Skip to content

WL — Constructed Wetland (Vådområde)

Summary

Conversion of agricultural land into a constructed wetland. The wetland retains N through denitrification and sedimentation processes. A permanent land-use change — the field leaves agricultural production.

Eligibility / Potential area

PotV(i,"WL") = Vaadomrpot(i) × Countcy(i) × IniPotV(i) / 5
Vaadomrpot(i) = fraction of field with wetland potential (from new_WL_2018.inc). Only fields with hydrological suitability for wetland creation have potential > 0.

N Effect

NEffM(i,"WL") = 90 kg N/ha/yr   [for all fields with Countcy(i) > 0]
⚠️ Changed 2026-01-20: Previously soil-differentiated: - Sandy soils (< 5): 120 kg N/ha/yr - Clay soils (≥ 5): 190 kg N/ha/yr

The flat 90 kg/ha/yr value is a consensus figure for VP3. The reason for moving to a single value (and why 90 rather than the prior range) should be confirmed.

P Effect

P_Effects_Total(i,"WL") = 0
⚠️ Changed 2023-11-18: Previously PotV(i,"WL") × 1.9 kg P per ha. Set to zero following a model review (email from Hans Estrup/Berit). The rationale is unclear from the code — needs clarification.

Cost

CostM(i,"WL"):
  livestock ≥ 0.8:  3,486 + prodcost(i) + 200  DKK/ha/yr
  livestock < 0.8:  3,486 + prodcost(i)          DKK/ha/yr
Components: - 3,486 DKK/ha/yr: fixed wetland construction/management cost - prodcost(i): opportunity cost of removing the field from production - +200 DKK/ha/yr: livestock surcharge (added 2021-11-05)

Retention type

NR (No retention) — WL converts the field itself into a N-processing unit. No landscape retention is applied because the denitrification happens at the wetland, not downstream.

Mutual exclusions

  • Member of lr1(j) (full land retirement) — contributes to me(i) at weight 1
  • Via mutexc5 and mutexc6: excludes all land management (lm) and buffer zone (bz) measures
  • Cannot be combined with IBZ (mutexc16)
  • VP2 WL on same field blocks further WL selection

Catalog source

DCA Rapport nr. 174 (Eriksen et al., 2020): Chapter "Etablering af vådområde" (p. 344–359). Authors: Carl Christian Hoffmann, Joachim Audet, Annette Baattrup-Pedersen et al.

WL in TargetEcon corresponds to "Etablering af vådområde" (wetland establishment on drained agricultural land, typically in valley areas or on fields with natural high water tables). The catalog distinguishes five wetland types; WL covers primarily drain-irrigated and valley-type wetlands.

N effect in catalog: Average ~150 kg N/ha/yr across wetland types (range: 136 ± 83 for drain-irrigated areas; 180 ± 56 for restored peatlands; 188 ± 97 for valley wetlands). The model's value of 90 kg N/ha (since 2026-01-20) is more conservative than the catalog average. The previous values (120/190 kg N/ha by soil type) were closer to the catalog range for specific wetland types.

Confidence level: *** (high — extensive monitoring of Danish wetlands)

Cost confirmation (Tabel 12, p. 356): The fixed establishment cost of 3,486 DKK/ha/yr in the model matches the catalog exactly: annual annuity of investigation + establishment costs for N-wetlands (assuming 20-year horizon at 4% discount rate). The opportunity cost (prodcost) is the "loss from agricultural use" component listed separately in the catalog.

Catalog CE (Tabel 13, p. 357): - Clay soil (190 kg N/ha effect): budget 34 DKK/kg N, welfare 44 DKK/kg N - Sandy soil (120 kg N/ha effect): budget 39 DKK/kg N, welfare 50 DKK/kg N

P effect in catalog: Mixed. P retention in drain-irrigated wetlands is often negative (P release from peat soil). Valley wetlands with flooding show positive P sedimentation (0.5–1.5 kg P per flooded ha per flooding day). Restored lakes show high variability (−1.4 to +16 kg P/ha/yr). The model's 2023 decision to set P effect = 0 is consistent with this uncertain/mixed evidence.

Climate: Complex — wetland re-establishment stops CO₂ losses from drained peat, but may increase CH₄ emissions. Net climate effect is site-dependent and not reliably quantified.

Data sources

  • Wetland potential: new_WL_2018.inc (2018 data; replaces earlier WetLandPot.inc)
  • N effect: hard-coded in TargetEcon 2026.gms (flat 90 kg N/ha since 2026-01-20)
  • P effect: hard-coded in TargetEcon 2026.gms (set to 0 since 2023-11-18)
  • Cost (prodcost(i)): Cost_new_avg.inc + MarkAccess2.inc

Recent changes & open questions

  1. N effect = 90 kg/ha (2026-01-20): Current figure for VP3. May be revised in future model versions as evidence evolves — value is not final.
  2. P effect = 0 (2023-11-18): Current assumption for VP3. May also be revised — was previously PotV × 1.9. Not permanent.
  3. The tripartite agreement mandates ≥7,500 ha of new wetlands — this constraint (Wetland_eq) applies only to catchments with N targets. Is this correct policy-wise?
  4. ⚠️ Both the N effect value (90) and P effect (0) should be revisited when new empirical evidence or policy guidance becomes available. Track updates in the log.

Scenario appearances

To be filled as scenarios are documented.

  • LRl — fellow NR measure (lower N effect; lower cost; lavbund-specific)
  • PWET — P-targeted wetland (separate measure; different biology and routing)
  • Retention types — WL uses NR (no landscape retention; denitrification occurs at the wetland)
  • Cost concepts — livestock surcharge applies; includes construction cost (3,486 DKK/ha)
  • N Reduction — NR set appears in core equation
  • Tripartite Agreement — WL floor: ≥7,500 ha new wetlands