How do you interpret a capillary rise curve and decide whether shallow groundwater is actually supplying water to roots? Capillary rise is the upward movement or support of soil water above a water table, but a diagram of maximum rise height is not a measurement of upward water flux. This guide explains capillary rise graphs, upward unsaturated flow, pressure-head gradients, water-table depth, soil texture and implications for evaporation, crops and salinity.
People looking for capillary rise versus groundwater depth, upward soil water flux calculation, water table contribution to evapotranspiration, soil capillary fringe chart or why fine soils draw water higher than sand often encounter apparently conflicting statements. One diagram shows a tall rise in clay; another shows an intermediate soil supplying the greatest sustained flow. The disagreement disappears when static height, conductivity and ongoing demand are separated.
A useful interpretation specifies which quantity the plotted curve represents, where its zero is measured and whether the system is at rest or transporting water. Height above a reservoir, depth below the root zone, water content, matric pressure and equivalent millimetres per day are different observations. Turning one into another requires a physical model or additional measurements, not a change of wording alone.
What capillary rise means in soil
Water interacts with mineral surfaces and pore geometry, creating pressure differences between water and air in unsaturated regions. Small pores can maintain liquid connections against gravity over some height. Above a sufficiently shallow water table, a region of higher moisture can extend upward through the soil matrix as a consequence of this capillary behaviour.
The familiar paper-towel demonstration is useful but incomplete. Soil contains pores of different sizes, connectedness and wettability; field conditions also involve roots, evaporation, salinity and heterogeneous horizons. Capillary rise is not a single vertical column moving intact. It is a description of water distribution and possible flow through a complex pore network.
Static height and dynamic supply are not the same
A capillary rise height describes how far water may be supported above a free-water level under a specified geometry and equilibrium assumption. Dynamic upward flux describes how much water actually crosses a horizontal plane per unit area and time. A soil can support moisture high above groundwater while transmitting only a small ongoing flux.
Conversely, a coarser soil may transmit substantial water close to a water table but fail to sustain meaningful upward delivery across a large vertical separation. Graph labels must say whether they show height, rate or concentration. An impressive static height should never be translated directly into daily crop water supply.
The water table is the reference boundary
In a simple unconfined groundwater description, the water table is the surface at which gauge water pressure is approximately atmospheric. Below that surface, pore-water pressure is generally positive; above it, matric pressure is often negative where soil is unsaturated. The vertical coordinate needs a clear origin and sign.
Depth to the water table is usually measured downward from the land surface, whereas height above the water table is measured upward from the groundwater reference. A line showing a depth increasing from one to two metres represents a falling water table. A chart labelled elevation would show the opposite geometric trend for the same event.
The capillary fringe has a particular meaning
The capillary fringe describes a zone above the water table affected strongly by capillary saturation or near-saturation. Its extent depends on pore-size distribution, soil layering and the definition used. A broad unsaturated region with some groundwater-derived moisture above the fringe should not automatically be labelled fully saturated.
Different textbooks may use a narrow or broad description of capillary influence. The reporting task is to say whether a plotted boundary represents saturated pores, a chosen water-content threshold or detectable upward groundwater contribution. Each definition produces a different apparent rise height.
Fine pores explain the rise-height contrast
In an ideal cylindrical capillary, equilibrium rise height increases as pore radius decreases, all else equal. Real soils have complex pore networks, but the relation explains why fine-textured materials may retain capillary connections at greater heights than coarse sand. The ideal formula is a teaching model, not a substitute for soil hydraulic measurements.
The capacity to hold water high does not guarantee rapid transport. Small pore throats can severely restrict flow, especially as the soil dries and fewer paths remain hydraulically connected. This is the key to interpreting graphs that show high possible rise in a fine soil but limited crop supply at high demand.
Coarse soils may move water quickly over short distances
Sand typically has larger connected pores near saturation and can transmit water readily while remaining wet. However, capillary support across a large vertical distance is limited because those pores empty at relatively small suction. An upward-flow curve for sand may therefore decline rapidly as water-table depth increases.
Do not infer that all sandy profiles have identical critical depths. Grain-size distribution, layering and compaction matter, and roots may reach different depths. State the specific soil description and measured or modelled conductivity relation before applying a graph to a field.
Fine soils may sustain height without enough flux
A clay-rich soil can retain water under relatively strong matric suction, which supports a large static rise conceptually. Yet its unsaturated hydraulic conductivity can be low, especially along the drying path. A crop demanding several millimetres per day may therefore receive less upward water than a static diagram seems to promise.
This difference becomes obvious when a graph uses millimetres per day rather than centimetres of height. A reader must not carry a height conclusion into a supply conclusion. Useful analysis compares upward flux with evaporative or plant demand under the same conditions.
Medium-textured soils can behave differently
Loams and silty soils may combine appreciable capillary reach with conductivity sufficient for meaningful upward movement over certain depth ranges. That combination is why some field studies find notable groundwater contributions to crop evapotranspiration under shallow water tables. The result is contingent rather than universal.
A practical comparison therefore needs more than ranking sand, loam and clay by one property. Plot both the water-retention relationship and conductivity as soil becomes drier. Together they determine how much water can move through the potential gradient created by roots or evaporation.
Distinguish matric pressure from total hydraulic head
Water moves according to gradients of total hydraulic head, which include pressure head and elevation head. In a vertical profile, a larger suction magnitude at a shallower point does not by itself establish upward movement. Gravity contributes a head difference that must be included in the calculation.
A reader comparing two tensiometers should convert pressure units to consistent head units and define the vertical coordinate. Only then can the direction of the hydraulic gradient be assessed. The graph of suction against depth is a component of the hydraulic picture, not the complete driving-force curve.
Pressure-head and elevation-head signs
A common formulation uses total head H = h + z, where h is pressure head and z is elevation head measured upward from a stated datum. In unsaturated soil, h is often negative. The sign and direction of the resulting gradient determine the direction assigned to flow under the chosen convention.
A different coordinate system may measure depth downward and write the equation differently. That is acceptable if used consistently. The mistake is combining a downward-positive depth with an upward-positive elevation formula without translating signs. In technical writing, spell out the coordinate convention before interpreting the curve.
Darcy-type flow needs conductivity as well as gradient
Unsaturated Darcy or Darcy–Buckingham formulations relate water flux to hydraulic conductivity multiplied by a total-head gradient, with a sign convention specifying direction. Hydraulic conductivity changes strongly with soil wetness. The same gradient may correspond to very different fluxes in a wet and a dry profile.
Consequently, a potential profile cannot be converted into millimetres per day with a single fixed conductivity unless that assumption is justified. A well-supported flux estimate identifies the measured or fitted conductivity relation, relevant water contents and uncertainty. That makes the result reproducible instead of merely plausible-looking.
How to read an upward-flux versus depth curve
Many capillary contribution charts place water-table depth or distance beneath roots along the horizontal axis and possible upward flow along the vertical axis. Flux often decreases as the distance increases because the hydraulic path becomes longer and may include drier, poorly conductive soil.
Such curves may represent steady-state model calculations at specified root-zone suction or evaporative demand. They are not universal lookup tables. Read the assumed soil, root-zone condition, groundwater boundary and flux units. A curve made for one demand or hydraulic parameter set should not be applied blindly to another.
Depth below surface differs from depth below roots
A water table one metre below the land surface is not necessarily one metre below the effective root zone. If roots extend to seventy centimetres, the separation from the root-zone base to that water table is only thirty centimetres. This difference can dominate a predicted groundwater contribution.
Conversely, a shallow-rooted system may have a much longer unsaturated path to the same water-table depth. Every graph should make its depth datum explicit. When comparing studies, translate all depths to a common reference or clearly retain separate axes rather than treating unlike distances as interchangeable.
Worked example: equal water-table depths, different roots
Imagine two hypothetical crops growing above a water table one metre below the surface. Crop A uses an effective root depth of twenty-five centimetres, leaving seventy-five centimetres between the root-zone base and groundwater. Crop B uses a depth of seventy-five centimetres, leaving twenty-five centimetres.
The same groundwater depth can therefore yield different possible contributions, even in identical soil. This is not evidence that Crop B necessarily extracts more water; root physiology, salinity and demand still matter. The example shows why water-table depth alone does not define upward supply.
How atmospheric demand enters the problem
Evaporation from soil and transpiration through plants lower water potential in the upper profile and can help create conditions for upward movement from groundwater. On warm, dry, windy days, atmospheric demand may be high, but the actual upward supply is limited by soil conductivity and available hydraulic connection.
A graph with increased evapotranspiration does not prove the capillary flux increased by the same amount. Some additional demand may draw down soil storage or induce stress when hydraulic supply cannot keep pace. Compare demand with independently estimated or modelled flux before claiming that groundwater met the requirement.
The difference between demand and realised flux
Potential evapotranspiration and reference evapotranspiration represent atmospheric-demand-related quantities under specified assumptions. Realised crop evapotranspiration is affected by water availability, crop condition and management. Upward groundwater flow is only one possible source contributing to that realised use.
When a chart plots groundwater contribution as a percentage of crop demand, identify whether the denominator is potential or actual evapotranspiration. Changing denominators can alter a percentage without changing the measured flux. A clear definition is essential for comparisons across experiments, climates or species.
A water balance reveals what is inferred
For a root-zone control volume, infiltration, irrigation and upward lower-boundary flow supply water, while evapotranspiration and downward drainage remove it; storage can change over time. The exact formula depends on the boundary and sign convention. A capillary contribution estimated as a residual may absorb errors from other terms.
A well-formed result states which variables were observed directly and which were inferred. If upward flux is calculated as the unexplained remainder, do not present it as a separate direct sensor measurement. Residual methods are useful, but uncertainty accumulates from rainfall, drainage, storage and evapotranspiration estimates.
Worked example: a daily water budget
Consider a hypothetical day with four millimetres of actual evapotranspiration, one millimetre of effective irrigation, no measured rain or downward drainage and a two-millimetre reduction in root-zone water storage. Under a closed control-volume assumption, an upward contribution of one millimetre would reconcile the balance.
That conclusion depends on whether lateral flow and storage measurements are sufficiently known. If the two-millimetre storage reduction is uncertain by a similar amount, the estimated upward contribution is weak. The example illustrates why a plausible arithmetic residual is not automatically strong physical evidence.
Reading soil-water content profiles
A moisture profile can show relatively wet lower horizons under a dry surface. That pattern is compatible with a shallow groundwater influence, but can also arise from prior infiltration, texture differences or perched water. Volumetric water content alone does not establish present flow direction.
To test an upward-flow hypothesis, examine matric-potential measurements and total-head gradients as well as the water-content pattern. A static profile describes state; flow is a process inferred from forces and transport properties. Translating one into the other demands additional information.
Water-content and matric-potential hysteresis
During drying and rewetting, the relation between water content and matric potential can differ. Soil water retention is not always a single-valued function independent of history. This matters when a model uses one retention curve to estimate potential from measured water content.
An upward wetting influence from groundwater can follow a different path from downward infiltration after rainfall. When field data show loops or unusual delays, consider hysteresis alongside installation and sensor response. Avoid interpreting the difference as measurement error without testing those alternatives.
Layering can create capillary barriers
Water encountering a fine-to-coarse or coarse-to-fine interface may behave differently from water in a uniform column because air-entry properties and pressure continuity change across layers. Under some conditions, water can accumulate above an interface before substantial crossing occurs. Layering also changes upward continuity.
A curve from a homogeneous soil model cannot capture every such effect. Record horizon boundaries, textures, bulk densities and conductivity estimates when translating a field profile into flow statements. A thin restrictive layer may alter water supply more than the average texture classification suggests.
Perched water is not always the regional water table
A perched saturated zone may develop above a low-permeability layer while the main groundwater table remains deeper. A sensor near that zone can show capillary influence even when a monitoring well elsewhere reports a deeper regional water table. The relevant hydraulic boundary may be local.
Before using a water-table-depth graph, establish whether observations refer to a continuous groundwater body or a transient perched feature. Combining them without distinction can produce conflicting depth estimates and incorrect upward-flow assumptions. The phrase shallow groundwater needs a documented measurement location and definition.
Piezometers and observation wells provide context
A monitoring well can provide water-level observations, but screen depth, hydraulic connection, datum and response lag matter. In stratified ground, different wells may measure heads in distinct hydrostratigraphic units rather than one simple water-table surface. A smooth water-level line should not erase that complexity.
Combine water-level monitoring with soil sensors where the question concerns root-zone supply. State whether the plotted groundwater level is measured, interpolated or modelled. A well result cannot by itself specify unsaturated conductivity or prove the rate of upward delivery across the vadose zone.
Why a falling water table is ambiguous
A declining groundwater level may reflect extraction, drainage, seasonal recharge change or upward loss to plants and evaporation. The shape alone does not identify the dominant process. In a large aquifer, plant water use can be a small part of the measured head change.
Conversely, groundwater may supply some crop demand even if the measured water table appears nearly constant because lateral groundwater inflow replenishes the source. Interpret water-level trends with regional water budgets and site conditions. Avoid equating water-table fall numerically with crop evapotranspiration without a storage coefficient and other corrections.
Diurnal groundwater fluctuations
Some shallow groundwater systems display day-night water-level changes related in part to plant water use. However, barometric effects, pumping cycles, nearby water bodies and sensor temperature can also create or modify repeated fluctuations. A rhythmic curve is a clue rather than an identification certificate.
Methods estimating groundwater evapotranspiration from diurnal fluctuations require assumptions about lateral inflow, specific yield and recovery. Readers should check whether those assumptions are satisfied. A figure may support a hypothesis of plant uptake, but the quantitative rate needs more than observing a daytime dip.
Static equilibrium profiles have a different meaning
At static hydraulic equilibrium without net vertical flow, total head can be approximately constant with depth in the connected profile. Matric pressure becomes more negative with elevation above the water table to balance gravitational head. That variation in matric pressure does not itself imply ongoing upward flow.
This is a particularly important conceptual trap. Seeing stronger suction near the surface can look like a pump drawing water up, but the gradient may be exactly balanced by gravity. Only total-head gradient, not matric-pressure gradient considered alone, establishes the driving force for net flow.
How to identify a likely upward-gradient profile
With properly placed sensors, convert measured matric pressures into pressure head and add elevation head at each depth. A total-head profile that decreases in the upward direction is consistent with an upward driving force under the usual flow convention. Then ask whether conductivity is sufficient for meaningful water movement.
Temperature, sensor equilibration and depth measurement uncertainty can matter when the gradient is small. Do not infer exact flux from a noisy gradient alone. A well-presented chart includes uncertainty or at least a realistic description of the possible variation in head.
Zero-flux planes can shift
A soil profile can contain a depth at which the instantaneous vertical flux changes sign: water may move upward above that region under evaporation while moving downward below another part of the profile after infiltration. Such a zero-flux plane is a concept of the current flow field, not a permanent visible horizon.
Its location can change after rain, irrigation and shifts in demand. Inferring it requires hydraulic profiles and suitable assumptions. One water-content minimum does not automatically mark a zero-flux plane. A reliable explanation distinguishes a geometric feature on a plot from a calculated hydraulic boundary.
Capillary supply and soil evaporation
Groundwater-derived water can move upward and ultimately evaporate from a bare soil surface if the hydraulic connection persists. Evaporative demand at the surface can maintain a gradient, while conductivity through a drying near-surface layer can restrict the rate and lead to a falling evaporation stage.
A surface drying curve and an upward-flux curve can therefore move differently over time. Evaporation may decline even when groundwater remains shallow because the upper soil loses conductivity. Avoid explaining the system using only the water-table depth without considering the intervening transport pathway.
Capillary supply and plant transpiration
Roots can extract water from a zone supported by a shallow water table without necessarily extending into fully saturated groundwater. The importance of this supply depends on active rooting depth, soil conductivity, transpiration demand and salinity. The same physical contribution can be advantageous in one context and harmful in another.
A crop water-use chart showing reduced irrigation requirement does not prove improved water quality or long-term sustainability. Groundwater depletion and salt transport remain relevant. Interpret short-term plant access separately from the longer-term fate of groundwater and solutes.
Why waterlogging is a different risk
When the water table rises close to roots, oxygen availability may decline as pore space fills with water. Root respiration and nutrient processes can be affected. More nearby water is therefore not always better. A curve showing greater capillary contribution at shallow depth does not justify raising the water table without limit.
The optimum balance depends on crop species, root distribution, drainage and soil properties. A useful report identifies both the potential water-supply benefit and the possible aeration cost. The presence of two competing effects explains why groundwater management cannot be reduced to maximising upward flux.
Salinity is transported with water
Capillary rise can carry dissolved salts towards a root zone or soil surface. If water subsequently evaporates or plants take up water while salts remain, salinity may increase over time. The severity depends on groundwater chemistry, flux, leaching opportunities, drainage and plant tolerance.
A graph showing millimetres per day of groundwater contribution is not a complete salt-balance diagram. It should not be described as beneficial irrigation without considering groundwater salinity. Where feasible, connect flux records to measured electrical conductivity or ion concentrations through the same period.
Worked example: water delivery versus salt load
Imagine a hypothetical upward flux of two millimetres per day over one square metre. That equals two litres of water per day crossing the relevant horizontal plane. If the groundwater has a specified dissolved-solids concentration, multiplying it by the transferred volume gives a first estimate of solute delivery, subject to transport and mixing.
The calculation is not evidence that all solute remains at the surface. Some may be transported laterally, exchanged with minerals or leached later. The example reminds readers to keep water flux and chemical concentration as separate measured quantities before combining them into an estimated mass balance.
Seasonal wetting can reverse the flow
Rain or irrigation can recharge surface soil and create downward-driving conditions. At other times, root water extraction and evaporation may favour upward supply from groundwater. A seasonal time series may therefore show both signs of lower-boundary flux, depending on convention and model outputs.
A report should not treat capillary rise as a constant addition to every day’s budget. Explain when the term is active and how the model determines its sign. Use distinct words for net upward flux and gross exchanges if the method distinguishes them. Direction matters as much as the total.
What a numerical model contributes
Unsaturated-flow models combine hydraulic conductivity, retention relations, initial states and boundary conditions to simulate time-dependent soil-water movement. They can estimate upward flux at locations that are difficult to instrument directly. However, a simulation is conditional on its parameters and structural assumptions.
Model agreement with one water-table record does not automatically validate every flux estimate. Stronger evaluation compares moisture profiles, matric head and independent water balances across contrasting conditions. Report the model family, boundary assumptions and calibration strategy so the result can be interpreted rather than merely displayed.
Steady-state and transient curves
A steady-state upward-flow curve assumes that conditions do not change over the time scale being represented and that flux through the relevant profile is constant in time. Real rainfall and irrigation events produce transient redistribution. The observed pattern may deviate strongly from a steady-state lookup curve after sudden wetting.
Use steady-state results as a conceptual or conditional benchmark, not as a substitute for event-based monitoring. If the water table drops rapidly, the path length and gradient can change throughout the period. A single curve cannot capture all those dynamics without updating its assumptions.
Worked example: a groundwater-depth curve
Suppose a hypothetical model for one particular loam shows potential upward supply declining from three to one millimetre per day as the separation between root-zone base and water table increases. The appropriate conclusion is that the model predicts less supply at greater separation under those fixed assumptions.
It does not prove that every loam supplies these exact values, nor that the plant’s realised transpiration changes by two millimetres. Soil conductivity, rooting and atmospheric conditions could be different. Label modelled flux and specify the boundary conditions instead of converting an illustrative curve into universal advice.
Comparing methods: tracer evidence
Stable isotopes or other tracers can help investigate whether water used by plants resembles groundwater or recent rainfall. Such evidence addresses possible source contributions, but interpretation depends on mixing, sampling, fractionation, end-member differences and root access. A tracer diagram is not itself a hydraulic flux meter.
When tracer-based and water-balance estimates disagree, examine the assumptions of both. A source fraction and an absolute depth of transpiration are different quantities. To obtain an absolute flux from a fraction, the total water-use denominator must also be independently established.
Comparing methods: thermal and moisture observations
Temperature profiles, soil water content and matric pressure records can constrain movement or reveal patterns consistent with groundwater influence. Each method observes a different aspect of the system. A sudden wetter profile is not a direct signature of upward flow because infiltration can also create moisture changes.
The strongest interpretation combines methods that test distinct hypotheses. Water-level observations establish a boundary; potential profiles indicate gradients; conductivity relations constrain rates; water balances test totals. The value lies in agreement across complementary evidence rather than in the visual complexity of one chart.
Remote sensing and groundwater influence
Satellite-derived vegetation indices, land-surface temperature or evapotranspiration estimates may show that vegetation remains active during dry periods. That pattern can motivate investigation of groundwater reliance, but it does not establish direct connection to a specific water table.
Deep roots, irrigation, stored rainfall and species differences can generate similar remotely sensed patterns. A map of apparent drought resilience needs field evidence before being labelled a capillary-rise map. Keep remote sensing as supporting context rather than substituting it for hydraulic measurements.
Spatial variability across a field
Water-table elevation, terrain, soil texture and root distribution can vary over surprisingly short distances. A piezometer in a depression may not represent an upland plot, and a uniform crop may occupy soils with very different capillary pathways. A single curve can conceal that heterogeneity.
When designing a comparison, record sensor coordinates, surface elevation, water-table datum and soil profiles. For modelling, describe the spatial units over which parameters are averaged. A local estimate should not become a fieldwide or regional rate without evidence that the sampled locations are representative.
Uncertainty in upward-flux estimates
Uncertainty may come from sensor calibration, water-table depth, hydraulic conductivity, root-zone boundary and evapotranspiration estimates. Because conductivity can change sharply with soil moisture, small parameter errors may have large effects on predicted flux. Showing a smooth curve does not remove that sensitivity.
An informative plot can display parameter ranges or alternative hydraulic assumptions rather than only a single line. If uncertainty bands are unavailable, describe the dominant limitations plainly. Do not quote precise daily upward millimetres when the supporting soil physics are known only approximately.
A disciplined interpretation sequence
Begin with graph type: capillary height, water table, water content, matric head, total head or upward flux. Check units and datum. Establish soil profile and effective root depth. Determine whether the data are observed, inferred by water balance or generated by a model.
Then ask whether the plotted evidence supports hydraulic connection, direction and magnitude separately. Compare demand, storage and salinity where relevant. Finish with a bounded statement: groundwater may support the measured root zone under the stated conditions, or the model predicts an upward flux within its uncertainty range. Avoid universal prescriptions.
Translation and meaning across languages
The words rise, head, suction, pressure, depth, recharge and contribution can be misleading when carried between disciplines or languages without definitions. Rise height and upward flux are different dimensions; recharge generally describes downward replenishment, while capillary contribution indicates possible upward delivery to a chosen soil volume.
Keep the datum and sign in every translation of a technical figure. A negative flux may represent upward motion in one software package and downward movement in another. Translating the arrow direction along with the sign makes the meaning clear even when notation differs.
Teaching activity: interpret two axes
Give learners one graph showing height above a water table and another showing upward flux in millimetres per day against water-table depth. Ask them to identify which one describes a possible equilibrium extent and which addresses water supplied over time. Both may involve capillarity but answer different questions.
Then ask whether a one-metre height establishes one millimetre per day of delivery. It does not. Learners should identify the missing hydraulic conductivity and driving-gradient information. This exercise teaches dimensional reasoning and protects against a common error in scientific communication.
Teaching activity: include gravitational head
Provide two hypothetical measurement points at different elevations, each with a stated negative pressure head. Ask learners to add elevation head using one consistent datum and compare total head. Only then should they propose a likely flow direction, taking care with sign convention.
The activity demonstrates why stronger suction alone is insufficient to identify upward flow. A good answer names the required conductivity relation for quantitative flux. The same habits improve interpretation of electric potential, thermal gradients and other multi-component scientific measurements.
Frequently asked: does a higher capillary rise mean faster water supply?
No. Capillary rise height relates strongly to pore geometry and equilibrium suction, while upward flux depends on connected pathways, hydraulic conductivity and a driving gradient. Fine soils can support water at considerable height yet transmit it slowly when unsaturated.
A fast flux near groundwater may decline drastically as the soil profile dries or the vertical path becomes longer. To decide whether roots receive useful water, compare the predicted or measured rate with demand and root location. Do not choose a soil solely from the largest static height shown in a classroom table.
Frequently asked: can shallow groundwater replace irrigation?
Sometimes groundwater contributes substantially to crop use, but it cannot be assumed to replace a fixed percentage of irrigation everywhere. The contribution varies with water-table depth, crop, soil hydraulics, demand, groundwater recharge and salinity. Excessively shallow water tables can also cause waterlogging.
Any management claim requires site-specific monitoring, an honest water balance and a long-term view of salts and groundwater depletion. A curve is evidence about a physical possibility; it is not a universal promise of water savings.
Frequently asked: is a wet lower layer proof of capillary flow?
No. It may reflect previous rainfall, slow drainage, soil layering or a perched zone. The water-content state does not uniquely identify present flow direction. A hydraulic gradient based on total head and a suitable conductivity relation provide stronger evidence.
If the desired claim concerns groundwater origin, tracers or water balance may be required as well. A good interpretation distinguishes the origin of stored water, the direction of current movement and the fraction used by plants; those are separate scientific questions.
Sources and connected learning routes
Read the FAO account of capillary rise, soil type and groundwater management, its introductory soil-and-water explanation, UC Davis guidance on shallow groundwater for crop production and USGS research on unsaturated flow and groundwater evapotranspiration. These sources illustrate diverse conditions rather than a single worldwide capillary flux.
Within eduKateSG, connect hydraulic conductivity curves, soil retention, tensiometer traces and drainage lysimeter records. For precise cross-language terminology, use How English Works: Translation and Meaning Across Languages, Vocabulary Meaning and Precision and Sources, Attribution and Verification.