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BZ10 — Buffer Zone 10 metres (Randzoner 10 m)

Summary

Establishment of a 10-metre permanent vegetated buffer strip along watercourses. Intercepts surface runoff and reduces both N (surface flow) and P (erosion) reaching streams. A permanent land-use change on the buffer strip portion of the field.

Eligibility / Potential area

PotV(i,"BZ10") = BZ10pot(i) × Countcy(i) × IniPotV(i) / 5
BZ10pot(i) = fraction of field within the 10 m buffer zone potential (from PotentialBZ10.inc). Fields with no adjacent watercourse have BZ10pot = 0.

N Effect

NEffM(i,"BZ10") = prodeff(i) − 12 = leaching(i) − 12   [for Countcy(i) > 0]
The -12 correction was added 2021-06-29. Represents the residual leaching that occurs even in the buffer strip (from roots, soil mineralization). Effect is field-specific.

In the N reduction equation, BZ10 uses surface retention (SR): 

contribution to rr(ret) = PotV(i,"BZ10") × NEffM(i,"BZ10") × x(i,"BZ10") × (1 − SurfRet(i)/100)

P Effect

BZ10 has an erosion P effect, but only if significant buffer zone potential exists: 

P_Effects_erosion(i,"BZ10") = erosion_field(i) × 0.62   [if BZ10Pot(i) > 0.2]
The 0.2 threshold (20% of field within BZ potential) was added 2021-07-27. Reasoning: a BZ covering only a small fraction of the field edge is enough to protect the whole field's erosion losses, so we don't scale by potential. A 20% threshold is pragmatic but the appropriate value is still an open question.

Cost

CostM(i,"BZ10") = prodcost(i)   [for Countcy(i) > 0]
Pure opportunity cost — the buffer strip cannot be cropped, so the farmer loses the gross margin for that area. No fixed implementation cost.

Retention type

SR (Surface retention) — uses SurfRet(i) rather than TotRet(i), because buffer zones primarily intercept surface flows, not subsurface drainage.

Mutual exclusions

  • Member of bz(j) and mem(j) — at most one BZ measure per field
  • Cannot be combined with IBZ (mutexc11)
  • Cannot co-exist with land retirement (via mutexc6 and me(i))

Data sources

  • BZ potential: PotentialBZ10.inc
  • Surface retention: Surfret_2025on2018.inc
  • Leaching: leaching.inc
  • N + P effects, cost structure: hard-coded in TargetEcon 2026.gms

Catalog source

DCA Rapport nr. 174 (Eriksen et al., 2020): Chapter "Målrettede, brede og tørre randzoner" (p. 387–393). Authors: Brian Kronvang, Gitte Blicher-Mathiesen et al.

N effect status in catalog: The catalog explicitly states the N effect for delivery to watercourses is "Not assessed" — because there is no data to determine how much of the rootzone reduction in buffer zones actually reaches surface water. The effect depends heavily on local hydrology, soil type, and whether the zone is truly "dry" (not drained). International studies (Valkama et al., 2019 meta-analysis of 46 studies) show 57% reduction in total N in overland flow, but this doesn't translate directly to catchment-scale reduction.

The model's formula NEffM = leaching - 12 is grounded in the catalog's approach: the 12 kg N/ha represents the residual N leaching from a permanently fallow/brak area (confirmed in the catalog's "Permanent udtagning" chapter, Table 1). This is the minimum N leaching floor after taking land out of production.

P effect from catalog: The catalog strongly confirms P benefits: a buffer zone whose width is adapted to overland flow risk can retain 60–80% of the P delivered to the zone via overland flow. The primary mechanism is sedimentation of particle-bound P. Additionally, biomass harvesting can remove 5–24 kg P/ha/yr from the soil P pool (Hille et al., 2019).

Climate: Equivalent to permanent set-aside — reduction of ~1.6 t CO₂/ha/yr from ceased agricultural use.

Side-effects: Positive for pesticide reduction, nature/biodiversity, P retention. Mild positive for climate.

Notes & open questions

  1. BZ20 has the same N effect formula but higher P erosion reduction (0.75 vs 0.62). Why is the N effect identical despite the wider strip? (Catalog confirms N effect does not depend on strip width because the limiting factor is groundwater transport, not strip width.)
  2. The 20% threshold for P effects — should this be validated against empirical data?
  3. Randzonep2.inc — what does this file contain? Possibly related to BZ parameterization.

Scenario appearances

To be filled as scenarios are documented.

  • BZ20 — wider 20 m counterpart; same N effect, higher P erosion reduction (0.75)
  • Combined measures — NPB10_BZ10, NPB20_BZ10, NPB10_BZ20, NPB20_BZ20
  • Retention types — BZ10/20 use surface retention (SR)
  • P loss pathways — erosion pathway that BZ intercepts
  • N Reduction — where the surface retention equation appears
  • P Reduction — BZ P erosion effect equation