💧 Recharge Well Replenish Calculator

Estimate runoff available to a recharge well, route it through the system's daily hydraulic capacity, and calculate gross and net additional recharge for either a well screened into the saturated aquifer or a dry well terminating above the water table. The runoff engine follows the daily variable-retention Curve Number method used in the original calculator and is aligned with the VWBA 2.0 Recharge method.

Preliminary modelled estimate

ℹ️

Runoff supply and well acceptance are modelled separately. A saturated-aquifer well can use a tested rate or a Dupuit–Thiem screening estimate. A well ending above the water table can use a tested dry-well rate or a long-term infiltration rate applied to its wetted area. The latter estimates transfer into the vadose zone—not instantaneous arrival at the aquifer. This calculator does not design a well or establish that recharge is environmentally beneficial, legally permitted, or safe for groundwater quality. All populated parameter values are illustrative defaults and must be replaced with site-specific evidence.

📁 1. Upload P and PE/PET data

CSV format: Date, Precip (mm), PET (mm). The paired-station format used by the original Curve Number calculator is also accepted. Daily, consecutive data are required; at least three representative years are recommended.

🌧️ 2. Runoff inputs

Project / site name

P–PE/PET series

Contributing area (ha) Area draining to the interception point, not the well footprint.

Curve Number Use a composite CN if the contributing area contains several land covers.

🕳️ 3. Recharge well capacity

Well position relative to water table Use the seasonal-high—not only the current—water-table position.

Capacity basis

Number of identical wells

Sustainable capacity factor (%) Derates hydraulic capacity for clogging, skin/well loss, partial penetration, downtime and uncertainty.

Aquifer specific yield (0–1) Controls the modelled water-table rise per unit aquifer storage.

These inputs define the saturated well and the screening-level water-table response. When Dupuit–Thiem is selected they also determine acceptance capacity. The model assumes a homogeneous, isotropic, unconfined aquifer with radial flow and a fixed background head at the stated radius.

Well internal diameter (m)

Total well depth below ground (m)

Top of screened section (m below ground) The chart shows the screen from this depth to the top depth plus screen length.

Screen length within saturated aquifer (m)

Horizontal aquifer K (m/day) Use field-test data where possible; K commonly spans orders of magnitude.

Initial saturated aquifer thickness (m) Vertical height from the aquifer base to the initial water table.

Static depth to water (m below ground)

Minimum freeboard during recharge (m) Maximum water-level rise = static depth to water − freeboard.

Fixed-head / influence radius (m) Outer radius used for both radial outflow and transient mound storage; requires justification.

This dry-well option estimates exfiltration into the unsaturated formation. It does not use the Dupuit equation and does not simulate wetting-front travel, perched water, or the time at which infiltrated water reaches the aquifer. Field testing at the proposed receiving horizon is preferred.

Dry-well internal diameter (m)

Dry-well base depth below ground (m)

Seasonal-high water-table depth (m below ground) Must be deeper than the dry-well base; verify from seasonal monitoring.

Maximum effective wetted depth (m) Maximum active sidewall height and water-storage depth used by this screening model.

Usable internal storage fraction (%) Use 100% for an open cylinder; use the effective void fraction if aggregate or media occupies the well.

Long-term design infiltration rate (m/day) Field-derived effective flux at the receiving formation, after an appropriate safety factor.

Effective bottom-area contribution (%) Set to 0 for a sealed or conservatively excluded base; sidewall area is always included.

The following aquifer parameters only drive the diagnostic water-table graph. They do not change the dry-well exfiltration capacity or the reported recharge volume.

Background water-table depth (m below ground) Starting level for the project-induced mound; normally at or below the seasonal-high level.

Horizontal aquifer K (m/day)

Initial saturated aquifer thickness (m)

Fixed-head / response radius (m) Screening radius over which the local recharge mound stores and dissipates.

Mean vadose-zone travel time (days) Applied as a first-order lag between dry-well exfiltration and arrival at the water table.

🔀 4. Capture, routing and additionality

Runoff diverted to system (%) Accounts for runoff that bypasses the inlet or is intentionally excluded.

Usable upstream buffer storage (m³) Storage upstream of the well. Dry-well internal storage is calculated separately from its geometry.

Accepted water ultimately reaching aquifer (%) For a dry well, this is the evidenced vadose-zone transmission fraction within the accounting horizon.

Without-project recharge of same water (%) Fraction that would have reached the relevant aquifer without the project.

📊 Results

Modelled project-induced water-table response

Depth below ground at the well, relative to the user-defined constant background level. This is a screening hydrograph of recharge mounding, not a simulation of natural seasonal groundwater variation.

Modelled end-of-day water level in the well

Representative-well water level on the same depth-below-ground scale as the well and its screened or active section.

Water-table response diagnostics

Average annual water balance

Component	Value	Unit	Interpretation

Infiltration-rate sensitivity

Annual net replenish volume recalculated across a range of long-term design infiltration rates, with every other input held fixed. The curve rises while the well is capacity-limited and flattens once runoff supply, not acceptance, becomes the binding constraint.

Capacity diagnostics

Calendar-year results

Partial years are retained and labelled; do not compare them directly with complete years.

Year	Days	Rainfall (mm)	CN runoff (mm)	Runoff supply (m³)	Gross recharge (m³)	Net additional recharge (m³)	Overflow (m³)

📐 Method, inputs and limitations

1. Daily Curve Number runoff supply

The calculator uses the same Neitsch/SWAT variable-retention routine as the source calculator. PET adjusts the daily retention parameter between wet and dry limits. For each day:

Q = 0, when P ≤ 0.2S; otherwise Q = (P − 0.2S)² / (P + 0.8S)

Daily runoff depth is converted to volume as Q × area × 10, because 1 mm over 1 ha equals 10 m³. This is a runoff-volume model; it does not estimate a storm hydrograph or peak inlet flow.

2. Well acceptance capacity

A sustained field-tested rate is preferred for either configuration. For a well screened into an unconfined saturated aquifer, the optional Dupuit–Thiem theoretical per-well capacity is:

Qwell = πK[(h₀ + Δh)² − h₀²] / ln(R / rw)

Here K is horizontal hydraulic conductivity, h₀ is initial saturated thickness, Δh is the permitted rise in head at the well, R is the fixed-head or influence radius, and rw is well radius. The resulting rate is multiplied by the number of wells and the sustainable capacity factor.

The equation assumes steady, horizontal radial flow and full penetration.
It does not represent transient groundwater mounding, vertical anisotropy, layered aquifers, boundaries, neighbouring wells, screen loss or clogging explicitly.
For episodic recharge, partial penetration or sensitive receptors, use an injection test and a transient analytical or numerical groundwater model.
It is not applicable to an unsaturated dry well or infiltration shaft unless saturated radial-flow conditions are demonstrated.

With the head-coupled option, daily acceptance is recalculated as Qt = πK[(h₀ + Δhmax)² − (h₀ + mt−1)²] / ln(R/rw), so capacity declines towards zero as the previous day's modelled project mound consumes the permitted head rise. This keeps the acceptance and water-table models mutually consistent; it remains a quasi-steady screening treatment.

For a dry well that remains above the seasonal-high water table, the wetted-area screening option is:

Aw = πDLw + βπD²/4 and Qdry well = fdAw

Here D is internal diameter, Lw is maximum effective wetted sidewall depth, β is the included fraction of bottom area, and fd is a long-term field-derived design infiltration rate. The same effective rate is applied to sidewall and included bottom area. This is a constant-flux screening model; it does not calculate unsaturated hydraulic conductivity from aquifer K. An optional dynamic wetted-area mode limits the sidewall term to the previous day's modelled standing-water column, which is usually more realistic when the well is often nearly empty.

The rate represents water leaving the dry well and entering the vadose zone—not proof that the same volume reaches the relevant aquifer.
The aquifer-recharge result therefore applies a separate user-defined vadose-zone transmission fraction.
Layering, anisotropy, declining head, wetting-front interaction, perched diversion and clogging are not simulated.
Use an instrumented infiltration/drawdown test or a variably saturated model where these controls matter.

3. Project-induced water-table response

The graph uses a screening-level, well-centred radial storage model. It starts from a constant user-defined background water table and calculates only the incremental mound attributable to project recharge:

Sy Aeff dm/dt = Qin − πK[(h₀ + m)² − h₀²] / ln(R/rw)

Here m is mound height at the well, Sy is specific yield, Qin is the daily accepted (injected) volume per well for a saturated well — or lagged post-transmission recharge for a dry well — and the second term is quasi-steady radial Dupuit outflow. Aeff approximates storage beneath a logarithmic radial mound between the well radius and the stated fixed-head radius. The equation is solved implicitly at a daily time step.

For the above-water-table option, recharge is passed through a first-order vadose reservoir with the user-defined mean travel time before it enters the aquifer model. The reported recharge volume is unchanged; the lag affects only the physical timing and water-table graph.

For a saturated well, the mound is driven by the full injected volume; the recharge-efficiency factor reduces only the accounted recharge benefit, not the physical mound. The modelled well water level equals the formation head at the representative well, so the separate well-level chart is omitted as identical to the water-table series. Wellbore storage, screen or skin losses and the pressure head needed to sustain injection are not added; those effects require a measured head–rate relationship or a transient multi-aquifer-well model.

The graph is an absolute depth referenced to a constant assumed background, but it excludes natural seasonal and long-term groundwater fluctuations.
It is a diagnostic screen, not a substitute for Hantush, MODFLOW or another calibrated transient groundwater model.
Layering, anisotropy, boundaries other than the radial fixed-head boundary, regional gradients, well interference and unsaturated preferential flow are not represented.

4. Daily routing and additional recharge

Diverted runoff is combined with water already in routing storage. For the dry-well configuration, calculated usable internal storage (nφπD²Lw/4) is added to the user-entered upstream buffer, where φ is the usable storage fraction. Daily infiltration or injection is the lesser of water available and effective well capacity. Remaining water fills storage; any excess is overflow. Gross aquifer recharge is accepted water multiplied by the recharge-efficiency or vadose-zone transmission factor. Water remaining in routing storage at the end of the series is reported as a net storage change in the water balance, so diverted water reconciles exactly with acceptance, overflow and storage.

To estimate an end-of-day level in a dry well, remaining combined routing storage is allocated to the calculated dry-well void first, equally among identical wells, and then to upstream buffer storage. Stored volume per well is divided by φπD²/4 to obtain a standing-water column. A plotted level at the well base means the well is dry. This conservative allocation does not resolve within-storm peak water level or drawdown.

Net additional recharge = Gross recharge × (1 − without-project recharge fraction)

This implements the VWBA 2.0 principle that VWB is the difference between with- and without-project recharge. A zero counterfactual must be justified: intercepted runoff may otherwise recharge downstream or support surface-water users and ecosystems.

5. Important exclusions

Sub-daily rainfall intensity, runoff travel time, inlet hydraulics and first-flush diversion.
Groundwater quality, source-water compatibility, geochemistry, mobilised contaminants and pretreatment performance.
Natural background groundwater variation, detailed three-dimensional mounding, groundwater–surface-water interaction and effects on receptors or neighbouring abstractions.
Dry-well separation and siting compliance: the seasonal-high water table and limiting layers must satisfy locally applicable criteria.
Permits, land ownership, construction safety, maintenance requirements and monitoring design.
Whether recharge addresses the relevant shared water challenge or company impact in the right place and season.

Sources

WRI et al. (2025), Volumetric Water Benefit Accounting 2.0, Recharge method D-4.
Neitsch et al. (2011), SWAT Theoretical Documentation, Curve Number and variable-retention equations.
USGS Circular 1104, limitations and numerical treatment of radial flow at injection wells.
USGS MODFLOW 6 Multi-Aquifer Well package guidance, distinguishing simulated well head and aquifer exchange flow.
Philadelphia Water, Stormwater Management Guidance Manual, dry-well testing and wetted-area capacity method.
USGS Unsaturated-Zone Flow guidance, routing infiltration through the vadose zone to recharge.
Carleton (2010), USGS SIR 2010–5102, controls, analytical methods and limitations for groundwater mounding beneath infiltration structures and dry wells.