Retention Types¶
N leaves a field and must travel through the landscape before reaching the coast. Along the way, it is removed by denitrification in soils, riparian zones, and groundwater. The fraction lost in transit is called retention. The model uses two field-level retention parameters and classifies measures into three groups based on which (if any) retention applies.
The three retention classes¶
TR — Total retention (TotRet(i))¶
The full N retention the field experiences as water travels through groundwater, ditches, and streams to the coast.
Applies to: CCS, CCW, EC, IC, SA, FO, EW, LRh, N20, N10
In the equation:
Data source:Totret_2025on2018.inc — field-level total retention values (updated 2025 using 2018 field boundaries).
Intuition: A measure on a field with 80% total retention (e.g. a field far from the coast with slow groundwater flow) delivers only 20% of its gross N reduction to the coastal zone. The model still selects it if it's cheap enough per kg reaching the coast.
SR — Surface retention (SurfRet(i))¶
The retention that applies only to surface flow pathways. Lower than total retention in most cases because surface flow bypasses deep groundwater that has the highest retention.
Applies to: BZ10, BZ20 (and BZ components of comb2 measures)
In the equation:
Data source:Surfret_2025on2018.inc
Intuition: Buffer zones intercept surface runoff and shallow lateral flow — they do not affect deep groundwater. So only the surface-flow portion of landscape retention applies.
NR — No retention (1.0)¶
No landscape retention factor is applied. The full N effect is credited as arriving at the coast.
Applies to: WL (constructed wetland), LRl (land retirement, low-lying)
In the equation:
Intuition: These measures operate on wet/low-lying land where water moves laterally near the surface. The assumption is that N processed on these fields would have arrived at the coast with no significant further retention — or, equivalently, that the wetland/retirement is itself the retention mechanism and should get full credit.
Mini-wetland retention¶
Mini-wetlands (MW) use SurfRet_ID15(ret) — the surface retention of the entire ID15 catchment:
Retention and cost-effectiveness¶
Retention profoundly affects cost-effectiveness rankings: - A cheap measure in a low-retention catchment (near the coast) delivers almost its full gross effect → highly cost-effective. - The same measure in a high-retention catchment delivers only a fraction of its gross effect → less cost-effective. - Expensive, high-effect measures (e.g. WL) compete better in high-retention catchments because NR measures don't suffer the retention penalty.
This means the optimal measure mix differs by catchment — a key feature of the spatial explicitness of the model.
Key data files¶
| Parameter | File |
|---|---|
TotRet(i) |
Totret_2025on2018.inc |
SurfRet(i) |
Surfret_2025on2018.inc |
SurfRet_ID15(ret) |
SurfRet_ID15.inc |
Related pages¶
Model structure: - N Reduction equations - Measures overview
NR measures (no retention): - WL — constructed wetland; NR because it is itself the retention mechanism - LRl — low-lying land retirement; NR because lateral near-surface drainage
SR measures (surface retention): - BZ10 — 10 m buffer zone - BZ20 — 20 m buffer zone
TR measures (total retention): - CCS — catch crop on sandy soil - CCW — catch crop on clay soil - EC — energy crops - FO — afforestation - IC — interrow cover - EW — extended winter cover - SA — short-term set-aside (spatially differentiated) - LRh — high-land retirement (spatially differentiated) - N10 — 10% reduced N application - N20 — 20% reduced N application
Mini-wetlands (special case — surface retention at ID15 level): - MW