A vegetated riparian buffer removes pollutants from overland runoff mainly two ways: physical filtration/settling (stems and litter slow the sheet flow so suspended sediment drops out) and biogeochemical uptake + denitrification (roots take up dissolved nitrate, and anoxic buffer soils host bacteria that convert it to N₂ gas). Both are rate processes that act on however long a parcel actually spends inside the buffer — its residence time τ — which is the standard first-order model used in USDA-NRCS and EPA riparian-buffer design guidance:
C_out / C_in = exp( -k · τ ), τ = W / v
W = buffer width (m)
v = runoff velocity through the buffer (m/s)
k = removal-rate constant (1/s), set by vegetation type
τ = residence time (s) — how long a parcel spends inside the buffer
Sediment settles faster than nitrate is denitrified, so k_sediment > k_nitrate for the same vegetation — sediment trapping saturates at a fairly narrow buffer width while nitrate keeps improving out to wider buffers. Denser vegetation raises both k values by slowing flow and adding root/microbial surface area.
- Buffer width — more metres to cross, so (at a given velocity) more residence time and more removal.
- Vegetation density — sets k for sediment and nitrate; dense trees + shrub understory filter and denitrify fastest.
- Slope & rainfall intensity — both raise runoff velocity v, which shortens τ = W/v — the real trade-off that limits how much a fixed-width buffer can do during an intense storm: the same buffer removes less because parcels simply spend less time in contact with it.
Note on this model: every parcel decays at rate k per second of time actually spent inside the buffer (not per metre travelled) — that is what makes velocity matter at all. A distance-only decay law would make removal depend solely on W and be blind to v entirely, which would contradict the very point of this trade-off, so the per-second law is the one used here throughout, in the readouts and in the chart below alike.