🌿 Aquatic Invasive Plant Clearance Calculator

Estimate the change in evaporative loss from clearing invasive aquatic vegetation. Cleared area and mat cover are combined with a species evapotranspiration ratio referenced to open-water evaporation, a regional monthly evaporation profile, and an explicit regrowth trajectory, to give a monthly and annual water balance for the treated area. Clearing some species reduces evaporation; clearing others increases it, and the tool reports either outcome.

Preliminary modelled estimate
⚠️
The sign of the benefit is not guaranteed. Whether a floating or emergent stand evaporates more or less than the open water it covers depends on canopy height and roughness, albedo, shading of the water column, and the scale of the stand. Dense low mats such as duckweed and Azolla suppress evaporation, so clearing them costs water. Tall rough canopies such as Typha, papyrus and water hyacinth usually increase it, but the published ratios span a wide range and the highest values come from small isolated plots that are not representative of a large continuous mat. All populated parameter values are illustrative defaults and must be replaced with site-specific evidence.

🗺️ 1. Water body and clearance extent

Determines how a reduction in evaporation translates into a usable benefit; see the counterfactual field below.
Plan area actually treated, measured or mapped, not the area of the water body.
Share of the treated area genuinely occupied by the mat. Open lanes, boat channels and thin edges are not covered.
Sets the point in the evaporation year at which the benefit begins to accrue.
Period over which the benefit is claimed. A one-off clearance without follow-up rarely justifies a horizon beyond the regrowth time.

🌱 2. Invasive stand and replacement cover

Use the ratio basis where the plant sits on water that would otherwise evaporate freely; use the differential basis where it occupies bank, bar or floodplain.
Prefilled with the central value for the selected species; replace with site or regionally measured evidence where available.
Retained share of the measured excess over open water. Tank and lysimeter values are inflated by advection from surrounding dry or open fetch; a large continuous mat cannot sustain the same ratio.
1.00 for open water. Raise it if a native emergent community re-establishes, or if clearance is followed by a submerged-weed or algal-dominated state with different surface properties.

☀️ 3. Open-water evaporation

A regional monthly profile is sufficient at screening level. Enter open-water evaporation directly, or enter reference evapotranspiration and apply an open-water coefficient. Values are millimetres per calendar month, not per day.

Fractional swing about the mean. Low in the humid tropics, high in temperate and continental climates.

🔁 4. Regrowth and follow-up control

Fragments, rooted stubs and the seed or propagule bank. A residual of zero implies no source of reinvasion and is not modellable.
Time for cover to return to 95 per cent of the pre-clearance level under logistic growth. Water hyacinth can close a surface within a single growing season.
Zero for a single one-off operation. Follow-up resets cover to the residual level below.
Follow-up on a light regrowth is usually more complete, and much cheaper, than the initial clearance.

⚖️ 5. Accounting and counterfactual

Share of the avoided evaporative loss that is retained in, or delivered by, the water body the claim relates to. Reduce it where the reservoir spills, where the reach is not water-stressed, or where the water is lost by another pathway instead.
Rate at which cover within the treated area would have increased had nothing been done, capped at complete cover. This is the additionality term: it raises the baseline the project is measured against. Leave at zero unless expansion at the site is mapped and dated.

📚 Evidence base: ET of aquatic plants relative to open water

Ratios of stand evapotranspiration to open-water evaporation from the measurement literature. Ranges are wide because measurement method, plot size and climate all matter more than species identity in many studies. Values are a starting point for screening, not a substitute for site evidence.

Species or group Habit Low Central High Basis and confidence

📐 Method, inputs and limitations

1. The quantity being estimated

Clearing a floating or emergent stand does not remove a water demand in the way that ending an abstraction does. It replaces one evaporating surface with another. The benefit is therefore the difference between the evapotranspiration of the invaded surface and the evaporation of whatever replaces it, and it can be of either sign:

ΔETm = A × (φ − cm) × (Keff − Kpost) × Em × 10

Here A is the treated area in hectares, φ is the pre-clearance cover fraction, cm is the modelled cover fraction in month m, Em is open-water evaporation in mm for that month, and the factor of 10 converts hectare-millimetres to cubic metres. Keff and Kpost are the evaporation ratios of the invaded and post-clearance surfaces relative to open water.

On the differential water-use basis the same monthly structure is retained, but the term (Keff − Kpost) × Em is replaced by an annual differential water use in mm distributed across months in proportion to Em. That basis is appropriate for riparian and floodplain invaders whose water source is soil moisture and groundwater rather than the open water surface.

2. The ET ratio and why the published range is so wide

Reported ratios of macrophyte evapotranspiration to open-water evaporation range from below 0.8 to above 2.5 for the same species. Four mechanisms drive that spread, and they act in opposite directions:

  • Canopy roughness and height raise the aerodynamic conductance of a tall emergent stand well above that of a smooth water surface, increasing ET. This is the dominant effect for Typha, papyrus, Arundo and tall water hyacinth.
  • Shading and albedo reduce the radiation reaching the water and raise the reflected fraction, decreasing evaporation. This dominates for low closed mats such as duckweed and Azolla, which reduce total loss below open water.
  • Heat storage in an open water column sustains evaporation at night and into the autumn, whereas a vegetated surface with closed stomata does not. Bowen-ratio work on Phragmites found daytime ET around a quarter above open water but night-time open-water evaporation roughly three times higher, so the 24-hour ratio approached unity.
  • Advection — the oasis or clothesline effect — supplies sensible heat from surrounding warmer dry or open fetch to a small isolated stand, inflating its measured ET. Lysimeter coefficients above 2.5 have been reported alongside large-stand values roughly half as high at the same site.

The last of these is the reason for the scale adjustment. The excess over open water, not the whole ratio, is what advection inflates, so the adjustment is applied to that excess:

Keff = 1 + s × (Kinv − 1)

A value of s = 1 accepts the measured ratio as applying at project scale. Values around 0.6–0.8 are a defensible screening allowance where the ratio comes from tanks or small lysimeters and the project mat is large and continuous. The adjustment is intended for ratios above unity; where the ratio is below unity the mechanism is shading rather than advection, and s should normally be left at 1.

The break-even ratio at which clearance neither saves nor costs water is Kinv = 1 + (Kpost − 1) / s. Below it, clearance increases evaporative loss.

3. Regrowth, follow-up and the accounting horizon

Cover recovers logistically from the residual left by clearance towards the pre-clearance level, with the growth rate set so that cover reaches 95 per cent of that level after the stated recovery period. Each follow-up operation resets cover to the follow-up residual:

c(t) = φ / (1 + ((φ − r) / r) e−kt),   k = ln(((φ − r) / r) / 0.0526) / T

This matters more than the choice of ET ratio for most projects. A single clearance of a fast-growing floating species delivers a benefit that decays within one growing season, so claiming a full annual volume for a one-off operation overstates it substantially. The tool reports the mean avoided area over the horizon so that the difference between the treated area and the effective area is visible.

  • Logistic recovery is a description, not a mechanism. It does not represent seasonality of growth, herbicide persistence, biological control agents, nutrient limitation or hydrological disturbance.
  • Reinfestation from outside the treated area is not modelled separately; it is embedded in the residual and recovery period.
  • A residual of exactly zero cannot be modelled, because logistic growth from zero never starts. Setting a very small residual with a short recovery period is the usual way to represent aggressive reinvasion.
4. Counterfactual and realisation

Two separate reductions are applied. The realisation factor is the share of avoided evaporation that is retained in or delivered by the water body the claim relates to; the without-project spread term credits infestation that would have expanded beyond the treated area had nothing been done.

  • In a reservoir operating below spill, avoided evaporation is retained storage and realisation approaches 100 per cent. In a reservoir that spills in most years, water saved in the wet season is simply spilled and realisation should be much lower.
  • In a flowing channel, avoided ET appears as increased downstream flow. Whether that constitutes a benefit depends on whether the downstream reach is water-stressed and on who takes the additional water.
  • In a terminal lake or wetland, water not evaporated by the mat remains exposed as open water and continues to evaporate. The difference between the two rates is real, but the residence-time gain may be small and should be checked against a whole-lake balance rather than assumed.
  • The spread term compounds and is easily overstated. It should be left at zero unless there is a mapped, dated record of expansion at the site.
5. Important exclusions
  • Any effect on the water balance other than evaporation: interception, changes to inflow, seepage, and the water content of the removed biomass itself.
  • Regime shift after clearance. Removing a floating mat can release light and nutrients to submerged macrophytes or phytoplankton rather than producing bare open water, which changes both the surface properties and the ecological outcome.
  • Water quality: dissolved oxygen, nutrient release from decomposing biomass, and the fate of herbicide where chemical control is used.
  • Disposal of removed biomass, and the water and greenhouse-gas implications of leaving it to decompose in situ.
  • Ecological consequences for fish, invertebrates and waterbirds, which may be positive or negative and are not volumetric.
  • Navigation, hydropower, abstraction reliability and public health benefits, which are often the actual justification for clearance and are not captured by an evaporation volume.
  • Cost, and the strong dependence of long-run outcome on whether follow-up control is funded.
Sources
  • Snyder, R.L. and Boyd, C.E. (1987), Evapotranspiration by Eichhornia crassipes (Mart.) Solms and Typha latifolia L., Aquatic Botany 27, 217–227 — tank measurements giving E/E₀ of 1.31–2.52 (mean 1.75) and 1.05–2.50 (mean 1.62) respectively.
  • Van der Weert, R. and Kamerling, G.E. (1974), Evapotranspiration of water hyacinth (Eichhornia crassipes), Journal of Hydrology 22, 201–212 — water-balance measurements at Paramaribo and Brokopondo Lake giving 48 and 44 per cent above free-water evaporation.
  • Wondim, Y.K. et al. (2023), Evaluation of the evapotranspiration rate of lacustrine wetland macrophytes in Lake Tana, Ethiopia, Ecohydrology & Hydrobiology — dry-season lysimeter coefficients referenced to open water: E. crassipes 2.02 ± 0.67, Echinochloa stagnina 1.19 ± 0.24, papyrus intermediate.
  • Mohamed, Y.A., Bastiaanssen, W.G.M., Savenije, H.H.G., van den Hurk, B.J.J.M. and Finlayson, C.M. (2012), Wetland versus open water evaporation: an analysis and literature review, Physics and Chemistry of the Earth 47–48, 114–121 — demonstrates that the ratio is site-specific and a function of biophysical surface properties rather than a species constant.
  • Idso, S.B. and Anderson, M.G. (1988), and subsequent constructed-wetland work, on the oasis effect: small isolated stands give lysimeter coefficients around 2.5 where large continuous stands at the same site give roughly half that.
  • Burba, G.G., Verma, S.B. and Kim, J. (1999), A comparative study of surface energy fluxes of three communities (Phragmites australis, Scirpus acutus and open water) in a prairie wetland, Wetlands 19, 451–457 — daytime ET about 25 per cent above open water, night-time open-water evaporation about three times higher.
  • Kollah, B. et al. (2020), Floating Azolla cover influences evapotranspiration from flooded water surfaces, Wetlands 40 — floating cover significantly decreased ET relative to both open water and an artificial cover.
  • Le Maitre, D.C., Forsyth, G.G., Dzikiti, S. and Gush, M.B. (2013), and Morokong, T. et al. (2016), South African Journal of Economic and Management Sciences 19(5), 774–787 — differential water use of riparian invaders per condensed hectare: Pinus about 2,550, Eucalyptus about 1,250, Arundo donax about 1,150, Salix babylonica about 830 and Populus about 640 m³/ha/year.
  • WRI et al. (2025), Volumetric Water Benefit Accounting 2.0 — counterfactual treatment and the requirement to net against a without-project baseline.