WEPP-SWAT+

WEPP-SWAT+ combines two model layers in one workflow: WEPP generates daily hillslope runoff and sediment, and SWAT+ routes those hillslope loads through the channel network. Use it when plain WEPP is not enough because channel-routing behavior is part of the question you need to answer.

What Extends Beyond Standard WEPP

Standard WEPP already gives you hillslope results and, when enabled, watershed/channel routing within WEPP. WEPP-SWAT+ adds a second routing stage:

WEPP remains the hillslope generator,
SWAT+ receives WEPP daily outputs as recall inputs,
SWAT+ routes water and sediment through its channel network.

That means a WEPP-SWAT+ result should be read as:

hillslope generation from WEPP, then
channel routing from SWAT+.

What You Actually See In The UI

When a run is SWAT-enabled, the main control title becomes WEPP-SWAT+ and the main run button becomes Run WEPP-SWAT+.

The additional user-facing SWAT controls appear under SWAT+ Advanced Options:

Hydraulic-Sediment Parameters
- Channel Manning's n (mann)
- Floodplain Manning's n (fpn)
- Channel erodibility factor (erod_fact)
- Channel cover factor (cov_fact)
- Median bed material size (d50)
SWAT Print.prt
- header fields such as nyskip, day_start, yrc_start, day_end, yrc_end, interval
- per-object output toggles for daily, monthly, yearly, and avann
SWAT Exec
- Run SWAT+

If Bootstrap is enabled, users can also see:

Run SWAT+ Channel Routing

The visible SWAT controls are not per-channel editors. They are run-scoped controls layered on top of the existing WEPP form:

UI control	What the user is changing	Why it matters
`Channel Manning's n (mann)`	A uniform roughness override applied to all SWAT-DEG channel records in `hyd-sed-lte.cha`	Higher roughness generally slows routing and attenuates peaks
`Floodplain Manning's n (fpn)`	A uniform floodplain roughness override	Only matters when overbank routing is important; leaving it blank preserves the template default
`Channel erodibility factor (erod_fact)`	A uniform channel erodibility override	Higher erodibility generally raises routed sediment production
`Channel cover factor (cov_fact)`	A uniform SWAT channel cover/armoring parameter	This is calibration-sensitive and can materially change routed sediment behavior; treat it as a routing parameter, not a simple land-cover label
`Median bed material size (d50)`	A uniform bed-material-size override in millimeters	Larger material generally reduces transport efficiency and makes the bed harder to move
`nyskip`, `day_start`, `yrc_start`, `day_end`, `yrc_end`, `interval`	The output window and write cadence for `print.prt`	Controls how much SWAT output is written and for which years or dates
Per-object `daily`, `monthly`, `yearly`, `avann` checkboxes	Which time scales are written for each SWAT object row	Useful for QA and routing review; does not change model physics

The key assumption behind this UI is that the five hydraulic-sediment parameters are uniform overrides. The standard end-user surface does not expose per-order, per-reach, or per-material parameterization.

What The UI Actions Trigger

Several visible controls trigger different API-backed actions:

Run WEPP-SWAT+ This posts the current form to /rq-engine/api/runs/<runid>/<config>/run-wepp. In other words, the main combined button is still backed by the WEPP run route. In a SWAT-enabled run, the submitted form includes both WEPP settings and any SWAT channel-parameter overrides currently shown in the form.
Run SWAT+ This posts to /rq-engine/api/runs/<runid>/<config>/run-swat. Use it when WEPP hillslope outputs already exist and you want to rebuild/run the SWAT+ routing side without rerunning hillslopes.
Run SWAT+ Channel Routing in Bootstrap This uses /rq-engine/api/runs/<runid>/<config>/run-swat-noprep and assumes you already have a bootstrapped input checkout that you intentionally edited.
Changing SWAT Print.prt These edits are saved immediately, not only on full run. Object checkboxes post to /rq-engine/api/runs/<runid>/<config>/swat/print-prt, and the print header fields post to /rq-engine/api/runs/<runid>/<config>/swat/print-prt/meta.

The Most Important UI Controls

UI control	What it means	Why it matters
`Channel Manning's n (mann)`	Roughness for the routed channel	Higher values generally slow routing and dampen peaks
`Floodplain Manning's n (fpn)`	Roughness for floodplain flow when overbank routing matters	Higher values increase overbank resistance
`Channel erodibility factor (erod_fact)`	Ease of erosion in the routed channel	Higher values generally increase routed sediment production
`Channel cover factor (cov_fact)`	SWAT channel cover or armoring parameter	Treat this as a calibration-sensitive sediment-routing parameter rather than a simple "more cover/less cover" switch
`Median bed material size (d50)`	Representative bed-material size	Larger material generally makes transport and detachment harder
`SWAT Print.prt` header fields	Which years or dates are written and how often	Useful when you only need a subset of the record or want to avoid oversized outputs
`SWAT Print.prt` toggles	Which SWAT outputs are written and at what time scale	Useful for QA, routing review, and keeping outputs manageable

What The `SWAT Print.prt` Fields Actually Do

The SWAT Print.prt table is easy to misread because it looks technical and does not change routing directly.

nyskip controls how many early years SWAT skips before writing outputs.
day_start and yrc_start define the first day and year written.
day_end and yrc_end define the last day and year written.
interval controls the write interval for supported output frequencies.
The object rows are raw SWAT object names. Their daily, monthly, yearly, and avann checkboxes only control which text outputs SWAT writes on the next build.

Use these controls to inspect the WEPP-to-SWAT handoff, channel routing response, or recall outputs. Do not expect them to fix routing behavior by themselves.

How To Use It Well

Use the workflow in this order:

Make sure the WEPP hillslope setup is already defensible. If the hillslope forcing is wrong, better SWAT routing will not fix that.
Decide whether you need a combined run or a SWAT-only rerun. Use Run WEPP-SWAT+ when hillslope inputs or WEPP options changed. Use Run SWAT+ when the hillslope outputs already exist and you only changed SWAT-side settings. This distinction matters because the main combined button is still backed by the WEPP route, while Run SWAT+ is the explicit SWAT rerun surface.
Adjust only the SWAT channel parameters you can defend. Manning roughness, erodibility, cover factor, and D50 can change results substantially.
Turn on extra SWAT Print.prt outputs only when they answer a question. For example, enable recall outputs when you need to verify the WEPP-to-SWAT handoff.

Core Assumptions And Remaining Limits

The handoff from WEPP to SWAT+ is daily, not sub-daily. This is a routing workflow built on daily hillslope forcing.
WEPP provides the hillslope runoff and sediment time series; SWAT+ does not replace WEPP hillslope physics.
The current handoff focuses on hydrology and sediment. Nutrients, pesticides, and similar constituents are set to zero unless explicitly supported.
Channel connectivity is derived from the watershed abstraction and hillslope-to-channel mapping, not from a separate surveyed hydraulic network.
Channel geometry is partly derived from watershed geometry and empirical relationships; some channel properties still need literature-based selection or calibration.
The visible Hydraulic-Sediment Parameters are uniform overrides applied across the routed network. The default UI does not let you vary them by reach, order, or material class.
SWAT routing uses the SWAT-DEG channel object set. That is an implementation choice with its own assumptions about channel representation.
Changing print.prt changes what SWAT writes out, not how the model routes flow.

How To Interpret Results

A difference between WEPP and WEPP-SWAT+ does not automatically mean one model is "right" and the other is "wrong." It often means:

the hillslope forcing is the same, but
the channel-routing assumptions differ.

That is why the most useful interpretation is usually:

what changed after routing,
whether those changes are physically plausible,
whether the SWAT-side channel assumptions are strong enough for the decision you are making.