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PWET — Phosphorus Wetland (Fosforvådområde)

Summary

A constructed wetland specifically designed to retain P from agricultural drainage, distinct from the N-targeted WL wetland. Linked to specific stream reaches — cannot be combined with stream restoration on the same reach.

Eligibility

PotV(i,"PWET") = p_wet(i) × Countcy(i) × IniPotV(i) / 5
p_wet(i) from P_wet.inc — identifies fields suitable for P wetland construction.

P Effect

P_Effects_erosion(i,"PWET") = 15 × PotV(i,"PWET")
Flat rate of 15 kg P/ha/yr of wetland area (a strong P retention effect reflecting sedimentation in the wetland). No macropore or matrix pathway contribution.

N Effect

None (PWET is not in N effect matrices).

Cost

CostM(i,"PWET"):
  wtype 0, 1, 2:  prodcost(i) + 9,733   DKK/ha/yr
  wtype 3:        prodcost(i) + 13,200   DKK/ha/yr
wtype(i) = watercourse type — reflects the width and possibly other characteristics of the adjacent watercourse. Type 3 (wider/larger watercourses) has a higher construction cost (+13,200 vs +9,733 DKK/ha/yr), likely because P wetlands adjacent to larger watercourses require more infrastructure or land.

Constraints

Cannot co-exist with stream re-meandering or raising on the adjacent watercourse (via Pwet1_eq, Pwet2_eq, Pwet3_eq using binary indicator s(w)).

Total P reduction from all PWET in an upstream lake catchment ≤ Total_P_loss(up_lakecatch) — cannot claim more reduction than the catchment's total P load.

Mutual exclusions

Member of pho(j), mem(j), pho_notIBZ(j).

Data sources

  • wtype(i): P_waterways.inc (confirmed 2026-04-05) — watercourse type code per field; integer, most = 0. Separate from stream class (w_class) — field-level attribute for P wetland cost.
  • PWET eligibility (p_wet(i)): P_wet.inc
  • Total_P_loss(up_lakecatch): Total_P_loss.inc — baseline P load from upstream lake catchment (kg P/yr), ~448 entries (confirmed 2026-04-05).
  • P effect, cost structure: hard-coded in TargetEcon 2026.gms
  • WL — N-targeted wetland (different biology and routing; P effect = 0 for WL)
  • IBZ — in-stream buffer zone (also P-only; different construction; no mutual exclusion)
  • P loss pathways — PWET intercepts sedimentation (all P pathways via flat 15 kg/ha rate)
  • P Reduction — PWET catchment cap constraint; link to stream measures
  • Stream measures — PWET mutually exclusive with re-meandering/raising on adjacent reach
  • Cost concepts — prodcost + 9,733 or 13,200 DKK/ha depending on wtype