances · moderate · 0.4Impact +3 of 5. Click for details and evidence.Cracks, worm burrows and root channels form a continuous macropore network: makes Pore spacemakesRain intensity versus matrix infiltration capacity and soil wetness: uses WaterBypass flow delivers water and solutes to depth with little contact with the matrix: uses WaterBypass flow delivers water and solutes to depth with little contact with the matrix: uses NitrateFingered flow in water-repellent or layered sand: uses WaterMacropore-matrix exchange: imbibition slows the front and stores water in aggregates: uses WaterMacropore-matrix exchange: imbibition slows the front and stores water in aggregates: uses Pore spaceSwelling clay closes cracks as soil wets, sealing macropores: uses WaterSwelling clay closes cracks as soil wets, sealing macropores: uses Clay mineralsFast transport of Nitrate, pesticides and pathogens to drains and groundwater: uses WaterFast transport of Nitrate, pesticides and pathogens to drains and groundwater: uses Nitrateforms a connectedconduit networkPore space: forms a connected conduit networkexceeds matrixinfiltration capacityWater: exceeds matrix infiltration capacitybypasses matrix to depthWater: bypasses matrix to depthleached with littlematrix contactNitrate: leached with little matrix contactfingers throughwater-repellent layersWater: fingers through water-repellent layersslowly imbibed intoaggregatesWater: slowly imbibed into aggregatesexchanges water withmatrixPore space: exchanges water with matrixswelling closes flowpathsWater: swelling closes flow pathsswell on wettingClay minerals: swell on wettingcarries solutes togroundwaterWater: carries solutes to groundwaterrapidly transported todrainsNitrate: rapidly transported to drainsStart: Preferentialflow and macroporebypassSTARTStart: Preferential flow and macropore bypassEnd: Preferential flowand macropore bypassENDEnd: Preferential flow and macropore bypassCracks, worm burrowsand root channels forma continuous macropo…📄1Cracks, worm burrows and root channels form a continuous macropore networkRain intensity versusmatrix infiltrationcapacity and so…📄1Rain intensity versus matrix infiltration capacity and soil wetnessBypass flow deliverswater and solutes todepth with litt…📄1Bypass flow delivers water and solutes to depth with little contact with the matrixFingered flow inwater-repellent orlayered sand📄1Fingered flow in water-repellent or layered sandMacropore-matrixexchange: imbibitionslows the front a…📄1Macropore-matrix exchange: imbibition slows the front and stores water in aggregatesSwelling clay closescracks as soil wets,sealing macropores📄1Swelling clay closes cracks as soil wets, sealing macroporesFast transport ofNitrate, pesticides andpathogens to drains a…📄1Fast transport of Nitrate, pesticides and pathogens to drains and groundwater
Line thickness = size of impact (1 to 5)% on a line = chance a fork takes that branch📄 = pieces of evidence on a stepNegative effectLoops backHypothesisResource used up (arrow into the step)Resource used, not used upResource made or released (arrow out of the step)
What am I looking at? The concepts on every map
Participant
Someone or something that takes part in a process or influences it: a plant, a microbe, a river, a machine, a force. Participants are the rows of a map.
Resource
Something a step uses or uses up: water, oxygen, sunlight, a tool. Each step has an arrow down to the resources it uses.
Step
One thing that happens in a process. Every step has at least one participant and carries evidence.
Decision fork
A step where the process can go several ways. Every outcome is labeled with a percentage, and the percentages add up to 100%. It is not limited to yes or no.
Loop
A line that carries the process back to an earlier step. We record whether it reinforces or balances, how strong it is (weak, moderate, strong), and its gain (0 to 1): the share of the effect that returns on each pass.
Size of impact
Every line says how much the earlier step affects the next one (1 to 5), in which direction (+, -, 0), and why. Thicker lines mean bigger impact.
Evidence
The support for a claim: a source and what it shows. Every step, line, change and vote needs it. Click any step or line to read its evidence.
Boundary
Where a process starts and where it ends. Every process, and every subprocess, has a start and an end.
Subprocess
A step that is itself a whole process, with its own boundary, steps and lines. Click a step marked ↘ to open it. Subprocesses nest as deep as the evidence goes.
Description
A plain-language summary of what a process is and what it covers.
Hypothesis
A proposed new connection between two parts of the map that nobody has linked. It carries a prediction, a way to test it, and a count of papers on each topic and on both. It is an unverified idea until someone tests it.

Every step, with its evidence

The same information as the map, as a list you can search and share. Click a participant or resource to see everywhere it appears.

Cracks, worm burrows and root channels form a continuous macropore network
MacroporesEarthwormsPlant rootsPore space
Evidence (1) · added by Fern
  • Beven & Germann 2013, Water Resources Research 49:3071 (Macropores and water flow in soils revisited): Review of how structured pore networks conduct water and solutes rapidly. (confidence 0.75)
⑂ Rain intensity versus matrix infiltration capacity and soil wetness
Rain and irrigationSoil matrixWater
Evidence (1) · added by Fern
  • Jarvis 2007, European Journal of Soil Science 58:523: Macropore flow is triggered when rain intensity or ponding exceeds matrix infiltration capacity; shares are estimates (+-10 points). (confidence 0.5)
Bypass flow delivers water and solutes to depth with little contact with the matrix
MacroporesDrainage waterWaterNitrate
Evidence (1) · added by Fern
  • Flury et al. 1994, Water Resources Research 30:1945: Dye tracing in field soils showed preferential flow in most of them regardless of texture. (confidence 0.65)
Fingered flow in water-repellent or layered sand
Hydrophobic organic coatingsSandy soilWater
Evidence (1) · added by Fern
  • Ritsema & Dekker 1994, Water Resources Research 30:2519: Water-repellent sand shows unstable fingers that conduct most water through a small fraction of the volume. (confidence 0.7)
Macropore-matrix exchange: imbibition slows the front and stores water in aggregates
Soil aggregatesSoil matrixWaterPore space
Evidence (1) · added by Fern
  • Gerke & van Genuchten 1993, Water Resources Research 29:305: Dual-porosity model shows water exchange between macropore and matrix domains controls how far bypass water travels. (confidence 0.7)
Swelling clay closes cracks as soil wets, sealing macropores
SmectiteClay mineralsWaterClay minerals
Evidence (1) · added by Fern
  • Bronswijk 1988, Journal of Hydrology 97:199: In cracking clay soils, crack volume shrinks as soil swells, so bypass flow ends when the soil is wet. (confidence 0.65)
Fast transport of Nitrate, pesticides and pathogens to drains and groundwater
Drainage waterGroundwaterNitrateWater
Evidence (1) · added by Fern
  • Kladivko et al. 1991, Journal of Environmental Quality 20:264: Pesticides reached tile drains within days of rainfall through macropores in silt loam. (confidence 0.7)