Capillary Rise & Credit the Water Table
Estimates capillary upflow
A shallow water table feeds your crop for free by capillary rise — water wicks up through the soil into the root zone. Enter your soil texture, water-table depth and root depth to see how many mm/day it contributes, the irrigation it offsets, and the depth where the credit dies.
Enter your soil & water table
Next: credit the 3.5 mm/day the table supplies — cut your irrigation depth by that amount (about 105 mm/month, 70% of demand) and watch the table: if it drops below 3 m from the surface, the credit disappears and you must restore the full rate.
Capillary upflow (mm/day) is interpolated from steady-state upflow tables by soil texture × depth of the water table below the root zone, capped at the crop's ETc (the soil can't deliver more water than the crop transpires). Sources: FAO-56 capillary-rise guidance; Doorenbos & Pruitt / Rijtema-Wesseling steady upflow curves. Planning values — confirm with a piezometer and local soil data.
Capillary rise — key facts
- What it is
- Upward water movement from a water table
- Driver
- Soil texture × depth below root zone
- Capped at
- Crop ETc (mm/day demand)
- Sand credit dies
- ≈ 0.6 m below roots
- Loam credit dies
- ≈ 1.8 m below roots
- Silt loam credit dies
- ≈ 2.5 m below roots
- Waterlogging zone
- ≈ 0.3–0.6 m above root base
- Silt loam @ 1 m below roots
- ≈ 3.5 mm/day
- Monthly offset (3.5 mm/day)
- ≈ 105 mm/month
- Privacy
- Runs in your browser; nothing uploaded
Capillary upflow (mm/day) by soil & depth below the root zone
The capillary credit is a race between how far the water table sits below the roots and how well the soil's fine pores carry water upward. These anchor fluxes per texture come from steady-state upflow tables; the tool interpolates between them. Notice how coarse soils collapse to zero within a metre while fine soils sustain useful upflow far deeper.
| Soil texture | Upflow vs depth below root zone (m → mm/day) | Waterlog above | Credit dies below |
|---|---|---|---|
| Sand | 0.2→4 · 0.4→1.2 · 0.6→0.2 · 0.8→0.05 · 1→0 | 0.3 m | 0.6 m |
| Loamy sand | 0.2→4.5 · 0.4→2 · 0.6→0.7 · 0.8→0.2 · 1→0.05 · 1.2→0 | 0.3 m | 0.8 m |
| Sandy loam | 0.3→5 · 0.6→2.5 · 0.9→1 · 1.2→0.4 · 1.5→0.15 · 2→0.03 | 0.4 m | 1.2 m |
| Loam | 0.5→4.5 · 1→2.5 · 1.5→1.2 · 2→0.5 · 2.5→0.2 · 3→0.08 | 0.4 m | 1.8 m |
| Silt loam | 0.5→5 · 1→3.5 · 1.5→2.2 · 2→1.3 · 2.5→0.6 · 3→0.3 · 3.5→0.1 | 0.5 m | 2.5 m |
| Clay loam | 0.5→3.5 · 1→2.2 · 1.5→1.2 · 2→0.6 · 2.5→0.25 · 3→0.1 | 0.5 m | 2 m |
| Clay | 0.5→2.5 · 1→1.5 · 1.5→0.8 · 2→0.35 · 2.5→0.15 · 3→0.05 | 0.6 m | 1.8 m |
Sources: FAO Irrigation & Drainage Paper 56 (Allen et al.) capillary-rise guidance; Doorenbos & Pruitt (FAO-24); Rijtema & Wesseling steady upward-flux curves; soil-physics upflow tables. Planning values — typical mid-range fluxes per texture class, not exact for any one soil.
Why your water table is free irrigation — until it isn't
Water budgets usually treat irrigation and rainfall as the only inputs and ignore the ground below. But in any field with a shallow water table, water climbs upward through the soil's fine pores — capillary rise — and feeds the root zone continuously. In many irrigated command areas this groundwater contribution genuinely covers a large share of crop water use, so counting it lets you irrigate less without stressing the crop. The contribution is biggest in loams and silt loams, where small pores keep an unbroken water film high above the table; in sand the link snaps within a few tens of centimetres and the credit is near zero.
The catch is that the credit is fragile in both directions. Let the table fall past the soil's critical depth and the upflow dies — often abruptly as the dry season deepens — leaving the crop short if you trimmed irrigation too hard. Let it rise too high, into the waterlogging zone near the root base, and the roots suffocate while evaporating upflow concentrates salts at the surface, the classic mechanism of secondary salinisation. This tool computes the steady upflow for your soil and table depth, caps it at the crop's demand, and flags both failure modes so the “free” water is credited safely.
How to use it
- 1Choose your soil texture — fine soils carry capillary upflow higher and deeper than sand.
- 2Enter the water-table depth below the soil surface, ideally measured with a piezometer.
- 3Enter the active root-zone depth for your crop and growth stage.
- 4Enter the crop's daily water use (ETc); the upflow credit is capped at this demand.
- 5Subtract the contributed mm/day from your irrigation, and keep watching the table so the credit doesn't silently vanish.
Frequently Asked Questions
How much water can a shallow water table contribute to my crop?+
It depends on the soil texture and how far the water table sits below the root zone. In silt loam with the table 1.0 m below the roots, steady capillary upflow is about 3.5 mm/day — enough to offset most of a 5 mm/day crop demand. In sand it dies within 0.6 m of the roots, contributing almost nothing. This calculator interpolates the upflow from steady-state soil-physics tables and caps it at your crop's ETc, because the soil can never deliver more water than the crop actually transpires.
What is capillary rise in soil?+
Capillary rise is water wicking upward through the fine pores of the soil from a water table, against gravity, by surface tension. The narrower the pores, the higher and farther water can climb — which is why clay and loam carry useful upflow much deeper than sand. Above the water table sits a moist 'capillary fringe'; when its top reaches the root zone, the crop can draw water directly from groundwater and you can irrigate less.
At what water-table depth does the capillary contribution stop?+
Each soil has a critical depth below the root zone where upflow falls under about 0.2 mm/day and is no longer worth crediting. For sand it is roughly 0.6 m, sandy loam about 1.2 m, loam about 1.8 m, and silt loam as deep as 2.5 m. Past that depth, treat groundwater as no help and irrigate to the full crop demand. The tool reports this critical depth for your soil and the surface depth at which the credit dies.
Is a high water table good or bad for my crop?+
A moderately shallow table is a free water source, but too shallow is dangerous. If the table is within roughly 0.3–0.6 m of the root base (the waterlogging depth, which varies by soil), the roots can suffocate and salts brought up by evaporating water concentrate at the surface. This tool flags that waterlogging zone separately from the active-credit zone, so a table that looks generous on paper isn't blindly counted.
How do I use the capillary credit to irrigate less?+
Subtract the contributed depth from your irrigation requirement. If your crop needs 5 mm/day and the table supplies 3.5 mm/day of capillary upflow, you only need to irrigate the 1.5 mm/day shortfall — about 45 mm less per month. Keep monitoring the table with a piezometer: if it drops below the critical depth, the credit vanishes and you must restore the full rate, often suddenly as the dry season deepens.
Why does soil texture change the capillary contribution so much?+
Pore size governs capillary rise. Sand has large pores, so water can only be pulled up a short distance before the link breaks — upflow dies within about 0.6 m of the roots. Silt loam and clay loam have fine pores that sustain a continuous water film much higher, delivering useful upflow 1.8–2.5 m below the root zone. That is why the same water-table depth can be worthless on sand and valuable on a loam.
Does the capillary contribution count toward evapotranspiration (ETc)?+
Yes — capillary upflow directly substitutes for irrigation in the soil-water balance. It is capped at the crop's ETc because the upward flux can only replace water the crop is transpiring; if the table could theoretically supply 5 mm/day but the crop only uses 2 mm/day, the credit is 2 mm/day and the offset is 100%. The calculator caps the raw flux at your entered ETc and reports the percentage of demand it covers.
What is the capillary fringe?+
The capillary fringe is the saturated-to-moist band of soil directly above the water table, held up by capillary forces. Its height above the table is larger in fine soils (tens of centimetres in clay) and tiny in sand. When the fringe reaches up into the root zone, roots draw water from it continuously. The widget shades this fringe band between the water table and the root base so you can see whether it actually connects to the roots.
Can capillary rise cause salinity problems?+
Yes. When a shallow water table feeds the surface and the water evaporates, dissolved salts are left behind and accumulate in the root zone — the classic mechanism of secondary salinisation in irrigated command areas. The risk is highest when the table is very shallow (the waterlogging zone) and the water is saline. If your soil is in the waterlogging band, treat the 'free' water with caution and watch electrical conductivity at the surface.
Is 3.5 mm/day a large capillary contribution?+
It is substantial — that is roughly 70% of a typical 5 mm/day peak crop demand and about 105 mm over a month, which is a meaningful share of an irrigation budget. It is the kind of figure you see in a loam or silt loam with the table about 1 m below the roots. On sand the same table depth might contribute under 0.2 mm/day, which is negligible. Always anchor 'large' to your crop's actual demand, which this tool does for you.
How accurate is this capillary rise estimate?+
These are planning-grade values interpolated from published steady-state upflow curves (FAO-56; Doorenbos & Pruitt; Rijtema-Wesseling), representing typical mid-range fluxes for each texture class, not an exact figure for one field. Real upflow depends on the actual unsaturated hydraulic conductivity, soil layering and how steady the table is. Use the result to decide whether the credit is worth pursuing, then confirm with a piezometer and local soil data before cutting irrigation hard.
How is the capillary contribution formula derived?+
Steady upward flux is governed by Darcy's law in the unsaturated zone: the flux equals the unsaturated hydraulic conductivity times the gradient of total head between the water table and the root zone. Because conductivity falls steeply as the soil dries with height above the table, the flux drops sharply with depth — fast in coarse soils, slowly in fine ones. Rather than solve that numerically per soil, the tool reads typical published flux-vs-depth anchor points per texture and interpolates between them, capping at ETc.
What root-zone depth should I enter?+
Use the depth of the active, water-extracting roots for your crop and growth stage — about 0.3–0.6 m for shallow-rooted vegetables and cereals early on, and 1.0–1.5 m for deep-rooted crops like alfalfa or mature maize. A deeper root zone shortens the distance to a given water table, which increases the capillary credit, so the same table helps a deep-rooted crop more than a shallow-rooted one.