SA — Set Aside (Udtagen jord / Braklægning)

Summary

Temporary retirement of agricultural land from production. Unlike LRl/LRh, this is not permanent — the land can return to production. Reduces N leaching by removing N inputs and harvesting.

Eligibility

PotV(i,"SA") = Countcy(i) × IniPotV(i) / 5

N Effect

Spatially differentiated by coastal catchment (k). Formula: NEffM(i,"SA") = prodeff(i) − X where X is the residual N leaching that persists even after set-aside, and varies by catchment:

X value	Catchments	Interpretation
−6	16, 17, 25, 29	Very low residual loss — high N retention
−7 to −8	Most catchments	Typical range
−10 to −12	56, 74, 80, 82, 90, 93, 103, 122–125, ...	Moderate residual loss
−13 to −18	106–131 (many island/coastal catchments)	Higher residual loss
−27	57, 139	Highest residual loss — low background retention

Range: corrections of −6 to −27 across 108 coastal catchments (updated 2026-04-05 to full national coverage; previously ~97 catchments).

Important: For catchments NOT listed in differentiation_effects.inc, NEffM(i,"SA") = 0. As of the 2026-04-05 update, all 108 coastal catchments are covered. Catchments 57 and 139 retain the highest correction (−27); catchments 16, 17, 25, 29 retain the lowest (−6).

The original default (now commented out in main .gms) was prodeff(i) − 27 for all catchments. The differentiation replaced this flat value with catchment-specific residual loss estimates.

The X correction represents the N that would still leach from set-aside land (background atmospheric deposition, soil N mineralization) — smaller X = less residual = larger actual N reduction.

P Effect

Not explicitly set in P effects matrices — SA does not appear in P_Effects_erosion, P_Effects_macropore, or P_Effects_matrix. SA has no P effect in the model.

Cost

CostM(i,"SA"):
  livestock ≥ 0.8:  prodcost(i) + 250 + 200  DKK/ha/yr
  livestock < 0.8:  prodcost(i) + 250          DKK/ha/yr

250 DKK/ha/yr fixed cost + opportunity cost + livestock surcharge.

Retention type

TR

Mutual exclusions

Member of mem(j), lm(j). Tripartite floor: ≥84,000 ha (the largest area floor in the agreement).

Catalog source

DCA Rapport nr. 174 (Eriksen et al., 2020): Chapter "Permanent udtagning og kortvarig brak i omdrift" (p. 115–127). Authors: Gitte Blicher-Mathiesen, Jørgen E. Olesen et al.

SA (Set Aside) in TargetEcon corresponds to the catalog's "kortvarig brak i omdrift" (temporary fallow in rotation) — land that is taken out of production temporarily.

N effect from catalog (Tabel 6, p. 125): - Permanent withdrawal: 49 kg N/ha/yr (reference leaching 61 kg N/ha → residual 12 kg N/ha on brak) - Kortvarig brak (temporary fallow, = SA in model): 34 kg N/ha/yr (slightly lower due to temporary nature and continued soil disturbance) - The catalog uses a flat national average; the model refines this to catchment-specific corrections (6–27 kg residual), consistent with the catalog's concept of residual leaching = 12 kg N/ha average.

The 12 kg N/ha residual confirmed by catalog: "brak land" (uncultivated land) leaches approximately 12 kg N/ha/yr on average, consistent with BZ10/BZ20 formula and SA N effect structure. The correction term X in NEffM = prodeff(i) - X represents the catchment-specific version of this 12 kg residual.

Cost from catalog (Tabel 6): - Budget: 42–96 DKK/kg N (sand 42, clay 96) - Welfare: 54–123 DKK/kg N - The 250 DKK/ha fixed cost in the model matches the catalog's "slåning" (mowing) cost of 250 DKK/ha/yr — required to maintain the fallow land.

Side effects (catalog): Positive for pesticides (no pesticides on SA), positive for nature/biodiversity, neutral for P (no effect unless erosion risk is reduced), positive for climate.

Open questions

1. The 84,000 ha tripartite floor is very large — is SA expected to be the dominant measure in scenarios where the tripartite constraints bind?
2. ✅ Scientific basis for catchment-specific corrections: derived from "YES Differentiation" project (May 2025), which spatially differentiated the 12 kg N/ha residual leaching estimate using N modeling by catchment.
3. What happens in the model when a catchment has no SA N effect (NEffM = 0) but the tripartite SA floor forces SA to be selected there? It would contribute to the floor but not to N reduction — effectively a pure cost.

Data sources

• N effect (spatially differentiated): differentiation_effects.inc (project root; updated 2026-04-05 to 108 catchments per Rolighed et al. 2025 September revision; file header shows May 2025 — generation artifact, content updated later)
• Cost (prodcost(i)): Cost_new_avg.inc + MarkAccess2.inc

Updated scientific source (2025)

Rolighed et al. (2025) — "Differentieret effekt af kvælstofvirkemidler," DCA advisory note for SGAV, revised 30.09.2025. See DCA_diff_effekt_2025.md for full details.

Key updates vs. 2020 catalog: - New BRAK project data (AU Foulum + Flakkebjerg 2021–2024) distinguishes three brak sub-types - National average SA effect: 34 kg N/ha (area-weighted across slåningsbrak 78% / blomsterbrak 22%), based on 2021 reference leaching of 58 kg N/ha - Slåningsbrak (grass): N leaching = 12 kg N/ha → effect = 46 kg N/ha - Slåningsbrak (volunteer crops): N leaching = 29 kg N/ha → effect = 29 kg N/ha - Blomsterbrak: N leaching = 34 kg N/ha → effect = 24 kg N/ha - Differentiated values for all 108 catchments (range: 24–62 kg N/ha depending on catchment and brak type) - The differentiation_effects.inc file (May 2025) was generated from a pre-delivery version; ⚠️ may differ slightly from the final September 2025 values

Related pages

• LRh — same differentiation_effects.inc source; permanent vs temporary retirement
• EC — related land conversion measure; also TR retention type
• DCA differentiation effects — Rolighed et al. (2025); source for per-catchment corrections
• Retention types — SA uses TR (total retention)
• Cost concepts — livestock surcharge applies; prodcost + 250 DKK/ha
• Tripartite Agreement — SA floor: ≥84,000 ha (largest floor)