💧 Ditch Level Raising Replenish Calculator

Estimate the additional groundwater stored between drainage ditches when ditch water levels are raised — with the water table capped at ground level

⚙️ 1. Geometry

The strip of aquifer between two parallel ditches, and the length of channel over which levels are raised. All elevations are metres above the impervious base.
Perpendicular distance between the two ditch centrelines
Longitudinal length of the ditch reach where levels are raised
Uniform ground surface elevation above the impervious base. Where the water table reaches zg, recharge is rejected and leaves as surface runoff
Raising one ditch in a parallel network typically raises the water table on both flanks; use 2 only if the strips either side are similar in spacing and boundary levels

⚙️ 2. Aquifer Parameters

Homogeneous aquifer on a horizontal impervious base (Dupuit assumption)
Clay <0.01 · silt 0.01–1 · sand 1–50 · gravel >50
Drainable porosity: peat 0.1–0.5 · sand 0.1–0.3 · silt/clay 0.01–0.1
Net uniform accretion at the water table; negative = net evapotranspiration loss

⚙️ 3. Ditch Water Levels

Baseline = current drained condition; raised = after intervention (e.g. ditch blocking, bunds, adjustable weirs). Levels above ground are treated as brim-full (clamped to zg).

Baseline

Raised

Set one side unchanged if only one ditch is raised

📊 Results

Additional Groundwater Stored –
Storage uplift per metre of channel– Mean water-table rise, Δh̄– Maximum water-table rise– Indicative time to new steady state– Water table vs ground surface Baseline crest– Raised crest– Seepage zone width — baseline– Seepage zone width — raised– Rejected recharge → surface runoff (over channel length × strips) Baseline– Raised– Change due to intervention–
Enter parameters and press Calculate Replenish.

📈 Water-Table Profiles

Baseline water table Raised water table Ground level Seepage / runoff zone Additional stored water (× Sy)
⬇️ Download Detailed Results (CSV)
🔬 Method, assumptions and how to interpret the result

Steady-state water-table profiles are computed for each scenario from the Dupuit–Forchheimer solution for a phreatic aquifer with uniform accretion between two fully penetrating ditches (Bear, Hydraulics of Groundwater, eq. 5-212):

h²(x) = h₀² + (h_L² − h₀²)·x/L + (N/K)·x·(L − x)

Ground-level cap (free-boundary solution). If this unconstrained profile would exceed ground level zg, the water table instead sits at the surface over an interior seepage zone, and recharge falling on that zone is rejected and leaves as surface runoff. Each flank then drains only its own local recharge, giving a smooth-tangency (zero-flux) contact with the seepage zone. The flank widths are:

a = √( K·(z_g² − h₀²) / N ) left flank b = √( K·(z_g² − h_L²) / N ) right flank h²(x) = h₀² + (N/K)·(2ax − x²) for 0 ≤ x ≤ a (mirrored on the right) h(x) = z_g for a ≤ x ≤ L − b (seepage zone)

Rejected recharge = N × (L − a − b) per metre of channel, reported as an annual runoff volume. Ditch levels above zg are clamped to zg (brim-full).

The replenish estimate is the increase in drainable groundwater storage between the two (capped) steady states:

V = S_y · Length · Strips · ∫₀ᴸ [h_raised(x) − h_baseline(x)] dx

Interpretation. The stored volume is a one-off stock gain, not an annual flux. The rejected-recharge figures, by contrast, are annual fluxes: if raising levels enlarges the seepage zone, part of the recharge that previously infiltrated is converted to surface runoff every year — a recurring negative term to weigh against the one-off storage gain in any replenish claim. In steady state the strip's total outflow (ditch drainage + rejected recharge) equals N·L in both scenarios.

The indicative equilibration time is the order-of-magnitude estimate t ≈ Sy·Δh̄/N (recharge-limited filling); actual response also depends on lateral inflow and seasonality.

Assumptions and limits:

  • Homogeneous, isotropic aquifer; horizontal impervious base; horizontal ground surface; steady state.
  • Dupuit assumption (horizontal flow) — small local errors near the crest, the seepage-zone edges and the ditches.
  • Surface water in the seepage zone runs off immediately: no ponded storage, no re-infiltration downslope, and no change to N elsewhere.
  • Ditches fully penetrate to the base and hold constant levels. For shallow field drains, use Hooghoudt's equivalent depth for the base position, or results will be biased.
  • Any change in recharge or evapotranspiration caused by the intervention (e.g. wetter soils increasing ET) is not modelled.