A tension disc infiltrometer measures controlled intake, not ordinary rainfall
A tension disc infiltrometer delivers water to a defined circular area of soil while controlling the water pressure head at the surface. It helps researchers translate near-saturated infiltration behaviour into evidence about which pathways conduct water under different applied heads. The instrument is not simply a funnel or a rain gauge: the contact medium, disc radius, imposed pressure, antecedent soil water and time-dependent inflow all matter to the outcome.
Reading tension disc infiltrometer data begins with questions that prevent false explanations. What was the supply pressure head at the membrane? Was it expressed as a negative pressure head or positive suction magnitude? Did the disc establish reliable hydraulic contact with the soil? How was the reservoir level or cumulative volume recorded? Was the result an early transient rate, a quasi-steady rate, or a conductivity inferred through an analytical or numerical model?
This manual focuses on supply head, infiltration flux, the role of large pores and the limitations of inferring macropore flow. Its worked numerical examples are constructed learning exercises, not actual field observations. The goal is to make the calculations reproducible while protecting the difference between observed infiltration, modelled hydraulic conductivity and interpreted pore contributions.
Why the applied pressure changes what conducts
When a disc imposes a negative pressure head relative to atmospheric pressure, the soil immediately below it is supplied under tension rather than a ponded positive head. This can restrict entry into some larger air-filled openings, depending on capillarity, contact conditions and the geometry of the system. Moving the imposed head closer to zero can recruit additional pathways. Yet the resulting response must be interpreted as a change in the hydraulic system under controlled boundary conditions, not a direct photograph of pore diameters.
For example, a test performed at minus 6 centimetres of water head and then at minus 2 centimetres may show a larger infiltration rate in the second step. A physically plausible interpretation is that lower tension permits greater participation of conductive pathways. The exact contribution of macropores, however, cannot be established merely by subtracting two measured volume rates. Differences in water content, transient wetting, lateral flow and instrument contact can contribute.
The sign convention is particularly important. An applied head of minus 2 centimetres is closer to zero than minus 6 centimetres; the first represents smaller suction magnitude. Translating it as ‘more strongly pulled’ would reverse the intended boundary. Always report both the sign and the reference in a methods explanation, especially when comparing articles using positive suction magnitude instead.
What the instrument has to control
A typical tension disc system includes a water reservoir, a control that establishes supply pressure head, and a porous disc or membrane that interfaces with the soil surface. The exact apparatus and pressure-regulating mechanism vary. The membrane must remain saturated and in satisfactory contact with the soil. If it lifts, leaks, wrinkles or bridges depressions, the recorded volume change may reflect the boundary setup instead of soil behaviour alone.
The disc radius defines the nominal contact area. The supply-head adjustment defines the intended hydraulic boundary, while a reservoir reading and the elapsed time allow calculation of water delivery. The operator should also record initial moisture, temperature, surface condition, slope, contact material, sequence of heads, and whether water escaped outside the intended infiltration footprint.
A small layer of fine contact material can improve hydraulic continuity where the native surface is rough, but its thickness and hydraulic resistance can influence measurements. It is therefore misleading to tell readers that any smooth disc pressed onto soil instantly provides a reliable measurement. Successful interpretation begins with a documented, physically plausible boundary rather than just a set of numbers downloaded from the instrument.
Disk area, delivered volume and apparent flux
For a circular disc of radius r, area A equals pi times r squared. A 10-centimetre radius gives area approximately 314.16 square centimetres. Suppose the reservoir delivers 90 millilitres over a 10-minute interval after the curve has become nearly linear. Since one millilitre is one cubic centimetre, the rate is 9 cubic centimetres per minute. Dividing by nominal disc area gives 0.02865 centimetres per minute, or approximately 17.2 millimetres per hour.
That calculation is an areal water delivery or infiltration flux, usually based on the observed reservoir decline and nominal contact area. It is not automatically the unsaturated hydraulic conductivity at that supply head. Flow spreads three-dimensionally below a circular disc, especially in the early stages, and the imposed boundary creates a combination of gravitational and capillary gradients. An analytical or numerical interpretation is required to recover a conductivity function under stated assumptions.
Another check is dimensional consistency. Volume divided by time gives cubic centimetres per minute; dividing by square centimetres gives centimetres per minute. Multiplying by ten millimetres per centimetre and sixty minutes per hour gives six hundred, so 0.02865 centimetres per minute becomes about 17.2 millimetres per hour. Confusing radius with diameter multiplies the area by four and can create a fourfold flux error without any change in observed volume.
Time series: why an early rate differs from a later rate
Cumulative infiltration is often plotted against elapsed time. The slope of the curve over a chosen interval is the volume rate for that interval; after division by area it becomes an areal rate. Early intake can reflect a relatively dry initial soil and strong capillary gradients. As the wetted region expands, the rate may evolve toward a comparatively steady value under sustained supply and conditions.
A single overall average can hide this change. Suppose 120 millilitres enter in the first ten minutes, followed by 90 millilitres in the next ten, with the same disc. The first nominal flux is about 22.9 millimetres per hour and the second is about 17.2 millimetres per hour. Combining both intervals gives 210 millilitres over twenty minutes, or about 20.1 millimetres per hour, which obscures the downward trend.
The interpreter should therefore state the interval used, show the cumulative-volume record, and explain whether the apparent steady phase was actually established. Short tests are not automatically invalid, but a short late interval with noisy readings is weak evidence for a precise steady-state rate. The best description also explains how stability was judged and whether the pressure head stayed controlled throughout.
The hydraulic model behind a conductivity estimate
Infiltration from a circular disc generally involves radial as well as vertical movement in the subsurface. Hydraulic conductivity depends on the pressure head and soil state, while geometry and capillary gradients influence the observed volume flux. A simple equation that sets conductivity equal to total reservoir outflow divided by contact area can therefore be inappropriate. Several analytical approaches and numerical inverse models are used, each with assumptions about homogeneous soil, initial conditions, contact and parameterization.
The USDA Agricultural Research Service has documented a DISC model for inverse analysis of transient tension-disc infiltration. In such a framework, the model solves a three-dimensional water-flow problem with radial symmetry and estimates soil hydraulic parameters from data under assumptions. This is a useful reminder that the instrument measures an infiltration response; recovering conductivity requires a declared interpretation method.
If a report simply says ‘hydraulic conductivity was measured with a disc infiltrometer’, ask for the radius, applied head, observation interval, parameter-estimation procedure and fit diagnostics. A transparent statement would explain that infiltration was measured and conductivity then estimated with a specified model. That sentence preserves the distinction between observation and inference without adding unnecessary jargon.
Interpreting pore recruitment without claiming to see every pore
Capillarity offers an illustrative connection between pressure head and the size of a simplified water-filled cylindrical opening. Under a strongly idealised capillary model with zero contact angle, radius can be related to surface tension, fluid density, gravity and head magnitude. Yet soil pores are not uniform vertical glass tubes. They have irregular geometry, dead ends, necks, partial filling and varied surface conditions. The calculation is a conceptual scale, not a direct census of hydraulic pathways.
At surface tension approximately 0.072 newtons per metre, water density 1000 kilograms per cubic metre and head magnitude 0.05 metres, the idealized capillary radius computed as two times surface tension divided by density times gravity times head is about 0.000294 metres, or 0.294 millimetres. That is a diameter of about 0.587 millimetres. This estimate depends on the assumed contact angle and model geometry. It must not be reported as an observed pore diameter.
Changing a disc supply head alters the pathways likely to admit and transmit water, but a simple radius threshold cannot capture unsaturated macropore flow in real structured soil. Preferential flow can occur through partially filled macropores, fingers, or focused regions, sometimes at water contents well below saturation. A responsible explanation distinguishes a capillary scale estimate from direct images or tracer evidence of active pathways.
Comparing two controlled heads
Imagine one instrument operated at a supply head of minus 6 centimetres and later at minus 2 centimetres. At the first head, the late-stage nominal flux is 7 millimetres per hour. At the second, it is 18 millimetres per hour. The difference is 11 millimetres per hour. The near-zero head produced more intake under those observations. However, 11 divided by 18 is not automatically a measured fraction of water travelling through macropores.
To claim that 61 percent of flow used macropores, one would need a clearly specified partitioning model and conditions that identify the relevant pathways. In reality the wetter second stage may have altered the moisture distribution and gradients; different pore classes may conduct in both tests. A portion of the difference can reflect changing geometry of the wetted bulb as well as pressure-dependent conductivity.
Translate the finding as evidence that infiltration is sensitive to the imposed tension and that additional flow pathways may participate near saturation, subject to model and measurement limits. If the purpose is a quantitative macropore contribution estimate, disclose the chosen method, assumptions, uncertainty and any independent tracer or imaging evidence. Do not turn an intuitive explanation into an unsupported percentage.
Sequence effects when testing multiple tensions
The order in which a site receives different supply heads can affect the apparent response. Testing first at a stronger negative head and later closer to zero may wet the soil progressively. Reversing the sequence does not necessarily retrace the same state because antecedent moisture and hysteresis have changed. This is a time-history problem, not simply a matter of instrument precision.
Suppose one team tests minus 10, minus 6 and minus 2 centimetres at the same location, while another begins at minus 2 and finishes at minus 10. Comparing final rates without reporting the order conceals differences in initial state and recent wetting. The first sequence may reduce disturbances associated with starting close to saturation, but it still needs a method-specific justification and stability checks.
A useful record states the exact sequence, start and stop times, cumulative infiltration at every change, and whether a new steady interval was achieved after each setting. If repeated locations are used instead, spatial variation must be considered. Readers need to know whether a head effect can be distinguished from an order effect before assigning a hydraulic explanation.
Surface contact can dominate an apparently scientific curve
An infiltrometer assumes the supply water passes through the intended boundary into soil. Rough surfaces can leave gaps beneath a membrane, especially over stones, crusts, roots or clods. A contact layer can fill depressions, but excessive material or unsuitable hydraulic properties add resistance or an extra storage reservoir. Neither outcome is a true measurement of the native soil matrix alone.
Consider an instrument that yields an unusually low intake rate at one patch. It may reflect soil compaction, but before calling it a compacted layer the operator should check whether the disc actually contacted the surface evenly. A second measurement after improving contact may rise without any soil structural change. This does not license choosing whichever rate looks plausible; it demands a documented experimental explanation.
Record contact medium thickness, wetting preparation, sealing condition, membrane saturation and any leakage. If water escapes around the rim or remains in a contact pad while the reservoir is read, the apparent infiltration volume does not have the intended denominator. A persuasive field report demonstrates contact quality as well as numerical repeatability.
Antecedent water content changes the transient response
Early infiltration often reflects the initial moisture state and the magnitude of the capillary gradient. A dry site and a moist site may have markedly different early uptake even if other hydraulic parameters are similar. It is therefore unsafe to compare total volumes over fixed short intervals as though they were direct measures of a single constant conductivity.
Suppose site A takes 150 millilitres in its first five minutes while site B takes 90 millilitres, both under the same disc and nominal head. Without initial water contents, contact checks, later slopes and site descriptions, the difference cannot be attributed solely to more connected macropores at A. A stronger initial suction gradient at A could contribute. If the two sites reach similar late-stage slopes, that itself changes the interpretation.
Pair infiltration records with initial and final water-content measurements where possible, noting the depth and sampling method. The objective is not to force two rates to match but to understand why they differ. A proper translation makes the initial condition part of the reported result rather than an invisible nuisance.
Puddling, sealing, compaction and cracking
The upper few millimetres of a soil can control the early infiltration response. A crust produced by raindrop impact may reduce intake even when deeper soil conducts readily. Compaction can alter pore size and connectivity. Surface cracking, biological channels or root holes can create concentrated pathways whose participation changes with head and contact conditions.
Imagine a cropped site measured before and after a rainfall event that caused surface sealing. A fall in apparent infiltration may be consistent with sealing, but the test should also consider changed antecedent moisture, contact and spatial sampling. An adjacent cracked patch could produce a high rate that is unrepresentative of the broader field. One disc footprint does not establish the average hydraulic behaviour of an entire catchment.
Use field notes, photographs, replicate positions and appropriately selected complementary methods to distinguish surface resistance from deeper flow. A conclusion should identify whether it concerns near-surface matrix intake, preferential pathways or a field-average estimate. These are different claims with different supporting evidence.
The footprint is more than the disc circle
The disc area provides a clear surface denominator, but the wetted soil volume extends beneath and often laterally beyond the circle. Its shape evolves with time, initial water content, head and hydraulic properties. This helps explain why observed infiltration is not simply the one-dimensional flow rate through a vertical cylinder the size of the disc.
A numerical flow model for a tension disc may assume radial symmetry, with distance from the centre and depth as relevant coordinates. A strongly layered or sloping soil, a nearby root channel, or an uneven contact layer can violate simplified symmetry assumptions. If the model does not represent the field geometry, fitting parameters more precisely may give a false sense of accuracy.
When comparing measurements across disc sizes, do not assume that dividing by area removes every geometric influence. Edge effects and lateral flow are part of the physics. Record the disc radius and ensure any conductivity-inversion method is appropriate for that radius, imposed heads and measurement sequence.
How to decide whether the rate has become steady
Cumulative infiltration often begins with a curved trajectory and later approaches a line over a useful observation interval. The slope of that line can estimate a quasi-steady inflow under the applied conditions. But one straight-looking segment is not proof of equilibrium throughout a large soil volume. The operator should check successive interval rates, reservoir stability and whether the pressure boundary remained controlled. Short-lived fluctuations may result from bubbles, reservoir movement or changes in contact.
Take consecutive ten-minute volumes of 96, 91, 89 and 90 millilitres. With a 10-centimetre radius disc, the last three intervals correspond to about 17.4, 17.0 and 17.2 millimetres per hour. Their similarity provides stronger support for a near-steady rate than one isolated final reading, although the acceptable stability criterion depends on the scientific objective and measurement resolution. The first interval is modestly higher and may represent continuing transient adjustment.
A careless researcher might average all four periods and call the result steady. A careful one distinguishes early and late behaviour, states a selection rule before analysing outcomes, and reports temporal variation. If the rate never stabilizes, the data may still be usable in a transient numerical model, but they should not be mislabelled as a reliable steady-state observation.
Reservoir readings and volume calibration
Some instruments measure a water-level change that must be converted into volume using a reservoir cross-section or calibration. A reading of 10 millimetres on a narrow tube does not mean 10 millimetres entered the soil beneath the disc. It is a height in the instrument, not automatically an areal infiltration depth. Calibration tables, reservoir geometry, bubble compensation and consistent meniscus-reading practice help bridge the two quantities.
Suppose the reservoir cross-sectional area is 12 square centimetres and its water level falls 4 centimetres. A simple cylinder model gives a volume decrease of 48 cubic centimetres or millilitres. With disc area 314.16 square centimetres, that volume corresponds to roughly 1.53 millimetres of areal water equivalent at the disc. If the interval lasts ten minutes, the average nominal flux is about 9.17 millimetres per hour.
That conversion assumes the reservoir geometry is accurately known and the fall represents water delivered through the disc without leaks or other storage changes. If an apparatus includes multiple connected chambers or a bubble system, follow its actual calibration, not the elementary cylinder formula. A clear data sheet separates raw level, converted volume, time interval, disc area and computed flux.
Checking a changing supply head
The intended supply head must remain sufficiently controlled throughout the observation. A pressure regulator or Mariotte-type arrangement can malfunction, leak or respond imperfectly to changing water level. If the pressure shifts during a test, changes in infiltration can reflect the boundary condition as well as soil properties. Stating only the initial setting hides a potentially important source of error.
Imagine a test labelled minus 6 centimetres in which the head drifts toward minus 2 centimetres over the last quarter of the run. An apparent rise in intake could be consistent with lower applied suction recruiting additional pathways. It would be unsound to present the higher late rate as a stable response at minus 6 centimetres. The pressure record should be checked before using the observation in a conductivity inversion.
Document the head calibration, its temporal stability, reservoir position and control-system checks. Exclude or model intervals affected by a known boundary failure; do not simply smooth them until a persuasive curve appears. A technical translation should say whether the supply condition was maintained rather than relying on the authority of the instrument name.
Air bubbles and equipment artefacts
An air bubble in the water supply, membrane or tubing can interrupt hydraulic continuity or change a pressure-control response. An abrupt drop in water delivery may therefore arise from an equipment artefact rather than a sudden change in pore structure. Conversely, a leak may make reservoir decline appear faster than water actually enters soil. Both errors can be mistaken for interesting soil physics.
A useful quality-control record notes whether bubbles were removed, the membrane fully wetted, supply lines checked and connections sealed. Reservoir readings should be inspected for unexpected steps or discontinuities. If a test includes a suspicious plateau followed by an abrupt surge, first search for an operational explanation before interpreting it as macropore activation or soil structural collapse.
Repeating a test after correcting a defect can help identify an artefact, but repeating at exactly the same point may change antecedent moisture. The repeat is not necessarily an independent experiment. State which evidence comes from equipment checks, which comes from replicate locations, and which remains an unresolved possibility.
Water temperature, viscosity and comparison
Water temperature affects viscosity and thus can influence flow through pores. Temperature differences between measurements can complicate comparison, especially when the scientific question concerns small changes in estimated hydraulic properties. A field report that compares winter and summer tests should not attribute all differences to compaction, biological channels or land management without checking temperature and moisture conditions.
For a basic teaching example, two identical-looking flux measurements at different temperatures cannot automatically be assigned identical intrinsic hydraulic properties. The movement of water depends on fluid behaviour as well as the porous medium. An appropriate correction method should be selected and documented if the goal is to normalize conductivity estimates across temperatures; the necessary assumptions vary with the method.
A plain-language explanation does not require inventing a universal conversion factor. It should communicate that recorded temperature, soil state and fluid properties form part of comparability. Where correction is not justified, report measurements as made and qualify the comparison instead of silently forcing agreement.
Pairing infiltration with other observations
A disc infiltrometer gives important near-surface hydraulic evidence, but moisture probes, dye tracing, soil profiles, bulk density and direct structural descriptions can clarify otherwise ambiguous interpretations. Time-domain reflectometry may help describe water-content changes, although probe placement relative to a wetted bulb can complicate measurement and inverse modelling. A dye pattern can reveal preferential pathways, but staining alone is not a complete measurement of all water flux.
Suppose the near-zero-head test produces much greater intake than the higher-tension test, and excavation shows dye in a few connected fissures. The combined evidence is more supportive of preferential pathways than the rate difference alone. Still, the fraction of total flow assigned to those fissures requires a defensible quantitative method. A brightly stained path is visible evidence of contact by tracer, not necessarily the complete volume of water transported.
Methods become complementary when each has a clear measurement target and uncertainty. The useful conclusion might be that large conductive structures influence near-saturated response, supported by pressure sensitivity and direct pathway observations. That is stronger and more honest than stating that the disc ‘measured the percentage of macropores’.
Spatial replication and field-scale interpretation
Soil properties are heterogeneous across centimetres, metres and landscapes. One disc test samples a local surface contact area and a nearby subsurface volume shaped by hydraulic conditions. Roots, cracks, stones, wheel tracks, crusts and tillage can cause sharp local variation. A site with three high readings and three low readings cannot be represented responsibly by reporting only the largest.
A field design should state how locations were selected and whether different management zones, soil horizons or disturbance classes were sampled. Replication is not merely repeating a reservoir reading at one spot; truly distinct positions help estimate spatial variation. The statistic reported should match the design. A median may describe a typical site robustly when the distribution is skewed, while a mean supports other questions, but neither repairs biased selection.
Readers should know whether a result concerns a single patch, an average of independent sites, or a modeled field-scale property. If the question is flood runoff or contaminant transport, the distribution and connectivity of high-conductivity patches may be more important than a simple average. A well-translated local measurement retains its spatial boundary.
Uncertainty in radius, area and infiltration rate
Because disc area is pi times the squared radius, a small radius error affects the calculated area and inferred areal flux. If a nominal 10-centimetre radius is actually 9.5 centimetres, the area is about 283.5 square centimetres rather than 314.2, nearly 9.8 percent smaller. For the same delivered volume per minute, the flux per area would be about 10.8 percent larger. That is a meaningful discrepancy even if the reservoir readings were perfect.
Volume calibration, elapsed time, seepage outside the contact, pressure stability, initial water state and uneven soil response can contribute additional uncertainty. It is not enough to add the individual percentage errors indiscriminately; their correlations and the way the final rate is computed matter. Repeated observations can describe short-term variability, while independent test locations describe an additional component of site heterogeneity.
When reporting a result such as 17.2 millimetres per hour, avoid implying it is accurate to a tenth if the disc radius, contact or measured volumes are uncertain. A credible methods report states the nominal geometry and actual precision, then shows whether a conclusion would change across a plausible error range. The reader needs a trustworthy inference more than an impressive decimal.
Estimating hydraulic conductivity requires a declared method
The saturated hydraulic conductivity of a soil is not simply interchangeable with its infiltration rate measured at a negative supply head. Near-saturated conductivity depends on the prevailing water potential and the conductive pore network, while the disc’s measured flux depends on geometry, capillary driving force and initial conditions. Some steady-state analytical methods convert fluxes under specified assumptions, while numerical inverse approaches fit hydraulic-function parameters to transient observations.
Suppose a paper reports water delivery of 9 cubic centimetres per minute under a 10-centimetre-radius disc. That yields an observed nominal flux near 17.2 millimetres per hour. Without a conductivity-inversion equation and the assumptions required by it, a reader should not relabel the value as K at the applied pressure head. The numerical coincidence of the units length per time does not make the two quantities physically identical.
A sound report distinguishes raw volume, normalized surface flux, and model-estimated hydraulic conductivity. It names the model, fitted parameters, boundary conditions, data range and fit performance. This avoids a subtle but serious translation error in which a measured quantity becomes an inferred material property without disclosure.
What a rapid bypass signal means for pollution risk
Preferential flow can matter because water and dissolved substances may move rapidly through selected pathways without contacting all surrounding soil. A highly conductive fissure may help transfer water from the surface to depth while much of the matrix remains relatively dry. But a disc infiltrometer alone does not directly quantify contaminant breakthrough, mixing, sorption, or transport speed at a groundwater interface.
If a site shows much greater intake near zero head than under stronger negative head, the result can motivate additional tracer and monitoring work. It does not prove that a particular pesticide reaches a well within a particular number of hours. Such a claim would require a site-specific transport analysis with appropriate boundary conditions and evidence. The existence of macropores is not itself a universal prediction of a measured travel time.
A careful translation communicates concern proportionately: controlled infiltration results can indicate pressure-sensitive transport pathways, and these pathways may be relevant to runoff, recharge or leaching. It also makes clear that chemical fate and breakthrough depend on more than hydraulic intake. This separation protects both technical accuracy and public understanding.
Comparing field rainfall with disc intake
A rainfall intensity of 25 millimetres per hour may exceed a local measured disc flux of 17 millimetres per hour under a particular negative supply head. That comparison does not prove exactly 8 millimetres per hour of surface runoff will occur. Rainfall acts over a wider area, the surface boundary under rainfall can shift toward ponding, and infiltration capacity changes with moisture and duration. Local connectivity, microtopography and runon also matter.
The arithmetic difference of 8 millimetres per hour is a comparison between two nominal rates. It is not a complete water budget. The applied disc tension may exclude pathways that become active under ponding or intense rainfall. A field could also contain patches with much higher or lower intake than the test spot.
To make the comparison useful, specify rainfall event duration, soil pre-wetting, surface cover, local slope, applied disc head and spatial representativeness. If the goal is predicting runoff, use a model or field observations suited to rainfall-driven boundary conditions. Present the disc result as supporting evidence, not as a one-line runoff calculator.
Troubleshooting an unexpectedly low rate
When a disc test returns a very small intake, proceed through competing explanations. Is the soil genuinely low in near-saturated conductivity at that head? Is a crust restricting entry? Is the membrane in good contact? Has the contact material become a bottleneck? Is a bubble interrupting the water column? Did the reservoir reading or time interval contain an error? Was the imposed pressure actually maintained?
For a hypothetical record, suppose a 10-centimetre-radius disc delivers only 12 millilitres in ten minutes. The volume rate is 1.2 cubic centimetres per minute and the nominal flux is about 2.29 millimetres per hour. The computation may be correct while the physical interpretation remains open. Field notes and repeatable equipment checks are needed before concluding that all subsurface pores have collapsed.
A repair protocol should document what changed and why: inspect membrane saturation, contact, bubbles, leaks, head control, initial water content and surface condition. Do not erase the first result simply because a later number is larger. Instead preserve both records and explain whether the difference is a genuine soil response or an identified measurement defect.
Troubleshooting an unexpectedly high rate
An unusually high reservoir decline may reflect a connected large pathway, but it may also indicate leakage around seals, water escaping outside the intended contact, incorrect area or a wrongly converted reservoir height. First perform a volume and unit audit. Next inspect the apparatus and surface condition. Then compare with independent nearby tests and the pressure-head response.
Imagine a rate calculation that used a disc diameter of ten centimetres as though it were a radius. The area would be inflated fourfold, causing the calculated flux to be four times too small. Conversely, treating a ten-centimetre radius as a five-centimetre radius would make the calculated flux four times too large. A simple geometry mistake can therefore masquerade as a striking difference in soil conductivity.
True preferential flow may be spatially concentrated, so not every outlier is an error. The remedy is not automatic removal. It is independent checking and an explanation of whether unusual values represent a meaningful local hydraulic feature, an equipment fault or a different soil unit. The summary should state what was verified, not merely report an attractive average.
Independent worked transfer tasks and answers
Exercise one: a disc has radius 8 centimetres and receives 64 millilitres over eight minutes. Its area is pi times 64, approximately 201.1 square centimetres. The volume rate is 8 cubic centimetres per minute, so nominal flux is 0.0398 centimetres per minute, approximately 23.9 millimetres per hour. This is the areal supply rate, not automatically hydraulic conductivity.
Exercise two: two negative supply heads are minus 8 centimetres and minus 1 centimetre. Which is closer to saturation? Minus 1 centimetre is closer to zero and has smaller suction magnitude. It may permit additional larger pores to participate, subject to contact and pore geometry. It is incorrect to say minus 8 represents stronger water supply simply because its absolute number is larger.
Exercise three: a cumulative record reads 100, 184, 266 and 349 millilitres at equally spaced ten-minute marks after a reference start. The successive additional volumes are 84, 82 and 83 millilitres. Their near equality supports an approximately stable late interval. Confirm actual timing and pressure before quoting a steady slope; do not use the cumulative values themselves as separate interval rates.
Exercise four: a researcher reports 60 percent macropore flow because flux rose from 8 to 20 millimetres per hour between two heads. The difference is 12, and 12 divided by 20 equals 0.60, but the interpretation requires an explicit partitioning model. Different moisture states and gradients may account for part of the change. The arithmetic is not evidence for an exact pathway percentage.
Exercise five: a rain intensity is 30 millimetres per hour and a tension-disc result is 18 millimetres per hour. Can runoff be announced as 12? No. The disc boundary differs from rainfall and its footprint is local. The proper answer explains which quantities are known and which boundary and spatial evidence is missing.
Frequently asked questions
Does a tension disc directly count macropores? No. It measures infiltration under a controlled boundary, while pore participation is inferred using theory, comparisons, and sometimes complementary tracer or structural evidence. A calculated capillary diameter is an idealized scale, not an observed pore inventory.
Is the steady disc flux identical to unsaturated hydraulic conductivity? Not generally. Disc geometry creates multidimensional flow and capillary gradients. Any conductivity estimate should identify the analytical or numerical method used to interpret the measured flux and its assumptions.
Should I always use the same sequence of supply heads? Use a documented, justified protocol suitable for your objective. Sequence changes antecedent water state, so comparisons must identify the order and stabilization after adjustments. Results from different sequences may not be directly interchangeable.
What if the water reservoir drops but no wetting appears below the membrane? Inspect for leakage, poor contact, unprimed porous material or a faulty reading. Reservoir decline is not by itself proof of water infiltration through the intended soil surface.
Does a higher rate prove greater recharge at depth? No. Recharge depends on profile storage, drainage, lateral flow, evapotranspiration and deeper boundary conditions. Near-surface infiltration is one link in a longer water-balance and transport chain.
Primary methodological sources and related articles
Primary and methodological references: USDA ARS DISC model and inverse infiltration analysis; Logsdon and Jaynes, tension infiltrometer methodology; USGS overview of preferential flow processes in the unsaturated zone; and USGS discussion of preferential flow under unsaturated conditions. All numeric calculations here are original teaching examples, not measurements from these publications.
For related interpretation techniques, see How to Read Soil Water Retention Curves, Air Entry, van Genuchten Parameters and Hysteresis and How to Read Soil Dye-Tracer Staining and Preferential Flow Image Masks. Build clear terminology through the eduKate Vocabulary Learning Hub.